The Pinatubo CO2 pause suggests a rapidly testable path to multi-Gt mCDR

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Burnham This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6960838/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract After Pinatubo erupted in 1991, atmospheric CO 2 levels stabilized during 1992–1993. Concentrations became ~ 2.25 ppm less than projected from contemporary emissions, corresponding to ~ 17.6 Gt of CO 2 removed. CO 2 did not return to projected levels in the following decades. Pinatubo ash fell into a downwelling eddy WSW of the volcano, which could explain the large, long-term CO 2 removal. Of nine eruptions since 1500 that led to significant atmospheric cooling, three located near frequent downwelling mesoscale eddies resulted in notable CO 2 -level reductions. The six that were distant from downwelling eddies had no notable impact on atmospheric CO 2 . Therefore, CO 2 reductions correlate with downwelling eddies rather than cooling. A CO 2 -pause hypothesis proposed by Sarmiento was that iron from the ashfall enabled a phytoplankton bloom that led to a large atmospheric CO 2 removal. Local downwelling and converging eddies would have held high iron concentrations in place long enough to allow nitrogen-fixing bacteria to grow and provide the nitrate required. This nitrogen-fixing Ocean Iron Fertilization (N-OIF) process is a testable hypothesis which, if scaled, could provide a multi-Gt/y method for atmospheric CO 2 removal. Earth and environmental sciences/Biogeochemistry Earth and environmental sciences/Climate sciences Earth and environmental sciences/Ocean sciences Earth and environmental sciences/Solid earth sciences Figures Figure 1 Figure 2 Figure 3 Introduction It is well accepted in the scientific community that human-related greenhouse gas emissions are increasing atmospheric CO 2 levels and cause global warming. Reduction of such emissions, balanced with some carbon capture and sequestration can stabilize CO 2 levels and thus global warming. However, these measures implemented alone will likely lead to atmospheric CO 2 levels of 500 ppm or more. IPCC scenario SSP2-4.5 ( https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FullReport_small.pdf ), which we are currently following approximately, would give about 600-ppm CO 2 levels and 2.7 °C warming by the end of the century. Maintaining CO 2 concentrations at less than 500 ppm for this scenario would require capture and long-term sequestration of 650 Gt of CO 2 . From Hansen et al., the long-term, equilibrium temperature increase at 500 ppm would still be about 6 °C. Consequently, there is a growing consensus that faster, larger-scale, and more cost-effective carbon sequestration processes are needed. A 2023 study by the National Academies of Science, Engineering, and Medicine explored the role of M arine processes for C arbon D ioxide R emoval (mCDR). It prioritized ocean nutrient fertilization, seaweed cultivation, and ocean alkalinity enhancement as worthy of further study on the basis of cost and risk. We address the evidence that Ocean Iron Fertilization (OIF) as occurred following three eruptions in the last 500 years can achieve large-scale CO 2 removal at an affordable price. OIF has been caused by both natural and human activities. Although the Milankovitch cycles are often cited as the trigger for the periodic ice ages in the last 800,000 years, the change in solar insolation is insufficient to cause such large temperature swings, which correlate closely with CO 2 levels. 1 The discovery of large amounts of dust on the seafloor and in ice cores led to an alternative hypothesis about the importance of wind-blown dust as a major source of nutrients for plankton growth in the ocean, , with dust-induced plankton growth therefore being the factor underpinning such large CO 2 removals and consequent temperature drops. John Martin subsequently hypothesized that insufficient levels of iron were a major factor limiting plankton growth in the ocean, and subsequent measurements of iron in ice cores and OIF experiments proved him correct. , , , Nature has supplied other examples of dust-catalyzed plankton growth. Blooms have been observed from dust transported from South America and Africa. They have also been observed after volcanic eruptions. , , , , , , But more than a plankton bloom is required for sustained carbon sequestration. Transport of the captured carbon to below 500 m is required for 100-year sequestration over most of the ocean. Without sufficient transport, the organic carbon can be ingested, metabolized, reoxidized to CO 2 and respired to the atmosphere. The Pinatubo eruption is unique among the various dust- or iron-induced plankton blooms by showing a sustained reduction in atmospheric CO 2 . Sarmiento 14 estimated a reduction of 1.5 ppm in the two years following Pinatubo. Our analysis indicates that the ultimate reduction was 2.25 ppm relative to the rise expected from global greenhouse gas emissions and that the reduction persisted for at least two decades. The obvious question is what made Pinatubo unique among recent volcanoes. Various hypotheses have been offered, but in this paper, we explore the premise that Pinatubo’s ash deposition over a downwelling eddy enhanced the transport of planktonic carbon to depths needed for multi-decadal storage. We also explore the concept that periodic doses of iron will induce the secondary growth of nitrogen-fixing cyanobacteria, thus sustaining the organic capture and storage mechanism past the original iron-fertilization event. The Case for Multi-Gt Sequestration Following Sarmiento 14 , we examined CO 2 concentration data available from NOAA ( https://gml.noaa.gov/ccgg/trends/data.html ) to estimate the amount of CO 2 captured after the Pinatubo eruption. Figure 1 shows a plot of the deseasonalized monthly CO 2 concentrations compared to three nonlinear projections described in the Methods section and linear fits to the data from 1980–1990 and 1995–2005. The variable-fraction projection is based on nonlinear regression to data from 1958 to 1990 using annual CO 2 emissions. Calculated concentrations were based on a two-step process of first having all the CO 2 emitted into the atmosphere and then making the amount absorbed proportional to the difference between the initially calculated CO 2 level and its pre-industrial level of 288 ppm (see Methods). The fraction of CO 2 absorbed varies each year within the range of 35 to 50%. Differences between that projection and the observed CO 2 concentrations were 1.76, 2.19, and 2.16 ppm at the beginnings of years 1993, 1994, and 1995, respectively, for an average of 2.04 ppm. The linear projection estimate is approximate, because the true functionality is curved, so the two lines are not parallel. Nevertheless, the resulting estimate of the CO 2 removal is similar. The differences between the two lines are 2.57 and 2.33 ppm at the beginning of 1992 and 1993, respectively, for an average of 2.45 ppm. This value represents the deviation at the center of the two linear fits, which is the most appropriate for this model. Another projection shown in Fig. 1 used a constant absorbed fraction of each year’s emission fitted to data from 1958 to 1990 and then extrapolated to 2024. That method gives an average reduction of 2.27 ppm over 1993-95. Looking at the three methods together, a reduction of 2.25 ppm appears reasonable. Given that the Earth's atmosphere contains 7.824 Gt per ppm CO 2 (1 ppm of the atmosphere’s mass times the relative molecular weights of CO 2 and air), the observed 1993-95 reduction represents 17.6 Gt removed from the atmosphere. Equally important is there is no evidence that the CO 2 sequestered by Pinatubo has re-entered the atmosphere. The most conservative algorithm used, namely the variable fraction model in Fig. 1 , remains above the measured values. Potential Mechanisms In addition to the OIF explanation offered by Sarmiento 14 , various other explanations for the Pinatubo CO 2 pause have been offered since 1993. Most hypotheses are based on effects of large eruptions. As we will show, atmospheric CO 2 decreased significantly in only three eruptions out of nine since 1500 that caused atmospheric cooling. Those eruptions occurred adjacent to significant downwelling eddies. This lack of CO 2 removal after most large eruptions excludes widely discussed explanations based on large-scale fertilization of land or sea flora, as well as surface cooling or increased diffuse light from scattering by stratospheric aerosols. Any of those mechanisms, if operating, would be expected to reduce CO 2 levels long-term following all or most similar-sized and larger eruptions, but don’t. The 0.4-Gt reduction in world CO 2 emissions following the fall of the USSR in 1991 constitutes only 2% of the observed 17.6-Gt reduction. Further, our analysis includes such variations in global CO 2 emissions. Increased CO 2 solubility in the ocean due to a global temperature decrease caused by an increase in the Earth’s albedo is neither large enough nor permanent. Instead, substantial changes are needed in either ocean circulation or biology to redistribute carbon in the ocean. Could the latter be OIF? Fay et al. explored possible reasons for Pinatubo’s effect, and they mentioned OIF as a possibility for future exploration. More popular are proposals that changes in terrestrial respiration or enhanced tree growth due to diffuse sunlight are responsible. , , However, CO 2 reductions following eruptions of Agung in 1963 and El Chichón in 1982 were short-lived, indistinguishable from noise, and less than 25% of the Pinatubo magnitude (see Fig. 1 b). Is a slightly larger and longer-lasting aerosol concentration 42 enough to explain this difference, or are Krakauer and Randerson correct? In summary, a large decrease in atmospheric CO 2 is generally not observed following large volcanic eruptions. Our purpose here is not to disprove other hypotheses but to explore the viability of a downwelling eddy hypothesis that accommodates 500 years of global CO 2 data. Various signs point to the likelihood of nitrogen-fixing OIF (N-OIF) as a key factor. , , As shown in Fig. 2 , deposition of Pinatubo ash occurred in a region of the South China Sea that is known for its strong downwelling eddies. A downwelling eddy has two potential benefits for carbon removal. First, the surface water flows are convergent, which helps minimize dispersion and dilution of the fine ash and its associated nutrients. Second, the typical downward flow of ~ 17 m/day (0.0002 m/s) for depths below 100 m as given in https://data.marine.copernicus.eu/viewer/expert accelerates the natural sinking of both detritus and mineralized CO 2 to depths below which organisms can significantly ingest and metabolize the carbon. This downward flow is more than often thought but is consistent by analogy with atmospheric highs and lows with a recent upwelling analysis and is similar to downward flows given by Boyd et al. A downwelling and converging eddy could have held high-iron concentrations in place long enough (2–5 months) to allow nitrogen-fixing bacteria (either cyanobacteria Crocosphaera or Trichodesmium, which grows well there ) to propagate and provide the nitrate required for sustained growth. Historical records suggest that Pinatubo is not unique in initiating nitrogen-fixing OIF. Recent high-temporal-resolution measurements of the Law Dome ice core shown in Fig. 3 indicate an 11-ppm decrease in CO 2 in the 30 years after the Billy Mitchell eruption. Most or all of that was due to Billy Mitchell, whose ash would have deposited in a region of the ocean prone to downwelling eddies. Huaynaputina may have contributed, but most of its ash fell on land or near shore. It is better known for briefly cooling the earth by sulfate aerosols. Minor (~ 0.1°C) cooling may have been induced by the CO 2 reduction following Billy Mitchell. Ice-core CO 2 levels rose 5 ppm over the next 40 years and then stabilized for a century, although there may have been a minor dip in CO 2 levels from the 1660 Long Island volcano. The solid green line in Fig. 3 is a hypothetical (“synthetic”) CO 2 curve similar to that used by Ahn et al. 38 with various multi-year averaging intervals to explain why other data sets did not see as sharp of a drop before 1600 and then a rise shortly thereafter. The dashed blue line is a symmetric 19-yr moving average of the green line. Table 1 shows additional volcanic history summarized from a variety of sources. , , , , , , , , , , , , A first observation is that estimates of world temperature reductions vary by a factor of two for a given volcano, so calculating temperature effects is difficult to do accurately. Tambora in 1815 had a major ash fall in an area prone to downwelling eddies and has been linked to a 6-ppm drop in atmospheric CO 2 . It also decreased temperature by 0.5 to 1.3 °C. 44 Krakatoa’s ash fall was northward away from downwelling eddies, 45 and no effect on atmospheric CO 2 is discernible from available CO 2 data. Agung in 1963 and El Chichón in 1982 led to similar (~ 0.2°C) reductions in global temperature as followed Pinatubo but did not leave a notable CO 2 reduction after a year, as shown in Fig. 1 b. Table 1 List of major volcanoes in the last 500 years and an assessment of their contribution to global cooling and atmospheric CO 2 reduction. Eruption Year Temperature reduction, °C km 3 of magma released CO 2 ppm reduced after 10 years Ocean eddy near ash fall? Pinatubo 1991 0.2 40 , < 0.3 41 , 0.5 42 , 0.5 43 , 0.15 52 5 e 2.3 Yes El Chichón 1982 < 0.2 40 , < 0.3 41 , 0.1 52 , 0.4 a 0.4 48 0 No: landlocked Agung 1963 < 0.2 40 , < 0.3 41 , 0.35 46 , 0.2 b 0.4 49 0 No: equatorial Santa Maria 1902 < 0.4 41 , 0.25 40 , 0.2 a 5 f 0 No: landlocked Krakatoa 1883 0.6 41,c , 0.3 51 20 45 0 No: ashfall away from eddies Tambora 1815 0.8-1.3 44 , 1.1 41 , 0.5 40 30 44 6 44 Yes Long Island 1660 0.9 41 , 0.3 40 30 g 0 No: equatorial Huaynaputina 1600 1.1 41 , 0.8 40 , 1.6 47 14 47 0 No: 120 km from coast Billy Mitchell 1580 0.5 41 , 0.4 40 , 0.25 d 14 50 11 38 Yes h a NASA earth observatory, https://earthobservatory.nasa.gov/images/77538/remembering-el-chichon b MetMatters, https://www.rmets.org/metmatters/mount-agung-and-its-potential-global-impacts c Natural History Museum, https://www.nhm.ac.uk/discover/the-1883-krakatau-eruption-a-year-of-blue-moons.html d Volcano Discovery, https://www.volcanodiscovery.com/billy_mitchell.html e U.S. Geological Survey Fact Sheet 113 − 97, https://pubs.usgs.gov/fs/1997/fs113-97/ f Oregon State University, https://volcano.oregonstate.edu/santa-maria g Volcano Discovery, https://www.volcanodiscovery.com/long_island.html h https://earth.nullschool.net/#current/ocean/surface/currents/orthographic=-209.13,-5.23,2704/loc=-91.646,12.561 Discussion The case is strong for the Pinatubo eruption removing and effectively sequestering 15–20 Gt of atmospheric CO 2 in the two years following its eruption. This observed removal rate is roughly an order of magnitude larger than is currently discussed as possible from OIF. We propose that the circulation caused by a stationary mesoscale downwelling eddy helped contain the nutrients at concentrations needed to enable cyanobacteria to grow and fix nitrogen, thereby prolonging the effect of the iron deposition (nitrogen-fixing OIF). Examination of CO 2 concentrations over the past five years identified two eruptions (Billy Mitchell in 1580 and Tambora in1815) that had an even larger drawdowns of CO 2 , which means that this mechanism is rare but not unique. Nitrogen-fixing OIF (N-OIF) is a testable hypothesis of how fertilizing a downwelling eddy can greatly increase the amount and durability of marine carbon capture and sequestration. Thus N-OIF warrants replication tests. While laboratory tests will help understand the detailed chemical and biological mechanisms, only large-scale field tests will be able to reproduce the potentially complex interactions of those mechanisms with the marine biology and hydrodynamics associated with a downwelling eddy. If one assumes, for example, that the buildup and phase out of fossil fuels is a symmetric curve and that we achieve peak emissions in 2028, we can expect CO 2 to rise another 125 ppm by early next century in the absence of large-scale CO 2 removal. That scenario, which is similar to IPCC scenario SSP2-4.5, implies a potential ultimate rise to about 580 ppm without large-scale CO 2 removal and a net increase of 2350 Gt in atmospheric CO 2 from the 280-ppm pre-industrial level. Clearly, large-scale CO 2 removal technologies are needed to restore and sustain pre-industrial ecosystems, independent of the future rate of emission reduction. The 15–20 Gt/y removal event by Pinatubo in 1992–1994 suggests that mCDR could be a major improvement over others being pursued today. Important issues include what annualized gross primary productivity (GPP) can be achieved and what fraction of that productivity can be transported to multi-decade storage depths. A 5-month average GPP of 4.3 gC/m 2 /d was observed in the Humboldt current, with a daily maximum of 20 gC/m 2 /d. Managed OIF may be able to achieve closer to the maximum, but not all of that carbon will reach storage depths. A typical 17-m/d downward eddy velocity would improve storage efficiency. An average net storage value of 10 gC/m 2 /d over a 300-km diameter eddy would remove ~ 1 Gt of CO 2 /y. Ten such locations constitute 0.2% of the ocean surface and would remove ~ 10 Gt CO 2 /y. Whether that level of mCDR can be obtained is debatable, 8 but it is certainly conceivable. Although this is nearly equal to the net annual oceanic capture and storage of CO 2 , net storage is the difference between two large numbers, so it represents only a 3.2% change in the gross CO 2 uptake of the ocean. Further, CO 2 uptake is highly variable across the globe and seasons, so the 3.2% value is a better metric for the feasibility of mCDR at optimal sites. Only large-scale and longer-term experiments will be able to determine the true efficiency and cost of mCDR in general and N-OIF in particular. If successful, implementation at large scale could provide an important tool for restoring atmospheric CO 2 to levels of a century ago. Methods All data analyses were performed in Excel. Fits to the Keeling Curve from 1958–1990 were accomplished either by native regression tools or by a nonlinear regression grid search using an objective function defined as the sum of squares of the difference between the model function and the measured value. CO 2 concentrations after 1957 were downloaded from the NOAA CO 2 emission site ( https://gml.noaa.gov/ccgg/trends/data.html ) and before 1958 from the University of Exeter Global Carbon Budget ( https://globalcarbonbudget.org/ ). CO 2 emission history (annual and cumulative) was downloaded from Our World in Data ( https://ourworldindata.org/grapher/cumulative-co-emissions?country=~OWID_WRL , https://ourworldindata.org/co2-emissions ). Our fixed-fraction model is simpler than in McKinley et al. 24 but gets the same fraction of fossil-fuel CO 2 emissions absorbed prior to Pinatubo. The fixed-fraction method used the model function $$\:p{CO}_{2}\left(y+1\right)=p{CO}_{2}\left(y\right)+(1-a){M}_{CO2}\left(y\right)/7.824{10}^{9}$$ 1 where M is the annual mass of CO 2 emissions. The optimum value of a was 0.397, with an average squared residual value of 0.15 ppm 2 . The variable-fraction method used the functions $$\:{p}_{i}{CO}_{2}\left(y+1\right)=p{CO}_{2}\left(y\right)+{M}_{CO2}\left(y\right)/7.824{10}^{9}\:$$ $$\:p{CO}_{2}\left(y+1\right)={p}_{i}{CO}_{2}\left(y\right)-b({p}_{i}{CO}_{2}\left(y+1\right)-280)$$ 2 Here, p i CO 2 is the value that would have occurred with no absorption, and b is a mass transfer coefficient assuming CO 2 absorption is related to the excess pCO 2 since the industrial revolution. The optimum value of b was 0.0153, with an average squared residual of 0.25 ppm 2 . The exponential method used the function $$\:p{CO}_{2}\left(y\right)={c}_{1}+{c}_{2}exp\left({c}_{3}y\right)$$ 3 The optimum values of c 1 , c 2 , and c 3 were 288, 1.0373×10 − 2 , and 0.0287, respectively, with an average squared residual value of 0.25 ppm 2 . Declarations Author Contribution P.F. conceived this work, did the initial data analysis, and wrote much of the text. 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The atmospheric impact of the 1991 Mount Pinatubo Eruption, in Fire and Mud: Eruptions and Lahars of Mount Pinatubo , Philippines , U.S. Geological Survey, https://pubs.usgs.gov/pinatubo/self/ . McCormick, M. P., Thomason, L. W. & Trepte, C. R. Atmospheric effects of the Mt Pinatubo eruption. Nature 373 , 399–404 (1995). DOI: 10.1038/373399a0 . Kandlbauer, J., Hopcroft, P. J., Valdes, P. J. & Sparks, R. S. J. Climate and carbon cycle response to the 1915 Tambora volcanic eruption. JGR Atmos. 118 , 12301–12803 (2013). DOI: 10.1002/2013JD019767 . Self, S. & Rampino, M. R. The 1883 eruption of Krakatau. Nature 294 , 699–704 (1981). DOI: 10.1038/294699a0 . Angell, J. K. & Korshover, J. Surface Temperature changes following the six major volcanic episodes between 1780 and 1980. J. Appl. Meteorology and Climatology 24 , 937–951 (1985). DOI: 10.1175/1520-0450(1985)0242.0.CO;2 . White, S. et al. The 1600 CE Huaynaputina eruption as a possible trigger for persistent cooling in the North Atlantic region. Climate of the Past 18 , 739–757 (2022). DOI: 10.5194/cp-18-739-2022 Varekamp, J. C., Luhr, J. F. & Prestegaard, K. L. The 1982 eruptions of El Chichón Volcano (Chiapas, Mexico): Character of the eruptions, ash-fall deposits, and gasphase. J. Volcanol. Geotherm. Res. 23 , 39–68 (1984). DOI: 10.1016/0377-0273(84)90056-8 . Self, S. & Rampino, M. R. The 1963–1964 eruption of Agung volcano (Bali, Indonesia). Bull. Volcanol. 74 , 1521–1536 (2012). DOI: 10.1007/s00445-012-0615-z . Schabetsberger, R. et al. First Limnological characterization of crater lake Billy Mitchell (Bougainville Island, Papua New Guinea). Pacific Sci. 71 , 29–44 (2017). DOI: 10.2984/71.1.3 . Joshi, M. M. & Jones, S. G. The climatic effects of the direct injection of water vapour into the stratosphere by large volcanic eruptions. Atmos. Chem. Phys. 9 , 6109–6118 (2009) DOI: 10.5194/acp-9-6109-2009 . Fujiwara, M., Marineau, P. & Wright, J. S. Surface temperature response to the major volcanic eruptions in multiple reanalysis data sets. Atmos. Chem. Phys. 20 , 345–374 (2020). DOI: 10.5194/acp-20-345-2020 . Lebling, K. E., Northrop, E., McCormick, C. & Bridgwater, L. Toward Responsible and Informed Ocean-Based Carbon Dioxide Removal: Research and Governance Priorities (World Resources Institute, 2022). DOI: 10.46830/wrirpt.21.00090 . Daneiri, G. Dellarossa, V., Quinones, R., Jacob, G., Montero, P. & Ulloa, O. Primary production and community respiration in the Humboldt current system off Chile and associated oceanic areas. Mar. Ecol. Prog. Ser. 197 , 41–49 (2000). DOI: 10.3354/meps197041 . Friedlingstein, P. et al. Global carbon budget 2023. Earth Sust. Sci. Data 15 , 5301–5369 (2023). DOI: 10.5194/essd-15-5301-2023 . Additional Declarations No competing interests reported. Supplementary Files CO2riseratenonlinregMay2025r4.xlsx Cite Share Download PDF Status: Posted 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. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-6960838","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":484326957,"identity":"538a1e24-f5d5-4c1e-8fa5-398b8773e676","order_by":0,"name":"Peter Fiekowsky","email":"","orcid":"","institution":"DeepGreen Solutions","correspondingAuthor":false,"prefix":"","firstName":"Peter","middleName":"","lastName":"Fiekowsky","suffix":""},{"id":484326959,"identity":"9c4b747b-98ea-4d53-94d8-5febe1a7113c","order_by":1,"name":"Alan K. Burnham","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAyUlEQVRIiWNgGAWjYDACHh4QeYCBgb2BgZlELTwHSNYikUCkFv6es8ekK3fckded+cbwc0GFDQN/e3cCXi0SZ/vSJM+eeWa47XaOsfSMM2kMEmfObsBvzXkeM8nGtsOMQC0G0rxthxkMJHLxa5GHarHfdvOM8W+itBic7QFrSdx2g8eMOFsMz5xLtgRqSd52Jq3MmudMGg9Bv8idyT14E6jFdtvxw5tv81TYyPG39xLwPgJwGIBIHmKVgwD7A1JUj4JRMApGwQgCAI9mSxYAqns4AAAAAElFTkSuQmCC","orcid":"","institution":"DeepGreen Solutions","correspondingAuthor":true,"prefix":"","firstName":"Alan","middleName":"K.","lastName":"Burnham","suffix":""}],"badges":[],"createdAt":"2025-06-24 02:53:23","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6960838/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6960838/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":86804738,"identity":"93049a30-a4bf-42ae-9f4f-1111e3016fff","added_by":"auto","created_at":"2025-07-15 17:51:15","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":798463,"visible":true,"origin":"","legend":"\u003cp\u003eMeasured increase in atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentrations compared to linear and nonlinear fits and projections described in the Methods section.\u0026nbsp; (a) \u0026nbsp;Fits of a fixed-fraction model (Eq. 1), a variable-fraction model (Eq. 2), and an exponential function (Eq. 3) to data from 1958 to 1990.\u0026nbsp;A smooth reduction in slope would be expected for a sigmoidal relationship, but a sharp break in the growth trend is clear after Pinatubo. (b) Expanded view of the results from 1960 to 1985. Two other large volcanoes that occurred after the Mauna Loa CO\u003csub\u003e2\u003c/sub\u003e measurements started had either a negligible or small transient effect on CO\u003csub\u003e2\u003c/sub\u003e concentrations compared to El Niño effects. (c) Expanded view of the results from 1980 to 2005, including linear fits from 1980-1990 and 1995-2005. All three models indicate a net reduction in CO2 concentrations following Pinatubo.\u003c/p\u003e","description":"","filename":"Picture1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6960838/v1/1c943b77783c7e87d14da529.jpg"},{"id":86804739,"identity":"4d5db570-33ac-47db-ab00-fb2f745f9b95","added_by":"auto","created_at":"2025-07-15 17:51:15","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":531306,"visible":true,"origin":"","legend":"\u003cp\u003eOverlay of measured ash deposition from Pinatubo[i]with an Oct 29, 1992, downwelling eddy image downloaded from NASA Worldview (https://worldview.earthdata.nasa.gov/).\u003c/p\u003e\n\u003cp\u003e[i] Wiesner, M. G., Wetzel, A., Catane, S. G., Listanco, E. L. \u0026amp; Mirabueno, H. T. Grain size, areal thickness distribution and controls on sedimentation of the 1991 Mount Pinatubo tephra layer in the South China Sea. \u003cem\u003eBull. Volcanol.\u003c/em\u003e \u003cstrong\u003e66\u003c/strong\u003e, 226–242 (2004). DOI:10.1007/s00445-003-0306-x\u003c/p\u003e","description":"","filename":"Picture2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6960838/v1/975d6051e1b374a2992b54ff.jpg"},{"id":86804740,"identity":"4e9606f6-1cee-497a-97e7-2118a49c52b1","added_by":"auto","created_at":"2025-07-15 17:51:15","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":315306,"visible":true,"origin":"","legend":"\u003cp\u003eHigh-temporal-resolution data for CO\u003csub\u003e2\u003c/sub\u003e in the Law Dome ice core and its relationship to contemporary major volcanoes, adapted and simplified from Ahn et al.\u003csup\u003e38\u003c/sup\u003e The solid green line is a\u0026nbsp;hypothetical instantaneous CO\u003csub\u003e2\u003c/sub\u003e concentration, and the blue dashed line is a 19-yr moving average. PNG is Papua New Guinea.\u003c/p\u003e","description":"","filename":"Picture3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6960838/v1/6682a457f6dbe3d52aaa1ced.jpg"},{"id":91834489,"identity":"0252cbc4-50ed-48bf-8947-345ba21a502a","added_by":"auto","created_at":"2025-09-22 09:26:14","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2349139,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6960838/v1/cdd498b7-9d15-461e-9f4f-0ac1e76f13f2.pdf"},{"id":86805360,"identity":"4e71de6c-5e94-4a21-a861-d6c1d62c5620","added_by":"auto","created_at":"2025-07-15 17:59:15","extension":"xlsx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":539433,"visible":true,"origin":"","legend":"","description":"","filename":"CO2riseratenonlinregMay2025r4.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-6960838/v1/3744f385c691f06dc102b5f9.xlsx"}],"financialInterests":"No competing interests reported.","formattedTitle":"The Pinatubo CO2 pause suggests a rapidly testable path to multi-Gt mCDR","fulltext":[{"header":"Introduction","content":"\u003cp\u003eIt is well accepted in the scientific community that human-related greenhouse gas emissions are increasing atmospheric CO\u003csub\u003e2\u003c/sub\u003e levels and cause global warming. Reduction of such emissions, balanced with some carbon capture and sequestration can stabilize CO\u003csub\u003e2\u003c/sub\u003e levels and thus global warming. However, these measures implemented alone will likely lead to atmospheric CO\u003csub\u003e2\u003c/sub\u003e levels of 500 ppm or more. IPCC scenario SSP2-4.5 (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FullReport_small.pdf\u003c/span\u003e\u003cspan address=\"https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FullReport_small.pdf\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e), which we are currently following approximately, would give about 600-ppm CO\u003csub\u003e2\u003c/sub\u003e levels and 2.7 \u0026deg;C warming by the end of the century.\u003c/p\u003e \u003cp\u003eMaintaining CO\u003csub\u003e2\u003c/sub\u003e concentrations at less than 500 ppm for this scenario would require capture and long-term sequestration of 650 Gt of CO\u003csub\u003e2\u003c/sub\u003e. From Hansen et al.,\u003ca class=\"FNLink\" href=\"#Fn1\" id=\"#FNLinkFn1\"\u003e\u003c/a\u003e the long-term, equilibrium temperature increase at 500 ppm would still be about 6 \u0026deg;C. Consequently, there is a growing consensus that faster, larger-scale, and more cost-effective carbon sequestration processes are needed.\u003c/p\u003e \u003cp\u003eA 2023 study\u003ca class=\"FNLink\" href=\"#Fn2\" id=\"#FNLinkFn2\"\u003e\u003c/a\u003e by the National Academies of Science, Engineering, and Medicine explored the role of \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eM\u003c/span\u003earine processes for \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eC\u003c/span\u003earbon \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eD\u003c/span\u003eioxide \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eR\u003c/span\u003eemoval (mCDR). It prioritized ocean nutrient fertilization, seaweed cultivation, and ocean alkalinity enhancement as worthy of further study on the basis of cost and risk. We address the evidence that Ocean Iron Fertilization (OIF) as occurred following three eruptions in the last 500 years can achieve large-scale CO\u003csub\u003e2\u003c/sub\u003e removal at an affordable price.\u003c/p\u003e \u003cp\u003eOIF has been caused by both natural and human activities. Although the Milankovitch cycles are often cited as the trigger for the periodic ice ages in the last 800,000 years,\u003ca class=\"FNLink\" href=\"#Fn3\" id=\"#FNLinkFn3\"\u003e\u003c/a\u003e the change in solar insolation is insufficient to cause such large temperature swings, which correlate closely with CO\u003csub\u003e2\u003c/sub\u003e levels.\u003csup\u003e1\u003c/sup\u003e The discovery of large amounts of dust on the seafloor and in ice cores led to an alternative hypothesis about the importance of wind-blown dust as a major source of nutrients for plankton growth in the ocean,\u003ca class=\"FNLink\" href=\"#Fn4\" id=\"#FNLinkFn4\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn5\" id=\"#FNLinkFn5\"\u003e\u003c/a\u003e with dust-induced plankton growth therefore being the factor underpinning such large CO\u003csub\u003e2\u003c/sub\u003e removals and consequent temperature drops. John Martin subsequently hypothesized that insufficient levels of iron were a major factor limiting plankton growth in the ocean,\u003ca class=\"FNLink\" href=\"#Fn6\" id=\"#FNLinkFn6\"\u003e\u003c/a\u003e and subsequent measurements of iron in ice cores\u003ca class=\"FNLink\" href=\"#Fn7\" id=\"#FNLinkFn7\"\u003e\u003c/a\u003e and OIF experiments proved him correct.\u003ca class=\"FNLink\" href=\"#Fn8\" id=\"#FNLinkFn8\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn9\" id=\"#FNLinkFn9\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn10\" id=\"#FNLinkFn10\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn11\" id=\"#FNLinkFn11\"\u003e\u003c/a\u003e\u003c/p\u003e \u003cp\u003eNature has supplied other examples of dust-catalyzed plankton growth. Blooms have been observed from dust transported from South America\u003ca class=\"FNLink\" href=\"#Fn12\" id=\"#FNLinkFn12\"\u003e\u003c/a\u003e and Africa.\u003ca class=\"FNLink\" href=\"#Fn13\" id=\"#FNLinkFn13\"\u003e\u003c/a\u003e They have also been observed after volcanic eruptions.\u003ca class=\"FNLink\" href=\"#Fn14\" id=\"#FNLinkFn14\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn15\" id=\"#FNLinkFn15\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn16\" id=\"#FNLinkFn16\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn17\" id=\"#FNLinkFn17\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn18\" id=\"#FNLinkFn18\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn19\" id=\"#FNLinkFn19\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn20\" id=\"#FNLinkFn20\"\u003e\u003c/a\u003e But more than a plankton bloom is required for sustained carbon sequestration.\u003ca class=\"FNLink\" href=\"#Fn21\" id=\"#FNLinkFn21\"\u003e\u003c/a\u003e Transport of the captured carbon to below 500 m is required for 100-year sequestration over most of the ocean.\u003ca class=\"FNLink\" href=\"#Fn22\" id=\"#FNLinkFn22\"\u003e\u003c/a\u003e Without sufficient transport, the organic carbon can be ingested, metabolized, reoxidized to CO\u003csub\u003e2\u003c/sub\u003e and respired to the atmosphere.\u003c/p\u003e \u003cp\u003eThe Pinatubo eruption is unique among the various dust- or iron-induced plankton blooms by showing a sustained reduction in atmospheric CO\u003csub\u003e2\u003c/sub\u003e. Sarmiento\u003csup\u003e14\u003c/sup\u003e estimated a reduction of 1.5 ppm in the two years following Pinatubo. Our analysis indicates that the ultimate reduction was 2.25 ppm relative to the rise expected from global greenhouse gas emissions and that the reduction persisted for at least two decades.\u003c/p\u003e \u003cp\u003eThe obvious question is what made Pinatubo unique among recent volcanoes. Various hypotheses have been offered, but in this paper, we explore the premise that Pinatubo\u0026rsquo;s ash deposition over a downwelling eddy enhanced the transport of planktonic carbon to depths needed for multi-decadal storage. We also explore the concept that periodic doses of iron will induce the secondary growth of nitrogen-fixing cyanobacteria, thus sustaining the organic capture and storage mechanism past the original iron-fertilization event.\u003c/p\u003e\n\u003ch3\u003eThe Case for Multi-Gt Sequestration\u003c/h3\u003e\n\u003cp\u003eFollowing Sarmiento\u003csup\u003e14\u003c/sup\u003e, we examined CO\u003csub\u003e2\u003c/sub\u003e concentration data available from NOAA (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://gml.noaa.gov/ccgg/trends/data.html\u003c/span\u003e\u003cspan address=\"https://gml.noaa.gov/ccgg/trends/data.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) to estimate the amount of CO\u003csub\u003e2\u003c/sub\u003e captured after the Pinatubo eruption. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows a plot of the deseasonalized monthly CO\u003csub\u003e2\u003c/sub\u003e concentrations compared to three nonlinear projections described in the Methods section and linear fits to the data from 1980\u0026ndash;1990 and 1995\u0026ndash;2005.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe variable-fraction projection is based on nonlinear regression to data from 1958 to 1990 using annual CO\u003csub\u003e2\u003c/sub\u003e emissions. Calculated concentrations were based on a two-step process of first having all the CO\u003csub\u003e2\u003c/sub\u003e emitted into the atmosphere and then making the amount absorbed proportional to the difference between the initially calculated CO\u003csub\u003e2\u003c/sub\u003e level and its pre-industrial level of 288 ppm (see Methods). The fraction of CO\u003csub\u003e2\u003c/sub\u003e absorbed varies each year within the range of 35 to 50%. Differences between that projection and the observed CO\u003csub\u003e2\u003c/sub\u003e concentrations were 1.76, 2.19, and 2.16 ppm at the beginnings of years 1993, 1994, and 1995, respectively, for an average of 2.04 ppm.\u003c/p\u003e \u003cp\u003eThe linear projection estimate is approximate, because the true functionality is curved, so the two lines are not parallel. Nevertheless, the resulting estimate of the CO\u003csub\u003e2\u003c/sub\u003e removal is similar. The differences between the two lines are 2.57 and 2.33 ppm at the beginning of 1992 and 1993, respectively, for an average of 2.45 ppm. This value represents the deviation at the center of the two linear fits, which is the most appropriate for this model.\u003c/p\u003e \u003cp\u003eAnother projection shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e used a constant absorbed fraction of each year\u0026rsquo;s emission fitted to data from 1958 to 1990 and then extrapolated to 2024. That method gives an average reduction of 2.27 ppm over 1993-95. Looking at the three methods together, a reduction of 2.25 ppm appears reasonable.\u003c/p\u003e \u003cp\u003eGiven that the Earth's atmosphere contains 7.824 Gt per ppm CO\u003csub\u003e2\u003c/sub\u003e (1 ppm of the atmosphere\u0026rsquo;s mass\u003ca class=\"FNLink\" href=\"#Fn23\" id=\"#FNLinkFn23\"\u003e\u003c/a\u003e times the relative molecular weights of CO\u003csub\u003e2\u003c/sub\u003e and air), the observed 1993-95 reduction represents 17.6 Gt removed from the atmosphere. Equally important is there is no evidence that the CO\u003csub\u003e2\u003c/sub\u003e sequestered by Pinatubo has re-entered the atmosphere. The most conservative algorithm used, namely the variable fraction model in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, remains above the measured values.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003ePotential Mechanisms\u003c/h2\u003e \u003cp\u003eIn addition to the OIF explanation offered by Sarmiento\u003csup\u003e14\u003c/sup\u003e, various other explanations for the Pinatubo CO\u003csub\u003e2\u003c/sub\u003e pause have been offered since 1993. \u003ca class=\"FNLink\" href=\"#Fn24\" id=\"#FNLinkFn24\"\u003e\u003c/a\u003e Most hypotheses are based on effects of large eruptions. As we will show, atmospheric CO\u003csub\u003e2\u003c/sub\u003e decreased significantly in only three eruptions out of nine since 1500 that caused atmospheric cooling. Those eruptions occurred adjacent to significant downwelling eddies.\u003c/p\u003e \u003cp\u003eThis lack of CO\u003csub\u003e2\u003c/sub\u003e removal after most large eruptions excludes widely discussed explanations based on large-scale fertilization of land or sea flora, as well as surface cooling or increased diffuse light from scattering by stratospheric aerosols. Any of those mechanisms, if operating, would be expected to reduce CO\u003csub\u003e2\u003c/sub\u003e levels long-term following all or most similar-sized and larger eruptions, but don\u0026rsquo;t.\u003c/p\u003e \u003cp\u003eThe 0.4-Gt reduction in world CO\u003csub\u003e2\u003c/sub\u003e emissions following the fall of the USSR in 1991 constitutes only 2% of the observed 17.6-Gt reduction. Further, our analysis includes such variations in global CO\u003csub\u003e2\u003c/sub\u003e emissions. Increased CO\u003csub\u003e2\u003c/sub\u003e solubility in the ocean due to a global temperature decrease caused by an increase in the Earth\u0026rsquo;s albedo is neither large enough nor permanent. Instead, substantial changes are needed in either ocean circulation or biology to redistribute carbon in the ocean.\u003ca class=\"FNLink\" href=\"#Fn25\" id=\"#FNLinkFn25\"\u003e\u003c/a\u003e Could the latter be OIF? Fay et al.\u003ca class=\"FNLink\" href=\"#Fn26\" id=\"#FNLinkFn26\"\u003e\u003c/a\u003e explored possible reasons for Pinatubo\u0026rsquo;s effect, and they mentioned OIF as a possibility for future exploration.\u003c/p\u003e \u003cp\u003eMore popular are proposals that changes in terrestrial respiration or enhanced tree growth due to diffuse sunlight are responsible.\u003ca class=\"FNLink\" href=\"#Fn27\" id=\"#FNLinkFn27\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn28\" id=\"#FNLinkFn28\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn29\" id=\"#FNLinkFn29\"\u003e\u003c/a\u003e However, CO\u003csub\u003e2\u003c/sub\u003e reductions following eruptions of Agung in 1963 and El Chich\u0026oacute;n in 1982 were short-lived, indistinguishable from noise, and less than 25% of the Pinatubo magnitude (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Is a slightly larger and longer-lasting aerosol concentration\u003csup\u003e42\u003c/sup\u003e enough to explain this difference, or are Krakauer and Randerson\u003ca class=\"FNLink\" href=\"#Fn30\" id=\"#FNLinkFn30\"\u003e\u003c/a\u003e correct?\u003c/p\u003e \u003cp\u003eIn summary, a large decrease in atmospheric CO\u003csub\u003e2\u003c/sub\u003e is generally not observed following large volcanic eruptions. Our purpose here is not to disprove other hypotheses but to explore the viability of a downwelling eddy hypothesis that accommodates 500 years of global CO\u003csub\u003e2\u003c/sub\u003e data.\u003c/p\u003e \u003cp\u003eVarious signs point to the likelihood of nitrogen-fixing OIF (N-OIF) as a key factor.\u003ca class=\"FNLink\" href=\"#Fn31\" id=\"#FNLinkFn31\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn32\" id=\"#FNLinkFn32\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, deposition of Pinatubo ash occurred in a region of the South China Sea that is known for its strong downwelling eddies. A downwelling eddy has two potential benefits for carbon removal. First, the surface water flows are convergent, which helps minimize dispersion and dilution of the fine ash and its associated nutrients. Second, the typical downward flow of ~\u0026thinsp;17 m/day (0.0002 m/s) for depths below 100 m as given in \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://data.marine.copernicus.eu/viewer/expert\u003c/span\u003e\u003cspan address=\"https://data.marine.copernicus.eu/viewer/expert\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e accelerates the natural sinking of both detritus and mineralized CO\u003csub\u003e2\u003c/sub\u003e to depths below which organisms can significantly ingest and metabolize the carbon. This downward flow is more than often thought but is consistent by analogy with atmospheric highs and lows with a recent upwelling analysis\u003ca class=\"FNLink\" href=\"#Fn33\" id=\"#FNLinkFn33\"\u003e\u003c/a\u003e and is similar to downward flows given by Boyd et al.\u003ca class=\"FNLink\" href=\"#Fn34\" id=\"#FNLinkFn34\"\u003e\u003c/a\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA downwelling and converging eddy could have held high-iron concentrations in place long enough (2\u0026ndash;5 months) to allow nitrogen-fixing bacteria (either cyanobacteria Crocosphaera\u003ca class=\"FNLink\" href=\"#Fn36\" id=\"#FNLinkFn36\"\u003e\u003c/a\u003e or Trichodesmium, which grows well there\u003ca class=\"FNLink\" href=\"#Fn37\" id=\"#FNLinkFn37\"\u003e\u003c/a\u003e) to propagate and provide the nitrate required for sustained growth.\u003c/p\u003e \u003cp\u003eHistorical records suggest that Pinatubo is not unique in initiating nitrogen-fixing OIF. Recent high-temporal-resolution measurements of the Law Dome ice core\u003ca class=\"FNLink\" href=\"#Fn38\" id=\"#FNLinkFn38\"\u003e\u003c/a\u003e shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e indicate an 11-ppm decrease in CO\u003csub\u003e2\u003c/sub\u003e in the 30 years after the Billy Mitchell eruption. Most or all of that was due to Billy Mitchell, whose ash would have deposited in a region of the ocean prone to downwelling eddies. Huaynaputina may have contributed, but most of its ash fell on land or near shore. It is better known for briefly cooling the earth by sulfate aerosols.\u003ca class=\"FNLink\" href=\"#Fn39\" id=\"#FNLinkFn39\"\u003e\u003c/a\u003e Minor (~\u0026thinsp;0.1\u0026deg;C) cooling may have been induced by the CO\u003csub\u003e2\u003c/sub\u003e reduction following Billy Mitchell.\u003c/p\u003e \u003cp\u003eIce-core CO\u003csub\u003e2\u003c/sub\u003e levels rose 5 ppm over the next 40 years and then stabilized for a century, although there may have been a minor dip in CO\u003csub\u003e2\u003c/sub\u003e levels from the 1660 Long Island volcano. The solid green line in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e is a hypothetical (\u0026ldquo;synthetic\u0026rdquo;) CO\u003csub\u003e2\u003c/sub\u003e curve similar to that used by Ahn et al.\u003csup\u003e38\u003c/sup\u003e with various multi-year averaging intervals to explain why other data sets did not see as sharp of a drop before 1600 and then a rise shortly thereafter. The dashed blue line is a symmetric 19-yr moving average of the green line.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows additional volcanic history summarized from a variety of sources.\u003ca class=\"FNLink\" href=\"#Fn40\" id=\"#FNLinkFn40\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn41\" id=\"#FNLinkFn41\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn42\" id=\"#FNLinkFn42\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn43\" id=\"#FNLinkFn43\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn44\" id=\"#FNLinkFn44\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn45\" id=\"#FNLinkFn45\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn46\" id=\"#FNLinkFn46\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn47\" id=\"#FNLinkFn47\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn48\" id=\"#FNLinkFn48\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn49\" id=\"#FNLinkFn49\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn50\" id=\"#FNLinkFn50\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn51\" id=\"#FNLinkFn51\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca class=\"FNLink\" href=\"#Fn52\" id=\"#FNLinkFn52\"\u003e\u003c/a\u003e A first observation is that estimates of world temperature reductions vary by a factor of two for a given volcano, so calculating temperature effects is difficult to do accurately. Tambora in 1815 had a major ash fall in an area prone to downwelling eddies and has been linked to a 6-ppm drop in atmospheric CO\u003csub\u003e2\u003c/sub\u003e. It also decreased temperature by 0.5 to 1.3 \u0026deg;C.\u003csup\u003e44\u003c/sup\u003e Krakatoa\u0026rsquo;s ash fall was northward away from downwelling eddies,\u003csup\u003e45\u003c/sup\u003e and no effect on atmospheric CO\u003csub\u003e2\u003c/sub\u003e is discernible from available CO\u003csub\u003e2\u003c/sub\u003e data. Agung in 1963 and El Chich\u0026oacute;n in 1982 led to similar (~\u0026thinsp;0.2\u0026deg;C) reductions in global temperature as followed Pinatubo but did not leave a notable CO\u003csub\u003e2\u003c/sub\u003e reduction after a year, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eList of major volcanoes in the last 500 years and an assessment of their contribution to global cooling and atmospheric CO\u003csub\u003e2\u003c/sub\u003e reduction.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEruption\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eYear\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTemperature reduction, \u0026deg;C\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ekm\u003csup\u003e3\u003c/sup\u003e of magma released\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e ppm reduced after 10 years\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eOcean eddy near ash fall?\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003ePinatubo\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1991\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.2\u003csup\u003e40\u003c/sup\u003e, \u0026lt;\u0026thinsp;0.3\u003csup\u003e41\u003c/sup\u003e, 0.5\u003csup\u003e42\u003c/sup\u003e, 0.5\u003csup\u003e43\u003c/sup\u003e, 0.15\u003csup\u003e52\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5\u003csup\u003ee\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eYes\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eEl Chich\u0026oacute;n\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1982\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.2\u003csup\u003e40\u003c/sup\u003e, \u0026lt;\u0026thinsp;0.3\u003csup\u003e41\u003c/sup\u003e, 0.1\u003csup\u003e52\u003c/sup\u003e, 0.4\u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.4\u003csup\u003e48\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eNo: landlocked\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eAgung\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1963\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.2\u003csup\u003e40\u003c/sup\u003e, \u0026lt;\u0026thinsp;0.3\u003csup\u003e41\u003c/sup\u003e, 0.35\u003csup\u003e46\u003c/sup\u003e, 0.2\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.4\u003csup\u003e49\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eNo: equatorial\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eSanta Maria\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1902\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.4\u003csup\u003e41\u003c/sup\u003e, 0.25\u003csup\u003e40\u003c/sup\u003e, 0.2\u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5\u003csup\u003ef\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eNo: landlocked\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eKrakatoa\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1883\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.6\u003csup\u003e41,c\u003c/sup\u003e, 0.3\u003csup\u003e51\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e20\u003csup\u003e45\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eNo: ashfall away from eddies\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eTambora\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1815\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.8-1.3\u003csup\u003e44\u003c/sup\u003e, 1.1\u003csup\u003e41\u003c/sup\u003e, 0.5\u003csup\u003e40\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e30\u003csup\u003e44\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e6\u003csup\u003e44\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eYes\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eLong Island\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1660\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.9\u003csup\u003e41\u003c/sup\u003e, 0.3\u003csup\u003e40\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e30\u003csup\u003eg\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eNo: equatorial\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eHuaynaputina\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1600\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.1\u003csup\u003e41\u003c/sup\u003e, 0.8\u003csup\u003e40\u003c/sup\u003e, 1.6\u003csup\u003e47\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14\u003csup\u003e47\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eNo: 120 km from coast\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eBilly Mitchell\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1580\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003csup\u003e41\u003c/sup\u003e, 0.4\u003csup\u003e40\u003c/sup\u003e, 0.25\u003csup\u003ed\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14\u003csup\u003e50\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e11\u003csup\u003e38\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eYes\u003csup\u003eh\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003e\u003csup\u003ea\u003c/sup\u003eNASA earth observatory, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://earthobservatory.nasa.gov/images/77538/remembering-el-chichon\u003c/span\u003e\u003cspan address=\"https://earthobservatory.nasa.gov/images/77538/remembering-el-chichon\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003e\u003csup\u003eb\u003c/sup\u003eMetMatters, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.rmets.org/metmatters/mount-agung-and-its-potential-global-impacts\u003c/span\u003e\u003cspan address=\"https://www.rmets.org/metmatters/mount-agung-and-its-potential-global-impacts\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003e\u003csup\u003ec\u003c/sup\u003eNatural History Museum, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.nhm.ac.uk/discover/the-1883-krakatau-eruption-a-year-of-blue-moons.html\u003c/span\u003e\u003cspan address=\"https://www.nhm.ac.uk/discover/the-1883-krakatau-eruption-a-year-of-blue-moons.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003e\u003csup\u003ed\u003c/sup\u003eVolcano Discovery, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.volcanodiscovery.com/billy_mitchell.html\u003c/span\u003e\u003cspan address=\"https://www.volcanodiscovery.com/billy_mitchell.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003e\u003csup\u003ee\u003c/sup\u003eU.S. Geological Survey Fact Sheet 113\u0026thinsp;\u0026minus;\u0026thinsp;97, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://pubs.usgs.gov/fs/1997/fs113-97/\u003c/span\u003e\u003cspan address=\"https://pubs.usgs.gov/fs/1997/fs113-97/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003e\u003csup\u003ef\u003c/sup\u003eOregon State University, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://volcano.oregonstate.edu/santa-maria\u003c/span\u003e\u003cspan address=\"https://volcano.oregonstate.edu/santa-maria\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003e\u003csup\u003eg\u003c/sup\u003e Volcano Discovery, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.volcanodiscovery.com/long_island.html\u003c/span\u003e\u003cspan address=\"https://www.volcanodiscovery.com/long_island.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003e\u003csup\u003eh\u003c/sup\u003e\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://earth.nullschool.net/#current/ocean/surface/currents/orthographic=-209.13,-5.23,2704/loc=-91.646,12.561\u003c/span\u003e\u003cspan address=\"https://earth.nullschool.net/#current/ocean/surface/currents/orthographic=-209.13,-5.23,2704/loc=-91.646,12.561\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe case is strong for the Pinatubo eruption removing and effectively sequestering 15\u0026ndash;20 Gt of atmospheric CO\u003csub\u003e2\u003c/sub\u003e in the two years following its eruption. This observed removal rate is roughly an order of magnitude larger than is currently discussed as possible from OIF.\u003ca class=\"FNLink\" href=\"#Fn53\" id=\"#FNLinkFn53\"\u003e\u003c/a\u003e We propose that the circulation caused by a stationary mesoscale downwelling eddy helped contain the nutrients at concentrations needed to enable cyanobacteria to grow and fix nitrogen, thereby prolonging the effect of the iron deposition (nitrogen-fixing OIF). Examination of CO\u003csub\u003e2\u003c/sub\u003e concentrations over the past five years identified two eruptions (Billy Mitchell in 1580 and Tambora in1815) that had an even larger drawdowns of CO\u003csub\u003e2\u003c/sub\u003e, which means that this mechanism is rare but not unique.\u003c/p\u003e \u003cp\u003eNitrogen-fixing OIF (N-OIF) is a testable hypothesis of how fertilizing a downwelling eddy can greatly increase the amount and durability of marine carbon capture and sequestration. Thus N-OIF warrants replication tests. While laboratory tests will help understand the detailed chemical and biological mechanisms, only large-scale field tests will be able to reproduce the potentially complex interactions of those mechanisms with the marine biology and hydrodynamics associated with a downwelling eddy.\u003c/p\u003e \u003cp\u003eIf one assumes, for example, that the buildup and phase out of fossil fuels is a symmetric curve and that we achieve peak emissions in 2028, we can expect CO\u003csub\u003e2\u003c/sub\u003e to rise another 125 ppm by early next century in the absence of large-scale CO\u003csub\u003e2\u003c/sub\u003e removal. That scenario, which is similar to IPCC scenario SSP2-4.5, implies a potential ultimate rise to about 580 ppm without large-scale CO\u003csub\u003e2\u003c/sub\u003e removal and a net increase of 2350 Gt in atmospheric CO\u003csub\u003e2\u003c/sub\u003e from the 280-ppm pre-industrial level. Clearly, large-scale CO\u003csub\u003e2\u003c/sub\u003e removal technologies are needed to restore and sustain pre-industrial ecosystems, independent of the future rate of emission reduction.\u003c/p\u003e \u003cp\u003eThe 15\u0026ndash;20 Gt/y removal event by Pinatubo in 1992\u0026ndash;1994 suggests that mCDR could be a major improvement over others being pursued today. Important issues include what annualized gross primary productivity (GPP) can be achieved and what fraction of that productivity can be transported to multi-decade storage depths. A 5-month average GPP of 4.3 gC/m\u003csup\u003e2\u003c/sup\u003e/d was observed in the Humboldt current, with a daily maximum of 20 gC/m\u003csup\u003e2\u003c/sup\u003e/d.\u003ca class=\"FNLink\" href=\"#Fn54\" id=\"#FNLinkFn54\"\u003e\u003c/a\u003e Managed OIF may be able to achieve closer to the maximum, but not all of that carbon will reach storage depths. A typical 17-m/d downward eddy velocity would improve storage efficiency.\u003c/p\u003e \u003cp\u003eAn average net storage value of 10 gC/m\u003csup\u003e2\u003c/sup\u003e/d over a 300-km diameter eddy would remove\u0026thinsp;~\u0026thinsp;1 Gt of CO\u003csub\u003e2\u003c/sub\u003e/y. Ten such locations constitute 0.2% of the ocean surface and would remove\u0026thinsp;~\u0026thinsp;10 Gt CO\u003csub\u003e2\u003c/sub\u003e/y. Whether that level of mCDR can be obtained is debatable,\u003csup\u003e8\u003c/sup\u003e but it is certainly conceivable. Although this is nearly equal to the net annual oceanic capture and storage of CO\u003csub\u003e2\u003c/sub\u003e,\u003ca class=\"FNLink\" href=\"#Fn55\" id=\"#FNLinkFn55\"\u003e\u003c/a\u003e net storage is the difference between two large numbers, so it represents only a 3.2% change in the gross CO\u003csub\u003e2\u003c/sub\u003e uptake of the ocean. Further, CO\u003csub\u003e2\u003c/sub\u003e uptake is highly variable across the globe and seasons, so the 3.2% value is a better metric for the feasibility of mCDR at optimal sites. Only large-scale and longer-term experiments will be able to determine the true efficiency and cost of mCDR in general and N-OIF in particular. If successful, implementation at large scale could provide an important tool for restoring atmospheric CO\u003csub\u003e2\u003c/sub\u003e to levels of a century ago.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eAll data analyses were performed in Excel. Fits to the Keeling Curve from 1958\u0026ndash;1990 were accomplished either by native regression tools or by a nonlinear regression grid search using an objective function defined as the sum of squares of the difference between the model function and the measured value. CO\u003csub\u003e2\u003c/sub\u003e concentrations after 1957 were downloaded from the NOAA CO\u003csub\u003e2\u003c/sub\u003e emission site (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://gml.noaa.gov/ccgg/trends/data.html\u003c/span\u003e\u003cspan address=\"https://gml.noaa.gov/ccgg/trends/data.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) and before 1958 from the University of Exeter Global Carbon Budget (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://globalcarbonbudget.org/\u003c/span\u003e\u003cspan address=\"https://globalcarbonbudget.org/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). CO\u003csub\u003e2\u003c/sub\u003e emission history (annual and cumulative) was downloaded from Our World in Data (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://ourworldindata.org/grapher/cumulative-co-emissions?country=~OWID_WRL\u003c/span\u003e\u003cspan address=\"https://ourworldindata.org/grapher/cumulative-co-emissions?country=~OWID_WRL\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://ourworldindata.org/co2-emissions\u003c/span\u003e\u003cspan address=\"https://ourworldindata.org/co2-emissions\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). Our fixed-fraction model is simpler than in McKinley et al.\u003csup\u003e24\u003c/sup\u003e but gets the same fraction of fossil-fuel CO\u003csub\u003e2\u003c/sub\u003e emissions absorbed prior to Pinatubo.\u003c/p\u003e \u003cp\u003eThe fixed-fraction method used the model function\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:p{CO}_{2}\\left(y+1\\right)=p{CO}_{2}\\left(y\\right)+(1-a){M}_{CO2}\\left(y\\right)/7.824{10}^{9}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eM\u003c/em\u003e is the annual mass of CO\u003csub\u003e2\u003c/sub\u003e emissions. The optimum value of \u003cem\u003ea\u003c/em\u003e was 0.397, with an average squared residual value of 0.15 ppm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe variable-fraction method used the functions\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{p}_{i}{CO}_{2}\\left(y+1\\right)=p{CO}_{2}\\left(y\\right)+{M}_{CO2}\\left(y\\right)/7.824{10}^{9}\\:$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:p{CO}_{2}\\left(y+1\\right)={p}_{i}{CO}_{2}\\left(y\\right)-b({p}_{i}{CO}_{2}\\left(y+1\\right)-280)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eHere, \u003cem\u003ep\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e is the value that would have occurred with no absorption, and \u003cem\u003eb\u003c/em\u003e is a mass transfer coefficient assuming CO\u003csub\u003e2\u003c/sub\u003e absorption is related to the excess \u003cem\u003epCO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e since the industrial revolution. The optimum value of \u003cem\u003eb\u003c/em\u003e was 0.0153, with an average squared residual of 0.25 ppm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe exponential method used the function\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:p{CO}_{2}\\left(y\\right)={c}_{1}+{c}_{2}exp\\left({c}_{3}y\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe optimum values of \u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e, and \u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e were 288, 1.0373\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, and 0.0287, respectively, with an average squared residual value of 0.25 ppm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eP.F. conceived this work, did the initial data analysis, and wrote much of the text. A.K.B. did a more extensive data analysis, wrote the majority of the text, and prepared the figures. Both authors conducted literature searches, interpreted results, and reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe spreadsheet used to fit downloaded data to the curves shown in the figures is available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003e Hansen, J. E., et al. Global warming in the pipeline. \u003cem\u003eOxford Open Clim. Change\u003c/em\u003e \u003cb\u003e3\u003c/b\u003e, kgad008 (2023). 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DOI: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.5194/essd-15-5301-2023\u003c/span\u003e\u003cspan address=\"10.5194/essd-15-5301-2023\" targettype=\"DOI\" 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":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6960838/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6960838/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAfter Pinatubo erupted in 1991, atmospheric CO\u003csub\u003e2\u003c/sub\u003e levels stabilized during 1992\u0026ndash;1993. Concentrations became\u0026thinsp;~\u0026thinsp;2.25 ppm less than projected from contemporary emissions, corresponding to ~\u0026thinsp;17.6 Gt of CO\u003csub\u003e2\u003c/sub\u003e removed. CO\u003csub\u003e2\u003c/sub\u003e did not return to projected levels in the following decades. Pinatubo ash fell into a downwelling eddy WSW of the volcano, which could explain the large, long-term CO\u003csub\u003e2\u003c/sub\u003e removal. Of nine eruptions since 1500 that led to significant atmospheric cooling, three located near frequent downwelling mesoscale eddies resulted in notable CO\u003csub\u003e2\u003c/sub\u003e-level reductions. The six that were distant from downwelling eddies had no notable impact on atmospheric CO\u003csub\u003e2\u003c/sub\u003e. Therefore, CO\u003csub\u003e2\u003c/sub\u003e reductions correlate with downwelling eddies rather than cooling. A CO\u003csub\u003e2\u003c/sub\u003e-pause hypothesis proposed by Sarmiento was that iron from the ashfall enabled a phytoplankton bloom that led to a large atmospheric CO\u003csub\u003e2\u003c/sub\u003e removal. Local downwelling and converging eddies would have held high iron concentrations in place long enough to allow nitrogen-fixing bacteria to grow and provide the nitrate required. This nitrogen-fixing Ocean Iron Fertilization (N-OIF) process is a testable hypothesis which, if scaled, could provide a multi-Gt/y method for atmospheric CO\u003csub\u003e2\u003c/sub\u003e removal.\u003c/p\u003e","manuscriptTitle":"The Pinatubo CO2 pause suggests a rapidly testable path to multi-Gt mCDR","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-15 17:51:10","doi":"10.21203/rs.3.rs-6960838/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"4e3c0bf0-e388-445d-b4be-63f00bde0f8c","owner":[],"postedDate":"July 15th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":51428712,"name":"Earth and environmental sciences/Biogeochemistry"},{"id":51428713,"name":"Earth and environmental sciences/Climate sciences"},{"id":51428714,"name":"Earth and environmental sciences/Ocean sciences"},{"id":51428715,"name":"Earth and environmental sciences/Solid earth sciences"}],"tags":[],"updatedAt":"2025-09-22T09:25:43+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-15 17:51:10","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6960838","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6960838","identity":"rs-6960838","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}