Enhanced warming in net-zero scenarios that balance fossil fuel CO2 emissions with CO2 removals through reforestation

Enhanced warming in net-zero scenarios that balance fossil fuel CO2 emissions with CO2 removals through reforestation | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Enhanced warming in net-zero scenarios that balance fossil fuel CO2 emissions with CO2 removals through reforestation Alexander MacIsaac, Kirsten Zickfeld, Pierre Etienne Banville, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6355003/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 25 Feb, 2026 Read the published version in Communications Earth & Environment → Version 1 posted You are reading this latest preprint version Abstract Reforestation is increasingly considered an important nature-based climate solution for achieving net-zero CO 2 emissions as it has the potential to sequester and store substantial quantities of atmospheric CO 2 and generate co-benefits for ecosystems and ecosystem services. However, strategies using reforestation-based CO 2 removal to offset fossil fuel emissions may not lead to the same climate outcome as avoiding the fossil fuel emissions. Here, we use an Earth System model of intermediate complexity to compare the climate outcome of different pathways: a reference net-zero pathway, and net-zero pathways where additional fossil fuel CO 2 emissions relative to the reference pathway are balanced by reforestation-based CO 2 removals (“Net-zero pathways”). Results show that model simulations of Net-zero pathways yield a higher atmospheric CO 2 and warmer climate outcome than the reference simulation. The higher atmospheric CO 2 results from carbon cycle feedbacks, which induce an imperfect compensation between the net land CO 2 flux in reforestation areas that is accounted for as a removal, and the actual change in total land carbon storage. The additional global warming from higher atmospheric CO 2 is further amplified by biogeophysical effects of reforestation. Our research highlights the need for improved methods to measure and track the carbon cycle and climate effects of reforestation, particularly when used to balance or offset fossil fuel emissions in net-zero pathways. Earth and environmental sciences/Climate sciences/Climate change/Climate and Earth system modelling Earth and environmental sciences/Climate sciences/Climate change/Climate-change mitigation Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Meeting the Paris Agreement climate goal of limiting global warming to well below 2ºC and pursuing efforts to limit warming to 1.5ºC since the pre-industrial period will require the global realization of net zero carbon dioxide (CO 2 ) emissions before the end of this century 1 – 3 . Achieving this goal is possible only with deep reductions in the current levels of CO 2 emissions and will likely also require deliberate removals of CO 2 from the atmosphere to balance residual emissions which are difficult to eliminate 2 , 3 . A common assumption underlying net zero – a condition in which anthropogenic CO 2 emissions are balanced by anthropogenic CO 2 removals – is that it yields the same climate outcome as ‘true’ zero – a state where zero CO 2 emissions are achieved without removals 4 – 6 . However, the climate outcome of a net zero and a zero CO 2 emissions state could be different, which would have consequences for mitigation and policy frameworks seeking to stabilize warming at or near specific levels 6 . Here we test this assumption by quantifying the net climate effect of balancing fossil fuel CO 2 emissions with reforestation-based CO 2 removals and comparing it to the effect of a scenario where the addition fossil fuel CO 2 emissions are avoided. Many carbon dioxide (CO 2 ) removal (CDR) methods have been proposed, including direct air capture, bioenergy with carbon capture and storage, and a suite of so-called nature-based climate solutions (NbCS) 7 – 10 . NbCSs are interventions that seek to protect, sustainably manage or restore natural ecosystems with the aim of increased CO 2 sequestration or reducing greenhouse gas emissions while simultaneously benefitting the health of ecosystems and human well-being 10 , 11 . One prominently discussed solution is reforestation: a NbCS for CO 2 removal that is technologically feasible, cost-effective and yields co-benefits for biodiversity and ecosystem services alike 7 , 9 , 12 . These climate, economic, and co-benefits have made reforestation an attractive tool for national and international net zero CO 2 policies and programs. More pointedly, reforestation and afforestation activities currently account for almost all the CDR in deployment and it remains the dominant CDR method in pathways meeting the Paris Agreement 2 , 13 . The prevalence of reforestation in policies and climate change mitigation pathways makes this CDR method an ideal candidate to test the assumption that balancing CO 2 emissions with removals yields the same climate outcome as avoiding the emission. Indeed, recent studies have shown strong potential for reforestation to aide climate mitigation by lowering peak and end-of-century warming, and achieving net negative CO 2 emissions 14 – 18 . The climate benefit is achieved through sequestration and storage of CO 2 in plant biomass and a decrease in atmospheric CO 2 concentration 14 , 17 – 22 . These studies have shown that reforestation could remove 129–377 Gt CO 2 by the end-of-century depending on the extent of reforestation as well as socio-economic and sustainability constraints 14 – 17 . However, there are several reasons why using reforestation-based CO 2 removal to balance fossil fuel CO 2 emissions may not have the same climate effect as avoiding an equivalent quantity of emissions. On one hand these relate to the difficulty associated with estimating how much CO 2 is actually removed from the atmosphere when that carbon is moved to a vegetation carbon pool that itself responds dynamically to both climate changes and the level of CO 2 in the atmosphere 14 , 17 . The atmospheric CO 2 and temperature changes caused by reforestation elicit a carbon cycle feedback response which affects CO 2 fluxes globally 14 , 18 . When the CO 2 removed from the atmosphere is measured as a flux of carbon into reforested areas of land, it does not fully capture the dynamic system response to this removal flux. Consequently, the effective global net CO 2 flux could differ from the reforestation-based CO 2 removal as measured using the net land CO 2 flux in reforested areas. On the other hand, the passive CO 2 sink – i.e. the enhanced vegetation growth driven primarily by CO 2 fertilization – complicates the accounting of reforestation-based CO 2 removals within reforested areas 23 , 24 . If reforestation is occurring in a high CO 2 world, the quantity of CO 2 a forest can remove from the atmosphere is higher than it would be in a lower CO 2 world 23 , 24 . This negative feedback on changes in atmospheric CO 2 is a product of the carbon cycle, not simply the action of planting trees, which means standard reforestation CO 2 removal accounting potentially differs from the ‘true’ reforestation-based CO 2 removal 23 . In addition, reforestation at scale causes biogeophysical (non-carbon) effects that vary by latitude and that can change the impact of reforestation on climate 25 – 32 . For example, across all latitudes, reforestation will decrease surface albedo, which alone would increase surface air temperature (SAT) 17 , 18 . This effect, however, is balanced in the tropics and mid-latitudes by changes in evapotranspiration and sensible heat flux, such that the net effect – when accounting for CO 2 removal and biogeophysical effects – in the tropics is cooling and potentially negligible in the mid-latitudes 25 , 27 , 29 . In high latitudes, the decrease in surface albedo is amplified by snow-masking from evergreen trees in winter months, which yields a net warming in those regions and therefore has left recent studies to exclude high latitudes in their reforestation experiments and analysis 14 – 16 , 26 . As a result of these biogeophysical responses, the climate impact of a reforestation-based CO 2 removal will not balance exactly the climate impact of an emission of equal magnitude. Finally, the gain in biomass under reforestation does not guarantee the permanence of CO 2 storage and, as such, the CO 2 removed from the atmosphere could be subsequently returned 33 . Re-release of the sequestered CO 2 is possible because forests are susceptible to disturbance via drought, pests, wildfire, and future human interference 34 – 36 . Of particular concern are wildfires, which are increasing in frequency and intensity under current levels of climate change and are projected to increase further under future warming 35 – 39 . The consequence of the impermanence of this CO 2 storage mechanism is that re-release is expected to result in additional warming if reforestation is used to balance fossil fuel CO 2 emissions 17 . Given the combination of biogeophysical, carbon cycle, disturbance effects, we would expect that using reforestation to balance fossil fuel CO 2 emissions will have a different climate outcome than avoiding the fossil fuel CO 2 emissions 6 . However, this difference has not been quantified in an Earth system model that includes a representation of the carbon cycle and biogeophysical effects of reforestation and their feedbacks on climate. Here we fill this gap by comparing the climate outcome of different pathways: an idealized deep-mitigation pathway with zero CO 2 emissions achieved by 2050 (and maintained thereafter; Fig. 1 c. & f.) and no new land-use change from 2020 onwards (Baseline simulation), and two net-zero pathways with additional CO 2 emissions balanced by reforestation-based CO 2 removals (Reforestation Net-zero simulations; Fig. 1 ). We implement Global and Sustainable levels of reforestation in a coupled Earth system model of intermediate complexity (UVic ESCM 40 ; see Methods). Reforestation is implemented via the removal of agricultural area, to a global extent in the Global Reforestation simulation and to an extent that preserves agricultural area for an expected population demand for food in the Sustainable Reforestation simulation 8 (see Methods; Fig. 1 a. & d.). We measure reforestation-based CO 2 removals using the yearly net land CO 2 flux in reforested areas relative to the Baseline (Fig. 1 b. & e.). Fossil fuel CO 2 emissions of equal magnitude as this net land flux are then applied alongside the Global and Sustainable Reforestation scenarios (Fig. 1 c. & f.). First, we show the climate effect of reforestation alone compared to the Baseline and second, we compare the effect of using this calculated reforestation-based CO 2 removal as an offset for additional fossil fuel CO 2 emissions against the Baseline. In doing so, we show that balancing extra fossil fuel emissions with reforestation-based removals yields a warmer global temperature trajectory than a scenario where the fossil-fuel emissions are avoided. Results Reforestation simulations Change in forest cover in the UVic ESCM results from the path of implemented land-use change, changes in atmospheric CO 2 and SAT, and internal competition dynamics. By 2100, in the Global and Sustainable Reforestation simulations, these factors lead to 894 Mha and 295 Mha respectively of additional areal forest coverage compared to the Baseline (Supplementary Fig. 1a. & d.). Against a backdrop of the Baseline fossil fuel CO 2 emissions pathway, these increases in forest coverage yield an increase in global land carbon and decreases in both atmospheric CO 2 and SAT (Fig. 2 c. & d.). Globally, land carbon increases over the 21st century in both the Global and Sustainable Reforestation simulations such that by 2100 global land carbon is 189 and 64 Gt CO 2 higher than the Baseline in both cases respectively (Fig. 2 a. & b.; Supplementary Fig. 2). The gain in land carbon in these simulations reduces peak and end-of-century atmospheric CO 2 concentration relative to the Baseline (Fig. 2 c. & d.). Atmospheric CO 2 peaks at 477 ppm in the Baseline around 2042 and this peak is reduced by 6 and 4 ppm for the Global and Sustainable Reforestation cases respectively (Fig. 2 c. & d.). By 2100, as the atmosphere and ocean carbon pools equilibrate with the increase in land carbon (Supplementary Fig. 2), atmospheric CO 2 concentration is further reduced by 17 and 5 ppm relative to the Baseline for the Global and Sustainable Reforestation cases, respectively (Fig. 2 c. & d.). These differences in atmospheric CO 2 result in cooler SAT relative to the Baseline. In the Baseline simulation, SAT peaks at 1.6°C above the pre-industrial (1850–1900) mean with a return to 1.3°C relative to pre-industrial by 2100 (Fig. 2 e. & f.). Compared to the Baseline, Global Reforestation yields a decrease in peak and end-of-century SAT of 0.01°C and of 0.04°C (Fig. 2 e.). The Sustainable Reforestation case yields a similar decrease in peak SAT of 0.01 which remains approximately the same by 2100 (Fig. 2 f.). While the changes in atmospheric CO 2 are drivers of changes in SAT, the biogeophysical effects of reforestation also affect SAT both in and outside of reforested areas (Fig. 2 e. & f.). Globally, biogeophysical effects dampen the cooling effect of reforestation-based CO 2 removal (see Methods, Supplementary Fig. 3). This biogeophysical warming effect on SAT results from changes in the energy balance at the surface: a decrease in albedo in reforested areas leads to greater solar radiation absorbed, which, combined with the greater roughness of forests, leads to an increase in the sensible heat flux, which is only partially offset by a decrease in the latent heat flux (Supplementary Fig. 4). The resulting SAT changes are propagated to distant regions through atmospheric and ocean circulation and climate feedbacks 32 , 41 . Dynamics such as these illustrate the contrasting effects on SAT of reforestation-based CO 2 removals and the biogeophysical effects of reforestation. Without biogeophysical effects, peak SAT is reduced compared to the Baseline by 0.03 and 0.02°C for the Global and Sustainable Reforestation cases respectively (Fig. 2 e, & f.). By 2100, without biogeophysical effects, SAT is reduced by 0.11 and 0.04°C compared to the Baseline for Global and Sustainable Reforestation simulations (Fig. 2 e. & f.). Using the change in forest area relative to the Baseline as a spatial constraint, we calculate the rate of reforestation-based CO 2 removal as the change in the yearly net land CO 2 flux in reforested areas relative to the Baseline (Fig. 1 b. & e., see Methods). In the UVic ESCM, the net land CO 2 flux arises from the balance between net primary productivity (NPP), soil respiration (SR) and a land-use flux. In the Global Reforestation simulation, the rate of reforestation-based CO 2 removal exhibits an initial peak and decline and averages 3.8 Gt CO 2 /yr between 2050–2100, with a cumulative removal over the 21st century of 234 Gt CO 2 . In the Sustainable Reforestation case, the reforestation-based CO 2 removal rate averages approximately 2.1 Gt CO 2 /yr between 2020–2050 and declines to an average of 0.8 Gt CO 2 /yr between 2050–2100 leading to a cumulative removal from 2020–2100 of 88 Gt CO 2 (Supplementary Fig. 5–6). Throughout the 21st century, in both the Global and Sustainable Reforestation simulations, the global net land CO 2 flux relative to the Baseline is lower than this flux in reforested areas (Fig. 2 a. & b.; Supplementary Figs. 2 & 7). Due to the different responses of NPP and soil respiration to changes in atmospheric CO 2 and SAT, the decrease in NPP resulting from the lower CO 2 concentration is not balanced by a corresponding decrease in soil respiration, leading to outgassing of CO 2 in these regions. Outgassing of CO 2 in non-reforested areas implies that accounting for reforestation-based CO 2 removal as the net land CO 2 flux in reforested areas misses the full land carbon cycle response to reforestation, which is relevant to understanding the atmospheric CO 2 concentration response in the Net-zero simulations. Net-zero simulations Although the Net-zero simulations are designed such that the net land CO 2 flux in reforested areas balances the additional fossil fuel CO 2 emissions, atmospheric CO 2 concentration in these simulations is higher than in the Baseline. At its peak, the atmospheric CO 2 concentration in the Global Reforestation Net-zero simulation is approximately 2 ppm higher than in the Baseline, with the difference increasing to 5 ppm by end of the century (Fig. 3 a.). In the Sustainable Reforestation Net-zero case, atmospheric CO 2 is 0.3 ppm higher than in the Baseline at the peak and increases to 1.8 ppm above the Baseline by 2100 (Fig. 3 b.). There are two primary, competing, factors that contribute to the difference in atmospheric CO 2 between the Net-zero and the Baseline simulations. On one hand, the reforestation-based CO 2 removal in the Net-zero simulations is enhanced by the CO 2 fertilization effect. Because the atmospheric CO 2 concentration is higher in the Net-zero than in the Reforestation simulations (used to quantify the reforestation-based CO 2 removal), more CO 2 is sequestered in reforested areas than was accounted for as a removal and used to balance fossil-fuel CO 2 emissions in the Net-Zero simulations (Supplementary Figs. 7–8). On the other hand, as explained above, the global net land CO 2 flux in the Reforestation Simulations is lower than this flux in reforested areas and the same relationship carries into the Net-zero simulations (Fig. 2 e. & f.; Supplementary Fig. 7). The result of this difference is that the additional fossil fuel CO 2 emissions are incompletely balanced by the global net land CO 2 flux leading to higher atmospheric CO 2 concentration in the Net-zero simulations compared to the Baseline (Supplementary Fig. 8). That higher atmospheric CO 2 concentration, alongside the biogeophysical effects of reforestation, contribute to an increase in SAT in the Net-zero simulations relative to the Baseline. As a result, peak SAT is 0.03 and 0.01°C higher than the Baseline for the Global and Sustainable Reforestation Net-zero simulations respectively (Fig. 3 .c & d). By end-of-century the difference in SAT increases to 0.12 and 0.04°C higher than the Baseline for Global and Sustainable Reforestation Net-zero simulations (Fig. 3 c. & d.). As in the Reforestation simulations, biogeophysical effects of reforestation play an important role in amplifying the SAT difference relative to the Baseline. By 2100, without biogeophysical effects (see Methods), the warming above the Baseline would be 0.04 and 0.01°C for the Global and Sustainable Reforestation Net-zero simulations respectively (Fig. 3 c. & d., dotted lines). In the simulations discussed so far it was assumed that CO 2 sequestered through reforestation remains permanently stored. However, forests are susceptible to disturbance via changes in disturbance regimes such as wildfires, droughts, pests, and future deforestation that make carbon stored in vegetation biomass vulnerable to release 33 . We assess the effect of re-release of stored CO 2 in the Net-zero simulations by deforesting half of the reforested area over a twenty-year period centered around the year of peak CO 2 (2035–2055) (Methods; Supplementary Fig. 9). Loss of tree cover through disturbance causes a flux of previously removed CO 2 back into the atmosphere which increases atmospheric CO 2 relative to the Reforestation Net-zero simulations with permanent storage by 8 and 2 ppm in 2100 for the Global and Sustainable cases, respectively (Fig. 4 a. & b.). However, the biogeophysical effect of reverting land cover from forest to grassland almost entirely compensates for the increase in SAT from the increase in atmospheric CO 2 , resulting in only a negligible increase in the Sustainable Reforestation 50% permanent case and a temperature increase of 0.01°C in 2100 in the Global Reforestation 50% permanent case relative to the Reforestation Net-zero simulations (Fig. 4 c. & d.). Discussion and Conclusions We assessed the atmospheric CO 2 concentration and global SAT response in net-zero CO 2 pathways where additional fossil fuel CO 2 emissions are balanced by removals via reforestation. Our results show that using reforestation to balance additional fossil-fuel emissions and maintain a net zero CO 2 pathway yields a 2–5 ppm higher atmospheric CO 2 and a 0.04–0.12°C higher global surface air temperature by 2100 than would be realized in a pathway where the fossil fuel emissions were avoided. The implication is that achieving net-zero CO 2 emissions – where anthropogenic CO 2 removals and emissions balance – does not result in a situation in which the net effect of CO 2 emissions and removals on global mean surface temperature balance each other, as is often assumed. The magnitude of these differences is dependent on the global carbon cycle response to CO 2 emissions and removals, the biogeophysical effects of reforestation, and the permanence of CO 2 removal. Our accounting of the yearly CO 2 removal rate from reforestation is constrained to grid cells where reforestation occurs. This choice reflects common practice in carbon offset protocols used in the voluntary carbon market 402 top-down and book-keeping carbon accounting studies 8 , 16 , 43 , and Earth System Modeling studies 14 , 18 . By constraining the net land CO 2 flux to reforested areas, however, the accounting misses changes in CO 2 fluxes outside of reforested areas in response to changes in atmospheric CO 2 and SAT caused by reforestation (Fig. 1 b. & d.). To assess the effects of accounting for the reforestation-based CO 2 removal via the net land CO 2 flux in reforested areas versus the global flux, we conducted a sensitivity analysis where the global net land CO 2 flux relative to the Baseline was used to balance an equivalent amount of fossil-fuel CO 2 emissions (Supplementary Fig. 10). In these modified Global and Sustainable Reforestation Net-zero simulations, atmospheric CO 2 remains lower than in the Baseline throughout the simulation (Supplementary Fig. 10–11). Similar to the Net-zero simulations, as atmospheric CO 2 concentration is higher in these modified simulations than in the Reforestation simulations, the reforestation-based CO 2 removal is enhanced by the CO 2 fertilization effect (Supplementary Fig. 10), which contributes to why atmospheric CO 2 is lower than the Baseline. However, due to biogeophysical effects, SAT in both simulations remains higher than in the Baseline throughout the 21st century. These results show that even if additional fossil fuel emissions match the global net land CO 2 flux from reforestation, there is larger peak and year-2100 warming relative to a pathway (Baseline) without additional fossil fuel emissions. Recent work has highlighted the importance of the passive CO 2 sink in the context of net zero CO 2 emissions accounting frameworks 24 . By the physical science basis for net zero CO 2 , the passive CO 2 sink can be understood as the net land CO 2 flux that would occur in the absence of reforestation. While our Baseline represents such a counter-factual, it is possible that because our simulations are all coupled-emissions driven, there is a difference between the passive CO 2 sink in the Baseline and the Reforestation and Net zero simulations. However, as the research gap our study fills is to bridge common reforestation-based CO 2 removal accounting methods with net zero CO 2 frameworks, quantification of the passive CO 2 sink in reforestation pathways remains a challenge for future work.Our results show a strong biogeophysical effect of reforestation on SAT, as evidenced by the shaded areas in Figs. 2 and 3 . The effects occur both locally – in reforested areas – and non-locally – outside of reforested areas. While the response of biogeophysical effects is strong in our model, the representation of biogeophysical effects differs between ESMs and therefore the inter-model range of biogeophysical effects is highly uncertain 32 , 44 . Nevertheless, we expect the biogeophysical effects of reforestation to play an important role in net-zero pathways that seek to balance fossil fuel emissions with reforestation-based removals 6 , 26 – 32 , 41 , 44 . An additional methodological choice that could impact the results is the delay between when the CO 2 removal occurs and when it is accounted for as offset. In our study, there is no delay between removals and additional CO 2 emissions. Similar to practices in the voluntary carbon market 42 , it is reasonable to expect a delay between verification of reforestation-based CO 2 removals and the occurrence of additional CO 2 emissions the removal seeks to offset. To quantify the effect of these delays, we conducted a sensitivity analysis with additional CO 2 emissions occurring 1, 5, and 10 years after the removal. Delays between reforestation-based CO 2 removals and additional emissions postpones the timing of the effect of additional emissions on the climate system by the interval of the delay period. Against a backdrop of declining CO 2 emissions, delays between reforestation-based CO 2 removals and additional CO 2 emissions can reduce (10-year delay) peak warming but yields minimal differences in climate benefits by the end-of-the century (Supplementary Fig. 12). The Earth system model used for this study does not include a representation of natural disturbances. Carbon stored in aboveground biomass in our model simulations is therefore permanent if climate conditions support tree growth in a particular region. However, natural disturbance is an important process in assessing the climate benefit of reforestation because it affects the permanence of CO 2 storage 8 , 33 . To assess the effect of disturbances on carbon fluxes, we randomly deforest a fraction (50%) of reforested grid cells. While this prescription of disturbance is idealized, it is representative of randomized stand-replacing events. At a 50% disturbance fraction, our disturbance results are likely extreme and should therefore be viewed as an upper-bound. Regardless of the intensity of the disturbance, however, results from our model suggest that the biogeophysical cooling effect of changing forest cover to grasslands can potentially compensate for the warming induced by the re-release of CO 2 to the atmosphere. A novelty of our Net-zero simulations is that we simulate reforestation-based CO 2 removals and additional fossil fuel emissions occurring simultaneously in a coupled climate model. Results from these simulations reveal three important challenges for accurate accounting of the climate effect of reforestation-based net-zero pathways: 1) Standard reforestation carbon accounting practices that, by necessity, constrain the accounting of CO 2 removals to reforested areas, miss the net land CO 2 flux outside reforested areas. The CO 2 flux in these areas responds dynamically to changes in climate and atmospheric CO 2 induced by reforestation. Consequently, frameworks that limit CO 2 flux accounting to reforested areas miss CO 2 fluxes outside of reforested areas, potentially overestimating reforestation-based CO 2 removals. 2) In the absence of a standard methodology for disentangling the “passive” CO 2 sink due to CO 2 fertilization from the direct action of reforestation, it is possible that common practices of accounting reforestation-based CO 2 removals do not reflect the ‘true’ removal. Frameworks that use the same CO 2 removal accounting irrespective of the level of CO 2 fertilization potentially inaccurately report reforestation-based CO 2 removals. 3) Reforestation interventions entail biogeophysical effects that have the potential to counter a substantial portion of the cooling effect from reforestation-based CO 2 removals. These effects are not limited to regions of reforestation but also manifest in distant locations. The large uncertainty in biogeophysical effects and their non-local nature points to key challenges to the accurate representation of these effects in carbon accounting frameworks. Due to the combination of biogeophysical effects and the mismatch between reforestation-based CO 2 removal accounting and carbon cycle feedbacks, balancing additional fossil fuel CO 2 emissions with reforestation-based CO 2 removals yields a different climate-carbon cycle response than avoiding a fossil fuel CO 2 emission. As such, determining the effective reforestation-based CO 2 removal that would exactly balance the climate outcome of fossil fuel emissions remains a challenge for the research, policy, and voluntary offset market communities. Our results suggest the need to consider Earth system dynamics and feedbacks in net-zero CO 2 pathways and for improved methods to measure and track the climate-carbon cycle effects of reforestation so as to ensure that these effects are correctly accounted for. Research combining observations and Earth system model simulation is needed to reduce the uncertainty of the global biogeophysical effects of reforestation. Consideration should further be paid to reforestation-induced changes in CO 2 fluxes outside reforested areas as these fluxes also influence changes in atmospheric CO 2 and SAT. Future research should seek to include representation of natural disturbance as a climate change feedback, to account for the potential re-release of CO 2 into the atmosphere. Finally, research would benefit from assessment of Earth system effects of a diversity of Nature-based Climate Solutions in net zero CO 2 pathways. Methods Model summary All simulations were run with the University of Victoria Earth Systems Climate Model (UVic ESCM) version 2.10 40 . The UVic ESCM is an intermediate-complexity climate model with a resolution of 1.8 degrees latitude by 3.6 degrees longitude 45 . The atmosphere is represented as a single layer energy-moisture balance model with dynamical feedbacks. The ocean is represented by a 19-layer general circulation model which is coupled to a thermodynamic-dynamic sea ice model 40 . The UVic ESCM includes a land-surface model based on MOSES and the Dynamic Global Vegetation Model (DGVM) TRFFID (the Top-down Representation of Foliage and Flora Including Dynamics) 46 , 47 . Both the ocean and land components include an interactive carbon cycle. On land, the soil is represented by 8 layers with freeze-thaw processes to classify and demark permafrost and a multi-layer soil carbon diffusion model 48 – 50 . TRIFFID allows for five plant functional types (PFTs; broadleaf and needleleaf trees, C3 and C4 grasses, and shrubs) which compete for space via a prescribed dominance hierarchy where trees are the most competitive, followed by the shrub and then the grasses 46 , 47 . Crops are represented by C3 and C4 grasses through prescribed land-cover maps that inhibit the growth of trees and shrubs in agricultural areas 50 . In this fully-coupled arrangement, the UVic ESCM can be driven by prescribed fossil fuel CO 2 emissions and land-use patterns (as well as non-CO 2 forcers) to prognostically determine changes in the Earth system such as atmospheric CO 2 and surface air temperature 40 . CO 2 emissions from land-use changes are calculated internally by the model in response to the removal of forest plant functional types from prescribed agricultural areas. Half of the “harvested” forest carbon is accounted for as land-use emissions and half is transferred to the soil via litter-fall. Simulations The model was spun up over 10,000 years to establish a climate in equilibrium with pre-industrial forcings – including land cover, non-CO 2 radiative forcing, and atmospheric CO 2 concentration as described in CMIP6 protocols 52 . From the spin-up, an historical simulation (esm-hist) was run with observations-based CO 2 emissions, non-CO 2 forcing, and land use change to bring to the model up to 2015, again following CMIP6 protocols 52 . Forcing evolving according to SSP1-1.9 were then used to integrate the model up to the year 2020, which we use as the present-day starting point for future simulations 53 – 56 . All simulations described below were run for an additional 100 years, up to year 2120. The Baseline is an idealized simulation which serves as the reference case to all Reforestation and Reforestation Net-zero simulations. In the Baseline, prescribed fossil fuel CO 2 emissions decline linearly from 2020 to zero by 2050 for a cumulative total of 566 Gt CO 2 between 2020 and 2050 (Fig. 1 c. & f.). In this Baseline simulation, land-use patterns are held constant at the year 2020 extent. Non-CO 2 forcings are prescribed, following the SSP1-1.9 scenario, leading to a peak in combined non-CO 2 forcing at 2030 followed by declining forcing throughout the remainder of the simulation. The Baseline follows a 1.5°C overshoot trajectory in the UVic ESCM with atmospheric CO 2 concentration and global temperature peaking around 2040 and declining thereafter (Fig. 2 e. & f.). Two Reforestation simulations were run to quantify the CO 2 sequestration of reforesting all year 2020 agricultural lands ( Global Reforestation ) and reforesting an extent consistent with natural forest regrowth patterns and constrained by food security and biodiversity safeguards ( Sustainable Reforestation ) as per the Global Reforestation potential from Griscom et al . (2017) 8 and Cook-Patton et al. (2020) 22 . To achieve reforestation in the model, the prescribed agricultural areas were reduced to target extents linearly between 2020 and 2050 and held constant thereafter. In the UVic ESCM this intervention leads to ‘natural (re)growth’ of PFTs, with areal coverage of PFTs evolving based on climate, carbon availability and competition dynamics. In the Sustainable Reforestation case, land cover outside the target reforestation extent was retained as agricultural areas according to the year 2020 land-use patterns in SSP1-1.9. All other forcings (fossil fuel CO 2 emissions, non-CO 2 forcings) were specified as in the Baseline simulation. The reforestation-based CO 2 removals were determined by calculating the annual net atmosphere-to-land CO 2 flux (net land CO 2 flux) in reforested areas relative to Baseline and summing this across reforested grid cells with an increase in areal coverage of trees relative to the Baseline (Fig. 1 b. & e.). The net land CO 2 flux is calculated as the difference between net primary productivity (NPP) and soil respiration (SR), less a residual flux determined by the UVic ESCM to balance changes in land carbon. When agricultural area is prescribed in the UVic ESCM, the residual land CO 2 flux is positive into the atmosphere, as it accounts for both the changes in land cover and the ongoing repression of tree growth to maintain agricultural area. Removing agricultural areas reduces this residual CO 2 flux, and as such, our calculation of reforestation-based CO 2 removals the two-fold action of removing agriculture and allowing PFTs to regrow based on natural competition. Our calculation of reforestation-based CO 2 removals further includes both the direct (i.e. reforestation-induced) and passive CO 2 fluxes from tree growth (i.e. in response to atmospheric CO 2 concentration changes). Two Net-zero simulations were run whereby the yearly removal rate in the Reforestation simulations was applied as a fossil fuel emission on top of the Baseline emissions scenario (Fig. 1 c. & f.). Land-use change was applied as in the Global and Sustainable Reforestation simulations, with the assumption that CO 2 removal through reforestation balances exactly the additional fossil fuel emissions, achieving net zero CO 2 emissions. We prescribed the additional fossil fuel CO 2 to occur simultaneously with the reforestation-based CO 2 removals. Fossil fuel CO 2 emission and non-CO 2 forcings in these simulations evolved as in the Baseline and Reforestation simulations. To quantify the biogeophysical effect of reforestation, an additional set of simulations was run whereby the model was driven with the atmospheric CO 2 concentration as determined from Reforestation and Reforestation Net-zero simulations. Land cover in these simulations was held constant at the year 2020 SSP1-1.9 extent and CO 2 fluxes from land or ocean did not affect the atmospheric CO 2 concentration. This provides an experiment where the climate system responds to the atmospheric CO 2 concentration changes resulting from reforestation without the biogeophysical effects of reforestation. The difference between this set of simulations and the reforestation simulations corresponds to the biogeophysical effect of reforestation. Finally, we assessed how changes in future disturbance regimes could affect the permanence of CO 2 storage from reforestation. In both the Global and Sustainable Reforestation Net-zero cases, we randomly selected 50% of reforested grid cells and deforested the newly grown trees by increasing the agricultural area in those cells linearly between years 2035 and 2055 (Supplementary Fig. 9). The random selection of reforested grid cells was chosen to mimic the effect of stand-replacing natural disturbances, which are currently not represented in the UVic-ESCM, though our method here also captures the potential for changing human land-use decisions leading to the reversal of previous reforestation efforts. Declarations Competing interests The authors declare no competing interests. Additional information Supplementary information is available to editors and reviews in the supplementary information package supplied with this manuscript. Author contributions AJM developed the research questions, designed the study, performed model simulations, analyzed the model output, produced the figures, and led the writing of the manuscript. KZ conceived the research, assisted in data analysis and interpretation, and edited the manuscript. PEB and HDM provided editorial feedback throughout the writing process. References UNFCCC. The Paris Agreement. 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Federated Research Data Repository (2022). https://doi.org/10.20383/102.0552 Mathesius et al. CMIP6 scenarios’ radiative forcing of non-CO 2 greenhouse gases and aerosols for UVic ESCM simulations (1850–2500). Zenodo (2023). https://zenodo.org/records/11061151 Additional Declarations There is NO Competing Interest. Supplementary Files MacIsaacetal2025supplentaryinformation.docx Supplementary Information Cite Share Download PDF Status: Published Journal Publication published 25 Feb, 2026 Read the published version in Communications Earth & Environment → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6355003","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":438200512,"identity":"eaff0e98-e0f7-4314-94af-94a384268a2f","order_by":0,"name":"Alexander MacIsaac","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABEklEQVRIiWNgGAWjYDACdjDJDOUZgDkHGCrwaWHG1MKWwHCGeC1gwGOAVwt/M/MxiZ87rOXMG7gTP/woqJPnbz/z+cPBNgZ5/gbsWiQOsyUb9p5JN5Y5wLtZssfgsOGMM7nbJIBaDGccwGHNYR7DB7xthxNnMPBukGYwOMDYcIN3G/PHNoYEBhxa5A/zfzj4t+1wPVDL5t8MBnX282/wPAY5LEEehxaDwzyMj4G2JEgw8G4D2sKcuOEGDwPIYQkGOLQYHmYzNpZtSzcE2rLNEuiX5I1n0swkDpyTMNyIQ4vc8eZnkm/brOWBtmy+8eNPne2844cffzhQZiMvh8v7CF89QOFKEFI/CkbBKBgFowAPAACJ2lsr+1JSYgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-2677-3015","institution":"Simon Fraser University","correspondingAuthor":true,"prefix":"","firstName":"Alexander","middleName":"","lastName":"MacIsaac","suffix":""},{"id":438200513,"identity":"685f247c-2b36-4884-a508-7c8dee645892","order_by":1,"name":"Kirsten Zickfeld","email":"","orcid":"https://orcid.org/0000-0001-8866-6541","institution":"Simon Fraser University","correspondingAuthor":false,"prefix":"","firstName":"Kirsten","middleName":"","lastName":"Zickfeld","suffix":""},{"id":438200514,"identity":"b964ca24-ccda-4753-8949-f2857c7894dc","order_by":2,"name":"Pierre Etienne Banville","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Pierre","middleName":"Etienne","lastName":"Banville","suffix":""},{"id":438200515,"identity":"c01f3dc0-9486-41f8-a28b-2ef05b99a8ba","order_by":3,"name":"Damon Matthews","email":"","orcid":"https://orcid.org/0000-0003-3625-390X","institution":"Concordia University","correspondingAuthor":false,"prefix":"","firstName":"Damon","middleName":"","lastName":"Matthews","suffix":""}],"badges":[],"createdAt":"2025-04-01 16:40:46","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6355003/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6355003/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s43247-026-03329-x","type":"published","date":"2026-02-25T05:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":81449635,"identity":"3d004ea5-eaf6-45f1-8407-e27f9f5dead5","added_by":"auto","created_at":"2025-04-26 18:38:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":399556,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eCO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003e emissions pathways and the net land CO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003e flux for the Global and Sustainable Reforestation simulations\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e. Growth of forest coverage (green area, a. \u0026amp; d.) leads to an increase in the net land CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e flux in reforested areas relative to the Baseline in both the Global and Sustainable reforestation simulations (solid lines, b. \u0026amp; e.).\u0026nbsp;\u0026nbsp; These reforestation-based CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e removals balance fossil-fuel CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e emissions applied in addition to the Baseline CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e emissions (solid green and blue lines c. \u0026amp; f.) in Global and Reforestation net-zero simulations. Conceptually, CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e removals from reforestation and additional fossil fuel CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e emissions compensate each other, resulting in the same net CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e emissions as in the Baseline.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6355003/v1/3b4542b2cd8a440454177e20.png"},{"id":81449636,"identity":"5cb12378-eb04-4afa-aa4b-d01652151bce","added_by":"auto","created_at":"2025-04-26 18:38:24","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":305192,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eClimate and carbon effects in Reforestation simulations compared to the Baseline.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e Reforestation increases the net land CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e flux relative to the Baseline (a. \u0026amp; b.). That increase leads to a reduction in peak and end-of-century atmospheric CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e concentration (dashed lines c. \u0026amp; d.). Biogeophysical effects of reforestation reduce the impact that the carbon-effects alone would have on surface air temperature change relative to the pre-industrial (1850-1900) mean (shaded area of e. \u0026amp; f.).\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6355003/v1/88d21a6c8ed39e5ae3a1a7f6.png"},{"id":81449637,"identity":"c003a6d0-b2a6-4b1f-8b67-84a86f65cc89","added_by":"auto","created_at":"2025-04-26 18:38:24","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":286488,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eAtmospheric CO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003e and SAT from the Net-zero simulations compared to the Baseline.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e Atmospheric CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e (solid lines a, b) becomes higher in the Net-zero simulation by approximately 2035 and remains higher throughout the 21st century. SAT (solid lines, c \u0026amp; d) likewise becomes higher than the Baseline by approximately 2035 and remains higher throughout the 21st century. Dotted lines in c. and d. show that with the biogeophysical effects of reforestation removed (see Methods), SAT would reduce and become closer to the Baseline. Shaded areas represent the magnitude of the difference in SAT with and without biogeophysical effects of reforestation.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6355003/v1/d60d3813f158f84b31177d66.png"},{"id":81449776,"identity":"e0df37cb-0a5e-45d3-9645-efe065b38f4b","added_by":"auto","created_at":"2025-04-26 18:46:24","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":302887,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eThe effect of the permanence of CO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003e storage on the Net-zero simulation.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e \u0026nbsp;Atmospheric CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e concentration (a, b) and SAT (c, d) for Reforestation Net-zero simulations with random disturbance of forests across the globe (see Methods). Solid lines show results of standard Reforestation Net-zero simulations (100% permanent), and dashed lines show results of Reforestation Net-zero simulations with disturbance (50% permanent). Panels a. and b. show that disturbance of reforested areas has the potential to return previously removed CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e back to the atmosphere. Meanwhile, the effect of the permanence of removed CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e on SAT depends on the trajectory of atmospheric CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e and the biogeophysical effects (c. \u0026amp; d.), which leads to minimal change on SAT in the simulations with 50% permanence relative to the standard Net-zero simulations.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6355003/v1/f33103354591ac2b58c837af.png"},{"id":106391245,"identity":"bbf40c30-8e02-4bfb-8f6d-866399b27468","added_by":"auto","created_at":"2026-04-08 07:16:23","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1860328,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6355003/v1/a1acab4f-6cc2-4f59-8a06-ffea18803835.pdf"},{"id":81449777,"identity":"ba75c25d-0b07-4cd5-815e-2791cd7e57ea","added_by":"auto","created_at":"2025-04-26 18:46:24","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":5334149,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"MacIsaacetal2025supplentaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-6355003/v1/c35bcc210ac253ffd4824bcd.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Enhanced warming in net-zero scenarios that balance fossil fuel CO2 emissions with CO2 removals through reforestation","fulltext":[{"header":"Introduction","content":"\u003cp\u003eMeeting the Paris Agreement climate goal of limiting global warming to well below 2\u0026ordm;C and pursuing efforts to limit warming to 1.5\u0026ordm;C since the pre-industrial period will require the global realization of net zero carbon dioxide (CO\u003csub\u003e2\u003c/sub\u003e) emissions before the end of this century\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Achieving this goal is possible only with deep reductions in the current levels of CO\u003csub\u003e2\u003c/sub\u003e emissions and will likely also require deliberate removals of CO\u003csub\u003e2\u003c/sub\u003e from the atmosphere to balance residual emissions which are difficult to eliminate\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eA common assumption underlying net zero \u0026ndash; a condition in which anthropogenic CO\u003csub\u003e2\u003c/sub\u003e emissions are balanced by anthropogenic CO\u003csub\u003e2\u003c/sub\u003e removals \u0026ndash; is that it yields the same climate outcome as \u0026lsquo;true\u0026rsquo; zero \u0026ndash; a state where zero CO\u003csub\u003e2\u003c/sub\u003e emissions are achieved without removals\u003csup\u003e\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. However, the climate outcome of a net zero and a zero CO\u003csub\u003e2\u003c/sub\u003e emissions state could be different, which would have consequences for mitigation and policy frameworks seeking to stabilize warming at or near specific levels\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. Here we test this assumption by quantifying the net climate effect of balancing fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions with reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals and comparing it to the effect of a scenario where the addition fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions are avoided.\u003c/p\u003e \u003cp\u003eMany carbon dioxide (CO\u003csub\u003e2\u003c/sub\u003e) removal (CDR) methods have been proposed, including direct air capture, bioenergy with carbon capture and storage, and a suite of so-called nature-based climate solutions (NbCS)\u003csup\u003e\u003cspan additionalcitationids=\"CR8 CR9\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. NbCSs are interventions that seek to protect, sustainably manage or restore natural ecosystems with the aim of increased CO\u003csub\u003e2\u003c/sub\u003e sequestration or reducing greenhouse gas emissions while simultaneously benefitting the health of ecosystems and human well-being\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. One prominently discussed solution is reforestation: a NbCS for CO\u003csub\u003e2\u003c/sub\u003e removal that is technologically feasible, cost-effective and yields co-benefits for biodiversity and ecosystem services alike\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. These climate, economic, and co-benefits have made reforestation an attractive tool for national and international net zero CO\u003csub\u003e2\u003c/sub\u003e policies and programs. More pointedly, reforestation and afforestation activities currently account for almost all the CDR in deployment and it remains the dominant CDR method in pathways meeting the Paris Agreement\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe prevalence of reforestation in policies and climate change mitigation pathways makes this CDR method an ideal candidate to test the assumption that balancing CO\u003csub\u003e2\u003c/sub\u003e emissions with removals yields the same climate outcome as avoiding the emission. Indeed, recent studies have shown strong potential for reforestation to aide climate mitigation by lowering peak and end-of-century warming, and achieving net negative CO\u003csub\u003e2\u003c/sub\u003e emissions\u003csup\u003e\u003cspan additionalcitationids=\"CR15 CR16 CR17\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. The climate benefit is achieved through sequestration and storage of CO\u003csub\u003e2\u003c/sub\u003e in plant biomass and a decrease in atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan additionalcitationids=\"CR18 CR19 CR20 CR21\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. These studies have shown that reforestation could remove 129\u0026ndash;377 Gt CO\u003csub\u003e2\u003c/sub\u003e by the end-of-century depending on the extent of reforestation as well as socio-economic and sustainability constraints\u003csup\u003e\u003cspan additionalcitationids=\"CR15 CR16\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eHowever, there are several reasons why using reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal to balance fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions may not have the same climate effect as avoiding an equivalent quantity of emissions. On one hand these relate to the difficulty associated with estimating how much CO\u003csub\u003e2\u003c/sub\u003e is actually removed from the atmosphere when that carbon is moved to a vegetation carbon pool that itself responds dynamically to both climate changes and the level of CO\u003csub\u003e2\u003c/sub\u003e in the atmosphere\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. The atmospheric CO\u003csub\u003e2\u003c/sub\u003e and temperature changes caused by reforestation elicit a carbon cycle feedback response which affects CO\u003csub\u003e2\u003c/sub\u003e fluxes globally\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. When the CO\u003csub\u003e2\u003c/sub\u003e removed from the atmosphere is measured as a flux of carbon into reforested areas of land, it does not fully capture the dynamic system response to this removal flux. Consequently, the effective global net CO\u003csub\u003e2\u003c/sub\u003e flux could differ from the reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal as measured using the net land CO\u003csub\u003e2\u003c/sub\u003e flux in reforested areas.\u003c/p\u003e \u003cp\u003eOn the other hand, the passive CO\u003csub\u003e2\u003c/sub\u003e sink \u0026ndash; i.e. the enhanced vegetation growth driven primarily by CO\u003csub\u003e2\u003c/sub\u003e fertilization \u0026ndash; complicates the accounting of reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals within reforested areas\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. If reforestation is occurring in a high CO\u003csub\u003e2\u003c/sub\u003e world, the quantity of CO\u003csub\u003e2\u003c/sub\u003e a forest can remove from the atmosphere is higher than it would be in a lower CO\u003csub\u003e2\u003c/sub\u003e world\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. This negative feedback on changes in atmospheric CO\u003csub\u003e2\u003c/sub\u003e is a product of the carbon cycle, not simply the action of planting trees, which means standard reforestation CO\u003csub\u003e2\u003c/sub\u003e removal accounting potentially differs from the \u0026lsquo;true\u0026rsquo; reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn addition, reforestation at scale causes biogeophysical (non-carbon) effects that vary by latitude and that can change the impact of reforestation on climate\u003csup\u003e\u003cspan additionalcitationids=\"CR26 CR27 CR28 CR29 CR30 CR31\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. For example, across all latitudes, reforestation will decrease surface albedo, which alone would increase surface air temperature (SAT)\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. This effect, however, is balanced in the tropics and mid-latitudes by changes in evapotranspiration and sensible heat flux, such that the net effect \u0026ndash; when accounting for CO\u003csub\u003e2\u003c/sub\u003e removal and biogeophysical effects \u0026ndash; in the tropics is cooling and potentially negligible in the mid-latitudes\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. In high latitudes, the decrease in surface albedo is amplified by snow-masking from evergreen trees in winter months, which yields a net warming in those regions and therefore has left recent studies to exclude high latitudes in their reforestation experiments and analysis\u003csup\u003e\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. As a result of these biogeophysical responses, the climate impact of a reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal will not balance exactly the climate impact of an emission of equal magnitude.\u003c/p\u003e \u003cp\u003eFinally, the gain in biomass under reforestation does not guarantee the permanence of CO\u003csub\u003e2\u003c/sub\u003e storage and, as such, the CO\u003csub\u003e2\u003c/sub\u003e removed from the atmosphere could be subsequently returned\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. Re-release of the sequestered CO\u003csub\u003e2\u003c/sub\u003e is possible because forests are susceptible to disturbance via drought, pests, wildfire, and future human interference\u003csup\u003e\u003cspan additionalcitationids=\"CR35\" citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. Of particular concern are wildfires, which are increasing in frequency and intensity under current levels of climate change and are projected to increase further under future warming\u003csup\u003e\u003cspan additionalcitationids=\"CR36 CR37 CR38\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. The consequence of the impermanence of this CO\u003csub\u003e2\u003c/sub\u003e storage mechanism is that re-release is expected to result in additional warming if reforestation is used to balance fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eGiven the combination of biogeophysical, carbon cycle, disturbance effects, we would expect that using reforestation to balance fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions will have a different climate outcome than avoiding the fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. However, this difference has not been quantified in an Earth system model that includes a representation of the carbon cycle and biogeophysical effects of reforestation and their feedbacks on climate.\u003c/p\u003e \u003cp\u003eHere we fill this gap by comparing the climate outcome of different pathways: an idealized deep-mitigation pathway with zero CO\u003csub\u003e2\u003c/sub\u003e emissions achieved by 2050 (and maintained thereafter; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec. \u0026amp; f.) and no new land-use change from 2020 onwards (Baseline simulation), and two net-zero pathways with additional CO\u003csub\u003e2\u003c/sub\u003e emissions balanced by reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals (Reforestation Net-zero simulations; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). We implement Global and Sustainable levels of reforestation in a coupled Earth system model of intermediate complexity (UVic ESCM\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e; see Methods). Reforestation is implemented via the removal of agricultural area, to a global extent in the Global Reforestation simulation and to an extent that preserves agricultural area for an expected population demand for food in the Sustainable Reforestation simulation\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e (see Methods; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. \u0026amp; d.). We measure reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals using the yearly net land CO\u003csub\u003e2\u003c/sub\u003e flux in reforested areas relative to the Baseline (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. \u0026amp; e.). Fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions of equal magnitude as this net land flux are then applied alongside the Global and Sustainable Reforestation scenarios (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec. \u0026amp; f.). First, we show the climate effect of reforestation alone compared to the Baseline and second, we compare the effect of using this calculated reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal as an offset for additional fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions against the Baseline. In doing so, we show that balancing extra fossil fuel emissions with reforestation-based removals yields a warmer global temperature trajectory than a scenario where the fossil-fuel emissions are avoided.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eReforestation simulations\u003c/h2\u003e \u003cp\u003eChange in forest cover in the UVic ESCM results from the path of implemented land-use change, changes in atmospheric CO\u003csub\u003e2\u003c/sub\u003e and SAT, and internal competition dynamics. By 2100, in the Global and Sustainable Reforestation simulations, these factors lead to 894 Mha and 295 Mha respectively of additional areal forest coverage compared to the Baseline (Supplementary Fig.\u0026nbsp;1a. \u0026amp; d.). Against a backdrop of the Baseline fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions pathway, these increases in forest coverage yield an increase in global land carbon and decreases in both atmospheric CO\u003csub\u003e2\u003c/sub\u003e and SAT (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. \u0026amp; d.).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eGlobally, land carbon increases over the 21st century in both the Global and Sustainable Reforestation simulations such that by 2100 global land carbon is 189 and 64 Gt CO\u003csub\u003e2\u003c/sub\u003e higher than the Baseline in both cases respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. \u0026amp; b.; Supplementary Fig.\u0026nbsp;2). The gain in land carbon in these simulations reduces peak and end-of-century atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration relative to the Baseline (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. \u0026amp; d.). Atmospheric CO\u003csub\u003e2\u003c/sub\u003e peaks at 477 ppm in the Baseline around 2042 and this peak is reduced by 6 and 4 ppm for the Global and Sustainable Reforestation cases respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. \u0026amp; d.). By 2100, as the atmosphere and ocean carbon pools equilibrate with the increase in land carbon (Supplementary Fig.\u0026nbsp;2), atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration is further reduced by 17 and 5 ppm relative to the Baseline for the Global and Sustainable Reforestation cases, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. \u0026amp; d.).\u003c/p\u003e \u003cp\u003eThese differences in atmospheric CO\u003csub\u003e2\u003c/sub\u003e result in cooler SAT relative to the Baseline. In the Baseline simulation, SAT peaks at 1.6\u0026deg;C above the pre-industrial (1850\u0026ndash;1900) mean with a return to 1.3\u0026deg;C relative to pre-industrial by 2100 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee. \u0026amp; f.). Compared to the Baseline, Global Reforestation yields a decrease in peak and end-of-century SAT of 0.01\u0026deg;C and of 0.04\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee.). The Sustainable Reforestation case yields a similar decrease in peak SAT of 0.01 which remains approximately the same by 2100 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef.).\u003c/p\u003e \u003cp\u003eWhile the changes in atmospheric CO\u003csub\u003e2\u003c/sub\u003e are drivers of changes in SAT, the biogeophysical effects of reforestation also affect SAT both in and outside of reforested areas (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee. \u0026amp; f.). Globally, biogeophysical effects dampen the cooling effect of reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal (see Methods, Supplementary Fig.\u0026nbsp;3). This biogeophysical warming effect on SAT results from changes in the energy balance at the surface: a decrease in albedo in reforested areas leads to greater solar radiation absorbed, which, combined with the greater roughness of forests, leads to an increase in the sensible heat flux, which is only partially offset by a decrease in the latent heat flux (Supplementary Fig.\u0026nbsp;4). The resulting SAT changes are propagated to distant regions through atmospheric and ocean circulation and climate feedbacks\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. Dynamics such as these illustrate the contrasting effects on SAT of reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals and the biogeophysical effects of reforestation. Without biogeophysical effects, peak SAT is reduced compared to the Baseline by 0.03 and 0.02\u0026deg;C for the Global and Sustainable Reforestation cases respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee, \u0026amp; f.). By 2100, without biogeophysical effects, SAT is reduced by 0.11 and 0.04\u0026deg;C compared to the Baseline for Global and Sustainable Reforestation simulations (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee. \u0026amp; f.).\u003c/p\u003e \u003cp\u003eUsing the change in forest area relative to the Baseline as a spatial constraint, we calculate the rate of reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal as the change in the yearly net land CO\u003csub\u003e2\u003c/sub\u003e flux in reforested areas relative to the Baseline (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. \u0026amp; e., see Methods). In the UVic ESCM, the net land CO\u003csub\u003e2\u003c/sub\u003e flux arises from the balance between net primary productivity (NPP), soil respiration (SR) and a land-use flux. In the Global Reforestation simulation, the rate of reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal exhibits an initial peak and decline and averages 3.8 Gt CO\u003csub\u003e2\u003c/sub\u003e/yr between 2050\u0026ndash;2100, with a cumulative removal over the 21st century of 234 Gt CO\u003csub\u003e2\u003c/sub\u003e. In the Sustainable Reforestation case, the reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal rate averages approximately 2.1 Gt CO\u003csub\u003e2\u003c/sub\u003e/yr between 2020\u0026ndash;2050 and declines to an average of 0.8 Gt CO\u003csub\u003e2\u003c/sub\u003e/yr between 2050\u0026ndash;2100 leading to a cumulative removal from 2020\u0026ndash;2100 of 88 Gt CO\u003csub\u003e2\u003c/sub\u003e (Supplementary Fig.\u0026nbsp;5\u0026ndash;6).\u003c/p\u003e \u003cp\u003eThroughout the 21st century, in both the Global and Sustainable Reforestation simulations, the global net land CO\u003csub\u003e2\u003c/sub\u003e flux relative to the Baseline is lower than this flux in reforested areas (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. \u0026amp; b.; Supplementary Figs.\u0026nbsp;2 \u0026amp; 7). Due to the different responses of NPP and soil respiration to changes in atmospheric CO\u003csub\u003e2\u003c/sub\u003e and SAT, the decrease in NPP resulting from the lower CO\u003csub\u003e2\u003c/sub\u003e concentration is not balanced by a corresponding decrease in soil respiration, leading to outgassing of CO\u003csub\u003e2\u003c/sub\u003e in these regions. Outgassing of CO\u003csub\u003e2\u003c/sub\u003e in non-reforested areas implies that accounting for reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal as the net land CO\u003csub\u003e2\u003c/sub\u003e flux in reforested areas misses the full land carbon cycle response to reforestation, which is relevant to understanding the atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration response in the Net-zero simulations.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eNet-zero simulations\u003c/h3\u003e\n\u003cp\u003eAlthough the Net-zero simulations are designed such that the net land CO\u003csub\u003e2\u003c/sub\u003e flux in reforested areas balances the additional fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions, atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration in these simulations is higher than in the Baseline. At its peak, the atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration in the Global Reforestation Net-zero simulation is approximately 2 ppm higher than in the Baseline, with the difference increasing to 5 ppm by end of the century (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea.). In the Sustainable Reforestation Net-zero case, atmospheric CO\u003csub\u003e2\u003c/sub\u003e is 0.3 ppm higher than in the Baseline at the peak and increases to 1.8 ppm above the Baseline by 2100 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb.).\u003c/p\u003e \u003cp\u003eThere are two primary, competing, factors that contribute to the difference in atmospheric CO\u003csub\u003e2\u003c/sub\u003e between the Net-zero and the Baseline simulations. On one hand, the reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal in the Net-zero simulations is enhanced by the CO\u003csub\u003e2\u003c/sub\u003e fertilization effect. Because the atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration is higher in the Net-zero than in the Reforestation simulations (used to quantify the reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal), more CO\u003csub\u003e2\u003c/sub\u003e is sequestered in reforested areas than was accounted for as a removal and used to balance fossil-fuel CO\u003csub\u003e2\u003c/sub\u003e emissions in the Net-Zero simulations (Supplementary Figs.\u0026nbsp;7\u0026ndash;8).\u003c/p\u003e \u003cp\u003eOn the other hand, as explained above, the global net land CO\u003csub\u003e2\u003c/sub\u003e flux in the Reforestation Simulations is lower than this flux in reforested areas and the same relationship carries into the Net-zero simulations (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee. \u0026amp; f.; Supplementary Fig.\u0026nbsp;7). The result of this difference is that the additional fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions are incompletely balanced by the global net land CO\u003csub\u003e2\u003c/sub\u003e flux leading to higher atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration in the Net-zero simulations compared to the Baseline (Supplementary Fig.\u0026nbsp;8).\u003c/p\u003e \u003cp\u003eThat higher atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration, alongside the biogeophysical effects of reforestation, contribute to an increase in SAT in the Net-zero simulations relative to the Baseline. As a result, peak SAT is 0.03 and 0.01\u0026deg;C higher than the Baseline for the Global and Sustainable Reforestation Net-zero simulations respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.c \u0026amp; d). By end-of-century the difference in SAT increases to 0.12 and 0.04\u0026deg;C higher than the Baseline for Global and Sustainable Reforestation Net-zero simulations (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec. \u0026amp; d.). As in the Reforestation simulations, biogeophysical effects of reforestation play an important role in amplifying the SAT difference relative to the Baseline. By 2100, without biogeophysical effects (see Methods), the warming above the Baseline would be 0.04 and 0.01\u0026deg;C for the Global and Sustainable Reforestation Net-zero simulations respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec. \u0026amp; d., dotted lines).\u003c/p\u003e \u003cp\u003eIn the simulations discussed so far it was assumed that CO\u003csub\u003e2\u003c/sub\u003e sequestered through reforestation remains permanently stored. However, forests are susceptible to disturbance via changes in disturbance regimes such as wildfires, droughts, pests, and future deforestation that make carbon stored in vegetation biomass vulnerable to release\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. We assess the effect of re-release of stored CO\u003csub\u003e2\u003c/sub\u003e in the Net-zero simulations by deforesting half of the reforested area over a twenty-year period centered around the year of peak CO\u003csub\u003e2\u003c/sub\u003e (2035\u0026ndash;2055) (Methods; Supplementary Fig.\u0026nbsp;9).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eLoss of tree cover through disturbance causes a flux of previously removed CO\u003csub\u003e2\u003c/sub\u003e back into the atmosphere which increases atmospheric CO\u003csub\u003e2\u003c/sub\u003e relative to the Reforestation Net-zero simulations with permanent storage by 8 and 2 ppm in 2100 for the Global and Sustainable cases, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea. \u0026amp; b.). However, the biogeophysical effect of reverting land cover from forest to grassland almost entirely compensates for the increase in SAT from the increase in atmospheric CO\u003csub\u003e2\u003c/sub\u003e, resulting in only a negligible increase in the Sustainable Reforestation 50% permanent case and a temperature increase of 0.01\u0026deg;C in 2100 in the Global Reforestation 50% permanent case relative to the Reforestation Net-zero simulations (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec. \u0026amp; d.).\u003c/p\u003e"},{"header":"Discussion and Conclusions","content":"\u003cp\u003eWe assessed the atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration and global SAT response in net-zero CO\u003csub\u003e2\u003c/sub\u003e pathways where additional fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions are balanced by removals via reforestation. Our results show that using reforestation to balance additional fossil-fuel emissions and maintain a net zero CO\u003csub\u003e2\u003c/sub\u003e pathway yields a 2\u0026ndash;5 ppm higher atmospheric CO\u003csub\u003e2\u003c/sub\u003e and a 0.04\u0026ndash;0.12\u0026deg;C higher global surface air temperature by 2100 than would be realized in a pathway where the fossil fuel emissions were avoided. The implication is that achieving net-zero CO\u003csub\u003e2\u003c/sub\u003e emissions \u0026ndash; where anthropogenic CO\u003csub\u003e2\u003c/sub\u003e removals and emissions balance \u0026ndash; does not result in a situation in which the net effect of CO\u003csub\u003e2\u003c/sub\u003e emissions and removals on global mean surface temperature balance each other, as is often assumed. The magnitude of these differences is dependent on the global carbon cycle response to CO\u003csub\u003e2\u003c/sub\u003e emissions and removals, the biogeophysical effects of reforestation, and the permanence of CO\u003csub\u003e2\u003c/sub\u003e removal.\u003c/p\u003e \u003cp\u003eOur accounting of the yearly CO\u003csub\u003e2\u003c/sub\u003e removal rate from reforestation is constrained to grid cells where reforestation occurs. This choice reflects common practice in carbon offset protocols used in the voluntary carbon market\u003csup\u003e402\u003c/sup\u003e top-down and book-keeping carbon accounting studies\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e, and Earth System Modeling studies\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. By constraining the net land CO\u003csub\u003e2\u003c/sub\u003e flux to reforested areas, however, the accounting misses changes in CO\u003csub\u003e2\u003c/sub\u003e fluxes outside of reforested areas in response to changes in atmospheric CO\u003csub\u003e2\u003c/sub\u003e and SAT caused by reforestation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. \u0026amp; d.).\u003c/p\u003e \u003cp\u003eTo assess the effects of accounting for the reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal via the net land CO\u003csub\u003e2\u003c/sub\u003e flux in reforested areas versus the global flux, we conducted a sensitivity analysis where the global net land CO\u003csub\u003e2\u003c/sub\u003e flux relative to the Baseline was used to balance an equivalent amount of fossil-fuel CO\u003csub\u003e2\u003c/sub\u003e emissions (Supplementary Fig.\u0026nbsp;10). In these modified Global and Sustainable Reforestation Net-zero simulations, atmospheric CO\u003csub\u003e2\u003c/sub\u003e remains lower than in the Baseline throughout the simulation (Supplementary Fig.\u0026nbsp;10\u0026ndash;11). Similar to the Net-zero simulations, as atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration is higher in these modified simulations than in the Reforestation simulations, the reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal is enhanced by the CO\u003csub\u003e2\u003c/sub\u003e fertilization effect (Supplementary Fig.\u0026nbsp;10), which contributes to why atmospheric CO\u003csub\u003e2\u003c/sub\u003e is lower than the Baseline. However, due to biogeophysical effects, SAT in both simulations remains higher than in the Baseline throughout the 21st century. These results show that even if additional fossil fuel emissions match the global net land CO\u003csub\u003e2\u003c/sub\u003e flux from reforestation, there is larger peak and year-2100 warming relative to a pathway (Baseline) without additional fossil fuel emissions.\u003c/p\u003e \u003cp\u003eRecent work has highlighted the importance of the passive CO\u003csub\u003e2\u003c/sub\u003e sink in the context of net zero CO\u003csub\u003e2\u003c/sub\u003e emissions accounting frameworks\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. By the physical science basis for net zero CO\u003csub\u003e2\u003c/sub\u003e, the passive CO\u003csub\u003e2\u003c/sub\u003e sink can be understood as the net land CO\u003csub\u003e2\u003c/sub\u003e flux that would occur in the absence of reforestation. While our Baseline represents such a counter-factual, it is possible that because our simulations are all coupled-emissions driven, there is a difference between the passive CO\u003csub\u003e2\u003c/sub\u003e sink in the Baseline and the Reforestation and Net zero simulations. However, as the research gap our study fills is to bridge common reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal accounting methods with net zero CO\u003csub\u003e2\u003c/sub\u003e frameworks, quantification of the passive CO\u003csub\u003e2\u003c/sub\u003e sink in reforestation pathways remains a challenge for future work.Our results show a strong biogeophysical effect of reforestation on SAT, as evidenced by the shaded areas in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The effects occur both locally \u0026ndash; in reforested areas \u0026ndash; and non-locally \u0026ndash; outside of reforested areas. While the response of biogeophysical effects is strong in our model, the representation of biogeophysical effects differs between ESMs and therefore the inter-model range of biogeophysical effects is highly uncertain\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. Nevertheless, we expect the biogeophysical effects of reforestation to play an important role in net-zero pathways that seek to balance fossil fuel emissions with reforestation-based removals\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan additionalcitationids=\"CR27 CR28 CR29 CR30 CR31\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAn additional methodological choice that could impact the results is the delay between when the CO\u003csub\u003e2\u003c/sub\u003e removal occurs and when it is accounted for as offset. In our study, there is no delay between removals and additional CO\u003csub\u003e2\u003c/sub\u003e emissions. Similar to practices in the voluntary carbon market\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e, it is reasonable to expect a delay between verification of reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals and the occurrence of additional CO\u003csub\u003e2\u003c/sub\u003e emissions the removal seeks to offset. To quantify the effect of these delays, we conducted a sensitivity analysis with additional CO\u003csub\u003e2\u003c/sub\u003e emissions occurring 1, 5, and 10 years after the removal. Delays between reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals and additional emissions postpones the timing of the effect of additional emissions on the climate system by the interval of the delay period. Against a backdrop of declining CO\u003csub\u003e2\u003c/sub\u003e emissions, delays between reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals and additional CO\u003csub\u003e2\u003c/sub\u003e emissions can reduce (10-year delay) peak warming but yields minimal differences in climate benefits by the end-of-the century (Supplementary Fig.\u0026nbsp;12).\u003c/p\u003e \u003cp\u003eThe Earth system model used for this study does not include a representation of natural disturbances. Carbon stored in aboveground biomass in our model simulations is therefore permanent if climate conditions support tree growth in a particular region. However, natural disturbance is an important process in assessing the climate benefit of reforestation because it affects the permanence of CO\u003csub\u003e2\u003c/sub\u003e storage\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. To assess the effect of disturbances on carbon fluxes, we randomly deforest a fraction (50%) of reforested grid cells. While this prescription of disturbance is idealized, it is representative of randomized stand-replacing events. At a 50% disturbance fraction, our disturbance results are likely extreme and should therefore be viewed as an upper-bound. Regardless of the intensity of the disturbance, however, results from our model suggest that the biogeophysical cooling effect of changing forest cover to grasslands can potentially compensate for the warming induced by the re-release of CO\u003csub\u003e2\u003c/sub\u003e to the atmosphere.\u003c/p\u003e \u003cp\u003eA novelty of our Net-zero simulations is that we simulate reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals and additional fossil fuel emissions occurring simultaneously in a coupled climate model. Results from these simulations reveal three important challenges for accurate accounting of the climate effect of reforestation-based net-zero pathways:\u003c/p\u003e \u003cp\u003e1) Standard reforestation carbon accounting practices that, by necessity, constrain the accounting of CO\u003csub\u003e2\u003c/sub\u003e removals to reforested areas, miss the net land CO\u003csub\u003e2\u003c/sub\u003e flux outside reforested areas. The CO\u003csub\u003e2\u003c/sub\u003e flux in these areas responds dynamically to changes in climate and atmospheric CO\u003csub\u003e2\u003c/sub\u003e induced by reforestation. Consequently, frameworks that limit CO\u003csub\u003e2\u003c/sub\u003e flux accounting to reforested areas miss CO\u003csub\u003e2\u003c/sub\u003e fluxes outside of reforested areas, potentially overestimating reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals.\u003c/p\u003e \u003cp\u003e2) In the absence of a standard methodology for disentangling the \u0026ldquo;passive\u0026rdquo; CO\u003csub\u003e2\u003c/sub\u003e sink due to CO\u003csub\u003e2\u003c/sub\u003e fertilization from the direct action of reforestation, it is possible that common practices of accounting reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals do not reflect the \u0026lsquo;true\u0026rsquo; removal. Frameworks that use the same CO\u003csub\u003e2\u003c/sub\u003e removal accounting irrespective of the level of CO\u003csub\u003e2\u003c/sub\u003e fertilization potentially inaccurately report reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals. 3) Reforestation interventions entail biogeophysical effects that have the potential to counter a substantial portion of the cooling effect from reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals. These effects are not limited to regions of reforestation but also manifest in distant locations. The large uncertainty in biogeophysical effects and their non-local nature points to key challenges to the accurate representation of these effects in carbon accounting frameworks.\u003c/p\u003e \u003cp\u003eDue to the combination of biogeophysical effects and the mismatch between reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal accounting and carbon cycle feedbacks, balancing additional fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions with reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals yields a different climate-carbon cycle response than avoiding a fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emission. As such, determining the effective reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal that would exactly balance the climate outcome of fossil fuel emissions remains a challenge for the research, policy, and voluntary offset market communities.\u003c/p\u003e \u003cp\u003eOur results suggest the need to consider Earth system dynamics and feedbacks in net-zero CO\u003csub\u003e2\u003c/sub\u003e pathways and for improved methods to measure and track the climate-carbon cycle effects of reforestation so as to ensure that these effects are correctly accounted for. Research combining observations and Earth system model simulation is needed to reduce the uncertainty of the global biogeophysical effects of reforestation. Consideration should further be paid to reforestation-induced changes in CO\u003csub\u003e2\u003c/sub\u003e fluxes outside reforested areas as these fluxes also influence changes in atmospheric CO\u003csub\u003e2\u003c/sub\u003e and SAT. Future research should seek to include representation of natural disturbance as a climate change feedback, to account for the potential re-release of CO\u003csub\u003e2\u003c/sub\u003e into the atmosphere. Finally, research would benefit from assessment of Earth system effects of a diversity of Nature-based Climate Solutions in net zero CO\u003csub\u003e2\u003c/sub\u003e pathways.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eModel summary\u003c/h2\u003e \u003cp\u003eAll simulations were run with the University of Victoria Earth Systems Climate Model (UVic ESCM) version 2.10\u003csup\u003e40\u003c/sup\u003e. The UVic ESCM is an intermediate-complexity climate model with a resolution of 1.8 degrees latitude by 3.6 degrees longitude\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e. The atmosphere is represented as a single layer energy-moisture balance model with dynamical feedbacks. The ocean is represented by a 19-layer general circulation model which is coupled to a thermodynamic-dynamic sea ice model\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. The UVic ESCM includes a land-surface model based on MOSES and the Dynamic Global Vegetation Model (DGVM) TRFFID (the Top-down Representation of Foliage and Flora Including Dynamics)\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e,\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. Both the ocean and land components include an interactive carbon cycle.\u003c/p\u003e \u003cp\u003eOn land, the soil is represented by 8 layers with freeze-thaw processes to classify and demark permafrost and a multi-layer soil carbon diffusion model\u003csup\u003e\u003cspan additionalcitationids=\"CR49\" citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. TRIFFID allows for five plant functional types (PFTs; broadleaf and needleleaf trees, C3 and C4 grasses, and shrubs) which compete for space via a prescribed dominance hierarchy where trees are the most competitive, followed by the shrub and then the grasses\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e,\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. Crops are represented by C3 and C4 grasses through prescribed land-cover maps that inhibit the growth of trees and shrubs in agricultural areas\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. In this fully-coupled arrangement, the UVic ESCM can be driven by prescribed fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions and land-use patterns (as well as non-CO\u003csub\u003e2\u003c/sub\u003e forcers) to prognostically determine changes in the Earth system such as atmospheric CO\u003csub\u003e2\u003c/sub\u003e and surface air temperature\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. CO\u003csub\u003e2\u003c/sub\u003e emissions from land-use changes are calculated internally by the model in response to the removal of forest plant functional types from prescribed agricultural areas. Half of the \u0026ldquo;harvested\u0026rdquo; forest carbon is accounted for as land-use emissions and half is transferred to the soil via litter-fall.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eSimulations\u003c/h2\u003e \u003cp\u003eThe model was spun up over 10,000 years to establish a climate in equilibrium with pre-industrial forcings \u0026ndash; including land cover, non-CO\u003csub\u003e2\u003c/sub\u003e radiative forcing, and atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration as described in CMIP6 protocols\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e. From the spin-up, an historical simulation (esm-hist) was run with observations-based CO\u003csub\u003e2\u003c/sub\u003e emissions, non-CO\u003csub\u003e2\u003c/sub\u003e forcing, and land use change to bring to the model up to 2015, again following CMIP6 protocols\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e. Forcing evolving according to SSP1-1.9 were then used to integrate the model up to the year 2020, which we use as the present-day starting point for future simulations\u003csup\u003e\u003cspan additionalcitationids=\"CR54 CR55\" citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. All simulations described below were run for an additional 100 years, up to year 2120.\u003c/p\u003e \u003cp\u003eThe \u003cem\u003eBaseline\u003c/em\u003e is an idealized simulation which serves as the reference case to all Reforestation and Reforestation Net-zero simulations. In the Baseline, prescribed fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions decline linearly from 2020 to zero by 2050 for a cumulative total of 566 Gt CO\u003csub\u003e2\u003c/sub\u003e between 2020 and 2050 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec. \u0026amp; f.). In this Baseline simulation, land-use patterns are held constant at the year 2020 extent. Non-CO\u003csub\u003e2\u003c/sub\u003e forcings are prescribed, following the SSP1-1.9 scenario, leading to a peak in combined non-CO\u003csub\u003e2\u003c/sub\u003e forcing at 2030 followed by declining forcing throughout the remainder of the simulation. The Baseline follows a 1.5\u0026deg;C overshoot trajectory in the UVic ESCM with atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration and global temperature peaking around 2040 and declining thereafter (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee. \u0026amp; f.).\u003c/p\u003e \u003cp\u003eTwo \u003cem\u003eReforestation\u003c/em\u003e simulations were run to quantify the CO\u003csub\u003e2\u003c/sub\u003e sequestration of reforesting all year 2020 agricultural lands (\u003cem\u003eGlobal Reforestation\u003c/em\u003e) and reforesting an extent consistent with natural forest regrowth patterns and constrained by food security and biodiversity safeguards (\u003cem\u003eSustainable Reforestation\u003c/em\u003e) as per the Global Reforestation potential from Griscom \u003cem\u003eet al\u003c/em\u003e. (2017)\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e and Cook-Patton et al. (2020)\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. To achieve reforestation in the model, the prescribed agricultural areas were reduced to target extents linearly between 2020 and 2050 and held constant thereafter. In the UVic ESCM this intervention leads to \u0026lsquo;natural (re)growth\u0026rsquo; of PFTs, with areal coverage of PFTs evolving based on climate, carbon availability and competition dynamics. In the Sustainable Reforestation case, land cover outside the target reforestation extent was retained as agricultural areas according to the year 2020 land-use patterns in SSP1-1.9. All other forcings (fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions, non-CO\u003csub\u003e2\u003c/sub\u003e forcings) were specified as in the Baseline simulation.\u003c/p\u003e \u003cp\u003eThe \u003cem\u003ereforestation-based CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e \u003cem\u003eremovals\u003c/em\u003e were determined by calculating the annual net atmosphere-to-land CO\u003csub\u003e2\u003c/sub\u003e flux (net land CO\u003csub\u003e2\u003c/sub\u003e flux) in reforested areas relative to Baseline and summing this across reforested grid cells with an increase in areal coverage of trees relative to the Baseline (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. \u0026amp; e.). The net land CO\u003csub\u003e2\u003c/sub\u003e flux is calculated as the difference between net primary productivity (NPP) and soil respiration (SR), less a residual flux determined by the UVic ESCM to balance changes in land carbon. When agricultural area is prescribed in the UVic ESCM, the residual land CO\u003csub\u003e2\u003c/sub\u003e flux is positive into the atmosphere, as it accounts for both the changes in land cover and the ongoing repression of tree growth to maintain agricultural area. Removing agricultural areas reduces this residual CO\u003csub\u003e2\u003c/sub\u003e flux, and as such, our calculation of reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals the two-fold action of removing agriculture and allowing PFTs to regrow based on natural competition. Our calculation of reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals further includes both the direct (i.e. reforestation-induced) and passive CO\u003csub\u003e2\u003c/sub\u003e fluxes from tree growth (i.e. in response to atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration changes).\u003c/p\u003e \u003cp\u003eTwo \u003cem\u003eNet-zero\u003c/em\u003e simulations were run whereby the yearly removal rate in the Reforestation simulations was applied as a fossil fuel emission on top of the Baseline emissions scenario (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec. \u0026amp; f.). Land-use change was applied as in the Global and Sustainable Reforestation simulations, with the assumption that CO\u003csub\u003e2\u003c/sub\u003e removal through reforestation balances exactly the additional fossil fuel emissions, achieving net zero CO\u003csub\u003e2\u003c/sub\u003e emissions. We prescribed the additional fossil fuel CO\u003csub\u003e2\u003c/sub\u003e to occur simultaneously with the reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals. Fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emission and non-CO\u003csub\u003e2\u003c/sub\u003e forcings in these simulations evolved as in the Baseline and Reforestation simulations.\u003c/p\u003e \u003cp\u003eTo quantify the biogeophysical effect of reforestation, an additional set of simulations was run whereby the model was driven with the atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration as determined from Reforestation and Reforestation Net-zero simulations. Land cover in these simulations was held constant at the year 2020 SSP1-1.9 extent and CO\u003csub\u003e2\u003c/sub\u003e fluxes from land or ocean did not affect the atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration. This provides an experiment where the climate system responds to the atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration changes resulting from reforestation without the biogeophysical effects of reforestation. The difference between this set of simulations and the reforestation simulations corresponds to the biogeophysical effect of reforestation.\u003c/p\u003e \u003cp\u003eFinally, we assessed how changes in future disturbance regimes could affect the permanence of CO\u003csub\u003e2\u003c/sub\u003e storage from reforestation. In both the Global and Sustainable Reforestation Net-zero cases, we randomly selected 50% of reforested grid cells and deforested the newly grown trees by increasing the agricultural area in those cells linearly between years 2035 and 2055 (Supplementary Fig.\u0026nbsp;9). The random selection of reforested grid cells was chosen to mimic the effect of stand-replacing natural disturbances, which are currently not represented in the UVic-ESCM, though our method here also captures the potential for changing human land-use decisions leading to the reversal of previous reforestation efforts.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003c/p\u003e\u003cp\u003e \u003ch2\u003eAdditional information\u003c/h2\u003e \u003cp\u003e \u003cem\u003eSupplementary information\u003c/em\u003e is available to editors and reviews in the supplementary information package supplied with this manuscript.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eAJM developed the research questions, designed the study, performed model simulations, analyzed the model output, produced the figures, and led the writing of the manuscript. KZ conceived the research, assisted in data analysis and interpretation, and edited the manuscript. PEB and HDM provided editorial feedback throughout the writing process.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eUNFCCC. The Paris Agreement. United Nations Framework Convention on Climate Change. \u003cem\u003eUNFCCC\u003c/em\u003e (2016). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://unfccc.int/documents/184656\u003c/span\u003e\u003cspan address=\"https://unfccc.int/documents/184656\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRogelj et al. Mitigation Pathways Compatible with 1.5\u0026deg;C in the Context of Sustainable Development. In: \u003cem\u003eGlobal Warming of 1.5\u0026deg;C. 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CMIP6 scenarios\u0026rsquo; radiative forcing of non-CO\u003csub\u003e2\u003c/sub\u003e greenhouse gases and aerosols for UVic ESCM simulations (1850\u0026ndash;2500). \u003cem\u003eZenodo\u003c/em\u003e (2023). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://zenodo.org/records/11061151\u003c/span\u003e\u003cspan address=\"https://zenodo.org/records/11061151\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6355003/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6355003/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eReforestation is increasingly considered an important nature-based climate solution for achieving net-zero CO\u003csub\u003e2\u003c/sub\u003e emissions as it has the potential to sequester and store substantial quantities of atmospheric CO\u003csub\u003e2\u003c/sub\u003e and generate co-benefits for ecosystems and ecosystem services. However, strategies using reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removal to offset fossil fuel emissions may not lead to the same climate outcome as avoiding the fossil fuel emissions. Here, we use an Earth System model of intermediate complexity to compare the climate outcome of different pathways: a reference net-zero pathway, and net-zero pathways where additional fossil fuel CO\u003csub\u003e2\u003c/sub\u003e emissions relative to the reference pathway are balanced by reforestation-based CO\u003csub\u003e2\u003c/sub\u003e removals (\u0026ldquo;Net-zero pathways\u0026rdquo;).\u003c/p\u003e \u003cp\u003eResults show that model simulations of Net-zero pathways yield a higher atmospheric CO\u003csub\u003e2\u003c/sub\u003e and warmer climate outcome than the reference simulation. The higher atmospheric CO\u003csub\u003e2\u003c/sub\u003e results from carbon cycle feedbacks, which induce an imperfect compensation between the net land CO\u003csub\u003e2\u003c/sub\u003e flux in reforestation areas that is accounted for as a removal, and the actual change in total land carbon storage. The additional global warming from higher atmospheric CO\u003csub\u003e2\u003c/sub\u003e is further amplified by biogeophysical effects of reforestation. Our research highlights the need for improved methods to measure and track the carbon cycle and climate effects of reforestation, particularly when used to balance or offset fossil fuel emissions in net-zero pathways.\u003c/p\u003e","manuscriptTitle":"Enhanced warming in net-zero scenarios that balance fossil fuel CO2 emissions with CO2 removals through reforestation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-26 18:38:19","doi":"10.21203/rs.3.rs-6355003/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-earth-and-environment","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commsenv","sideBox":"Learn more about [Communications Earth and Environment](https://www.nature.com/commsenv/)","snPcode":"","submissionUrl":"","title":"Communications Earth \u0026 Environment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"26f21a14-3bfe-440c-86c1-8b05ef8f75db","owner":[],"postedDate":"April 26th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":46662048,"name":"Earth and environmental sciences/Climate sciences/Climate change/Climate and Earth system modelling"},{"id":46662049,"name":"Earth and environmental sciences/Climate sciences/Climate change/Climate-change mitigation"}],"tags":[],"updatedAt":"2026-04-08T07:16:18+00:00","versionOfRecord":{"articleIdentity":"rs-6355003","link":"https://doi.org/10.1038/s43247-026-03329-x","journal":{"identity":"communications-earth-and-environment","isVorOnly":false,"title":"Communications Earth \u0026 Environment"},"publishedOn":"2026-02-25 05:00:00","publishedOnDateReadable":"February 25th, 2026"},"versionCreatedAt":"2025-04-26 18:38:19","video":"","vorDoi":"10.1038/s43247-026-03329-x","vorDoiUrl":"https://doi.org/10.1038/s43247-026-03329-x","workflowStages":[]},"version":"v1","identity":"rs-6355003","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6355003","identity":"rs-6355003","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}