Integrating past variability in climate-driven Mediterranean fire hazard assessments for 2020-2100

Integrating past variability in climate-driven Mediterranean fire hazard assessments for 2020-2100 | 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 Research Article Integrating past variability in climate-driven Mediterranean fire hazard assessments for 2020-2100 Marion Lestienne, Boris Vannière, Thomas Curt, Isabelle Jouffroy-Bapicot, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-164555/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 4 You are reading this latest preprint version Abstract In the Mediterranean basin, Corsica (French island) harbours among the best-preserved Mediterranean forest ecosystems and its high biodiversity could be threatened by the climate and disturbance-regime changes due to the global warming. This study aims i) to estimate the future climate-related fire hazard in Corsica for the current century (2020–2100) based on two RCP scenarios (RCP4.5 and RCP8.5), and ii) to compare the predicted trends with the entire Holocene period for which fire hazard has previously been assessed. An ensemble of future climate simulations from two IPCC RCP scenarios has been used to compute the Monthly Drought Code (MDC) and the Fire Season Length (FSL) and to assess the level of fire hazard assessment. Here, we show that the MDC and the FSL would both strongly increase over the next decades due to the combined effect of temperature increase and precipitation decrease in the Corsica region. Moreover, the maximum Holocene FLS (7000 to 9000 years ago), will be reached (and even exceeded depending upon the scenario) after 2040. For the first time in the Holocene, we may be confronted to an increase in the number of fire-prone months driven by climate combined with many human-caused ignitions. This combination should increase the burned area from 15–140%. For the next 30 years, the game seems to be already played as both RCP scenarios resulted in similar increase in fire hazard intensity and duration. It is thus mandatory to reconsider fire-management and fire-prevention policy to mitigate the future fire risk, and its catastrophic consequences for ecosystems, population, and economy. Climatology Climate Analysis and Modeling paleofires fire season Monthly Drought Code (MDC) modelling Holocene Anthropocene Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction Over the past decades, a surge in the number of large and uncontrolled wildfires has occurred on all terrestrial ecosystems, and this can be observed irrespectively of national fire-fighting capacities or management tactics (Westerling et al. 2006 ; Lohman et al. 2007 ; Forsyth and Van Wilgen 2008 ; Bowman et al. 2009 ). The global warming predicted by climate models for the 21st century and beyond may amplify this trend, and threat most ecosystems worldwide (Giannakopoulos et al. 2005 ; Pachauri and Reisinger 2007 ; Varela et al. 2019 ). Located on a latitudinal gradient at the interface between temperate Europe and subarid North-Africa within the Mediterranean basin, the French island of Corsica has a climate characterized by a seasonal alternation between a warm and dry summer season with a marked drought period, and a cool season with high rainfall (Pausas 2004 ; Sá et al. 2017 ). This, together with the traditional use of pastoral fire, makes Corsica a fire hotspot (Keeley 2009 ; Leys et al. 2014 ). Along the last decades (1973–2019), 2 369 fires larger than 10 ha have occurred in Corsica and 305 063 ha (~ 67 779 ha.yr − 1 ) have been burned ( http://www.promethee.com/ ). Corsica harbours a high biodiversity, many endemic species (Médail and Verlaque 1997 ; Médail 2017 ), and preserved Mediterranean forest ecosystems (Medail and Quezel 1997 ; Vogiatzakis et al. 2016 ). However, this renders Corsica vulnerable to climate and disturbance-regime changes as shown its past environmental history and the predictions for the future (Giannakopoulos et al. 2005 ; Giorgi 2006 ; Vogiatzakis et al. 2016 ; Varela et al. 2019 ; Lestienne et al. 2020b ; Fargeon et al. 2020 ). Recent studies have suggested that the fire danger could be three times higher when summer temperature anomaly exceeds + 2°C (Fargeon et al. 2020 ) and that the fire danger could be doubled in Corsica (Varela et al. 2019 ). In this context, it is of importance to assess the future climate-related fire hazard in Corsica for the current century (2020–2100). To do this, we use 5 climate models with two RCP scenarios (4.5 and 8.5, respectively) and we compare these results to recent studies (Varela et al. 2019 ; Fargeon et al. 2020 ). Then, we compare the future trends in fire activity to those for the entire Holocene period based on our previous works (Lestienne et al. 2020a ). Indeed, values of the past fire hazard provide a valuable historical baseline and give valuable insights about thresholds and shifts in ecosystems responses related to past fire regime changes. Several climate-related indexes of fire-danger have been tested for the Mediterranean region among which the Fire Weather Index (FWI) (Wagner et al. 1987 ) and its sub-indexes, which all have overall showed their efficiency on current climate conditions (Aguado et al. 2003 ; Moriondo et al. 2006 ; Carvalho et al. 2008 ; Varela et al. 2019 ; Fargeon et al. 2020 ). However, most of these fire-danger indexes require daily weather forecast data to be computed, which can limit their use for past assessment and future prediction. The Monthly Drought Code (MDC), whose computation is derived from the computation of the Drought Code, i.e. a sub-index of the FWI, has also been created first in Canada (Girardin and Wotton 2009 ) and recently tested over Corsica (Lestienne et al. 2020a ). Its computation uses monthly means of maximum temperature and monthly precipitation, therefore providing an assessment of climate-related fire hazard without the constraint of daily data requirement. 2. Materials And Methods 2.1. Study area Corsica is a mountainous French island located in the western Mediterranean basin (Fig. 1). In spite of high mountains, there are no glaciers and no permanent snow on the island (Conchon 1986 ). During the early Holocene (i.e. 11,000 years ago), the vegetation was mainly composed by pinewood and heather. During the Neolithic (around 6000 years ago), a significant change occurred in land cover with the expansion of oak forests, which have dominated the island during most of the Holocene thereafter (Reille 1992 ; Reille et al. 1999 ; Lestienne et al. 2020b ). Nowadays, the ecological value of the region, the high population and settlement density on the island and the abandonment of traditional land uses (i.e. the subsequent fuel accumulation) represent important stakes that, combined with climate warming (Fig. 1) (NOAA 2011), induce an exponential increase in fire risk (Keeley 2009 ; Leys et al. 2014 ) in this fire-prone region (Appendix A). Figure 1 - Geographical context of Corsica, a French European island and changes in rainfall around the Mediterranean Basin between 1971–2010 and the comparison period 1902–2010 (NOAA, 2011). Figure adapted from NOAA/Earth System Research Laboratory. 2.2. Climate datasets for climate changes scenarios Refer to Fig. 2 for a summary diagram. Figure 2 - Synthesis of the climatic datasets extraction methodology. Current climate dataset was extracted from ERA-Interim ( https://www.ecmwf.int/ ). It is a daily global atmospheric reanalysis continuously updated in real time and allowing to obtain daily temperature and precipitation data since 1979. The spatial resolution is approximately 60 x 80 km (Corsica being covered by a total of five pixels) (Dee et al. 2011 ). Paleoclimate simulated datasets were extracted from simulations provided by HadCM3BL-M1, this model being a variant of the fully complex Hadley Centre climate model HadCM3, usually involved in IPCC assessment reports, and belonging to the HadCM3 family of climate models (Valdes et al. 2017 ). Based on orbital forcing parameters and some other prescribed variables for the ocean compartment, this intermediate-complexity model version performed a snapshot equilibrium simulation of the atmospheric compartment per millennium, from which climate normals (monthly means) for main climate variables ( i.e. air temperature and precipitation used here) are available. We therefore used these datasets of climate normals for the last eleven millennia (centered at changeovers 11, 10, …, 2, and 1 kilo years calibrated Before Present i.e. before 1950 – abbreviated thereafter ky cal BP), as well as for the control run representative of the pre-industrial period (AD 1750, considered as equivalent to 0 BP). Then, temperature and precipitation anomalies ( i.e . difference and rate of change for temperature and precipitation, respectively) have been computed and downscaled for each millennium changeover based on Ramstein et al. ( 2007 ). Using an inverse-distance weighting approach on the four closest HadCM3BL-M1 pixels from those covering Corsica, we downscaled these anomalies by applying them to the current period climate normal that we computed from the ERA-Interim dataset to reconstruct past climate for each Holocene millennium changeover at the whole island scale (Ramstein et al. 2007 ; Hély et al. 2010 ). As Holocene climate changes were mainly due to insolation changes, we also retrieved insolation values for the entire Holocene period using the “palinsol” R package (Crucifix 2016 ) which includes data from Berger and Loutre ( 1991 ). Future climate datasets are monthly means projections from simulations of five CMIP5 coupled models (Table 1 ) (Pachauri et al. 2015) under two RCP scenarios (4.5 and 8.5, respectively). In a preliminary step (Appendix B), we first compared them to the overall range of historical variability (1979–2016) in order to test their representativeness and to take into account the variability induced by models (Fargeon et al. 2020 ). Table 1 Summary of the climate models used in this study (based on the IPCC Fifth Assessment Report AR5 (Pachauri et al., 2015). Model Resolution References Atmospheric Oceanic Past HadCM3BL-M1 3,75° x 2,5° 3,75° x 2,5° Valdes et al. ( 2017 ) Future FGOALS_S2 1,65° x 2,8° 1° x 1° Bao et al. (2013) CanESM2 2,8° x 2,8° 1° x 1,4° Chylek et al. ( 2011 ) MIROC_ESM_CHEM 2,8° x 2,8° 1° x 1,4° Watanabe et al. ( 2011 ) MIROC5 1,4° x 1,4° 0,5° x 1,5° Watanabe et al. ( 2010 ) MRI_CGM3 1,1° x 1,1° 0,5° x 1° Yukimoto et al. (2012) 2.3. Climate fire hazard assessment The MDC (Girardin and Wotton, 2009 ) is unitless and captures the moisture content of deep and compact organic layers. As for the DC, the MDC indicates the effects of seasonal drought on forest fuels and the probability of smouldering in deep duff layers and in large logs, but they differ in their computation requirements. Although these indices were initiated in Canada, we showed in a previous study that the DC was efficient to discriminate fire days and no-fire days in Corsica for the period 1979–2016 and that the MDC was an efficient tool to reconstruct the Holocene climate-related fire hazard (Lestienne et al. 2020a ). The great advantage of the DC and the MDC is that they only require precipitation and temperature data (Wagner et al. 1987 ). Moreover, while the DC needs daily data, the MDC has been created to be directly computed from monthly means of precipitation and maximum temperatures, which makes it very powerful for simulated past and future datasets (Lestienne et al. 2020a ). The MDC has been computed at each millennium changeover for the entire Holocene (from 11 to 0 K years cal. BP) in order to compare with the coeval charcoal-inferred fire history reconstructed from lacustrine sediments by Lestienne et al. ( 2020a ). The 280-unit threshold in the DC appears meaningful to characterize extreme drought and a high probability of fire hazard in boreal forests (Girardin and Wotton 2009 ). In Corsica, we previously showed that this DC threshold is about 300 units (Lestienne et al. 2020a ). 2.4. Fire-Season Length assessment Based on the Holocene analysis of MDC values, and according to the method originally developed by Hély et al. ( 2010 ) for the DC and adapted to the MDC (Lestienne et al. 2020a ), the threshold value of 300 units was used to calculate the Fire-Season Length (FSL) for the past 11 millennia and for the future decades until 2100. Basically, for all months with MDC values above the 300-unit threshold, we considered the full month length (30 or 31 days) as part of the FSL. To define the starting (and ending) month of the FSL and to add the related number of starting (ending) days ( i.e. <30 or 31 for each month), we used a basic linear interpolation between months, reporting a MDC value lower than the 300-unit MDC, and the following (preceding) month with above-threshold MDC value (Lestienne et al. 2020a ). On the expected starting (ending) month, we associated the lower-than-threshold MDC value to the first (last) day and the above-threshold value to the last (first) day of this month to perform the computation. 3. Results 3.1. estimation of future fire hazard in a multi-millennial context In terms of fire hazard, the results showed that, by the end of the 21st century, the MDC will increase by 7.5% and up to 21% in Corsica, respectively for the RCP4.5 and RCP8.5 scenarii as compared to the 1979–2016 period reference (Fig. 3, Appendix C). Such future fire hazard values will be similar to those estimated for 7 ky cal BP, but they will not exceed the maximum Holocene MDC values. The duration of periods with high fire hazard (FSL), will increase higher (up to 25% i.e. 35 days for RCP4.5 and 32% i.e. 51 days for RCP8.5 by the end of the 21st century), and will exceed the Holocene maximum FSL value after 2040 from both RCP scenarii. Indeed, after 2040, FSL will stabilize around 130 days per a year (i.e. more than 4 months) for the RCP4.5 scenario, while it will exceed 150 days (i.e. more than 5 months) with the RCP8.5 scenario. Figure 3 - MDC (Monthly Drought Code) and FSL (Fire Season Length) changes in Corsica during the Holocene and future decades. All computations were performed on the 5 ERA-interim pixels delimitating Corsica. MDC computation was performed from May to October, and the FSL subsequently estimated based on the 300-unit MDC threshold (see Sect. 2.3). References to fires and human intensity of activities at the figure’s bottom come from a previous study based on microcharcoal, pollen and non-pollen palynomorphs analysis (Arborean/non-arborean pollen ratio for land openness; crop, ruderal pollen and dung fungal spores percentages for agropastoral activities) from the Bastani Lake in Corsica ( Lestienne et al., 2020b ). Past insolation has been calculated using the “palinsol” R package ( Crucifix 2016 ) which includes data from Berger and Loutre ( 1991 ). For comparison, the Holocene estimation of MDC and FSL values highlighted five periods (Fig. 3). Between 11 to 9 ky cal BP, the fire hazard index was relatively high, with MDC and FSL values above 600 units and 100 days, respectively. Afterwards, i.e. between 9 and 7 ky cal BP, the FSL values remained high with a peak of 126 days at 9 ky cal BP, while MDC values progressively decreased below 500 units. After 7 ky cal BP, FSL felt below 80 days, while MDC continued to decrease until 5 ky cal BP. After 5 ky cal BP, low fire hazard indexes lasted 4 millennia, characterized by minima values: below 400 units for MDC and between 54 et 77 days for FSL. Finally, between 1 and 0 ky cal BP the fire hazard switched back toward increasing trend due to both a slight increase in the MDC values, passing above 400 units, and a sharp increase in FSL values above 100 days, attesting change in fire-related climate conditions. 3.2. Intra-season variability The fact that the 21st century FSL curve follows also an increasing but amplified trend compared to the MDC curve (Fig. 3) is related to the different trends of monthly drought intensity, especially after 2050. Indeed, beyond the overall increase in monthly values of fire hazard intensity (MDC) for both RCP scenarii, fire-season months can be splitted in three groups (Fig. 4 ). The early fire-season (i.e. May and June) will have a higher fire hazard intensity than the Holocene maximum for both RCP scenarii, while it will be within (respectively higher than) the range of the reference period variability for the RCP4.5 (respectively RCP8.5 after 2050). Therefore, such drought intensity will induce at least a similar or an earlier onset of the fire-season (Appendix C). For the mid fire-season (i.e. July and August), only the RCP8.5 scenario will induce higher fire hazard intensity than the Holocene maximum after 2050, while both RCP scenarii will induce higher fire hazard intensity after 2050 than during the reference period. Such increase in mid fire-season hazard intensity would indirectly lead to an extension of the FSL if humidity of late summer and fall months would not compensate enough for the drought. This is the trend shown in the climate forecast (Appendix B), and that explains at least partially the extended fire-season end (Appendix C). Indeed, the late fire-season (i.e. September and October) is also extended, up to an extra month (i.e. November) towards the end of the 21st century in both RCP scenarii (Appendix C) due to on going temperature increase and still no extra rainfall (Appendix B). Figure 4 - MDC (Monthly Drought Code) distributions for each fire-season month over the next decades computed using the IPCC CMIP5 RCP scenarios. Red lines represent the 1979–2016 MDC distribution (modern reference period) from ERA-interim dataset with the plain line being the median and the dotted lines being the first and third quartiles values. Blue line represents the maximum MDC Holocene value estimated for Corsica over the last 11000 years BP using the HadCM3BL-M1 model. 4. Discussion 4.1. fire hazard changes for the next decades We showed that the MDC and the FSL will increase until ca. 2050 for both RCP scenarii. Then, these increases will slow down or stabilize with the RCP4.5 scenario, while they will continue their ascents with the RCP8.5 scenario. Such overall trends agree with those found from previous studies that used the FWI or one of its sub-indexes to analyse future changes in fire hazard in the Mediterranean basin. For instance, Moriondo et al. ( 2006 ), dealing with future trend in FSL in the Mediterranean basin but based on previous generation of IPCC scenarii showed that FSL will increase by ca. 40 and ca. 32 days for the scenario SRES A2 (close to RCP8.5) and the scenario SRES B2 (no RCP equivalent), respectively. These FSL values are in line with ours (from + 48 days or 50% to + 24 days or 25% for RCP8.5 and RCP4.5, respectively) despite different IPCC scenario generations, different current reference periods and extents (1961–1990 versus 1979–2016 for this study), different temporal projections (only one period for the future (2071–2100) for their study versus all decades until 2100 in this study), and differences in the mode of computation of FSL. Moreover, Moriondo et al. ( 2006 ) worked on six Mediterranean countries (i.e. Portugal, Spain, Greece, Italy, France, and Balkan), while ours focuses on Corsica, geographically located at the centre of the Mediterranean region covered in Moriondo et al. ( 2006 ). Our approach uses the most updated IPCC scenarii and the best temporal resolution allows better highlighting of changes in the next decades. The review of Giorgi and Lionello ( 2008 ), based on an ensemble of global and regional climate change simulations, showed that the next decades (2071–2100) will be characterized by a decrease in precipitation and an increase in temperature, and that these changes will mostly occur in summer. This is in line with our results showing a drought increase over the next decades, especially in summer (Fig. 4 , Appendix B). The causes for this large summer drying signal have been previously investigated by Rowell and Jones (2006), who examined four possible and non-exclusive mechanisms: i) the low spring soil moisture conditions leading to reduced summer convection; ii) the large land-sea contrast in warming condition leading to reduced relative humidity and precipitation over the continent; iii) the positive summer soil moisture precipitation feedback; iv) the remote influences (e.g. descending motions induced by the strengthening of the Asian monsoon). According to these authors, these changes will be similar among the RCP scenarii until 2050 and will therefore depend on the political and economic choices after 2050 (Pachauri and Reisinger 2007 ; Cuttelod et al. 2009 ; Planton et al. 2012 ). Such changes have been reported in most projections from both global and regional models and most IPCC scenarii (Kittel et al. 1997 ; Giorgi and Francisco 2000 ; Giorgi et al. 2001 ). More recent studies (Faggian 2018 ; Varela et al. 2019 ; Fargeon et al. 2020 ), using the same two RCP scenarii as ours, but based on the FWI (alone or in combination with ISI and DC indexes) showed that the fire danger will increase from 24 to 67% depending on the RCP scenario. It is worth noting that ISI and FWI require more inputs (i.e. wind speed and relative humidity) and therefore likely more uncertainties, and that authors did not systematically discriminated changes in fire danger intensity from changes in the fire-season length. 4.2. How much future fire hazard will depart from past? By comparing our Anthropocene future predictions with the Holocene values, we can see that the FSL will exceed the maximum values at Holocene from 2040, while MDC will reach in 2100 values never observed since 6 k years cal. BP. From these results, we can expect an increase in fire occurrences in the next decades, but the climatic conditions and the vegetation will differ in the future fire-prone periods as compared to the first half of the Holocene period and could also affect the fire occurrence. Indeed, the current insolation is lower than in the early Holocene (11 − 7 k years cal. BP), which corresponded to the maximum Holocene insolation (Berger and Loutre 1991 ) as shown in Fig. 3, especially for the months of June, July and August at 45°N latitude (Hély et al. 2010 ; Renssen et al. 2012 ). This high insolation in early Holocene induced a strong seasonality, with the highest temperature and lowest precipitation in summer, and inversely for winter as reconstructed by Dormoy et al. ( 2009 ) from Mediterranean marine pollen records. Here we showed that the FSL will increase in the future, due to both the increase in temperature as a response of increasing concentrations in Greenhouse Gas and the decrease in precipitation as a response of atmospheric dynamics over the Mediterranean region that will occur for every month (Appendix C), with most aridity in summer months. This difference in seasonality origin between early Holocene with high insolation and the 21st century with low insolation but high concentrations of Greenhouse Gas could explain the differences between the early-Holocene and the future fire hazard predictions in terms of intensity (MDC) and duration (FSL). Indeed, the MDC calculation needs monthly maximal temperatures and monthly precipitations data (Girardin and Wotton 2009 ). Because the early Holocene summer conditions was warmer and dryer, the MDC was high, but the strong seasonality with cooler and wetter conditions in both spring and autumn induced relatively short FSL. In the future, the climatic conditions in summer will not reach the maximal Holocene values, but the global aridification occurring for every month will extend the FSL much longer. Contrary to the early Holocene when insolation was the main climate driver, the global increase in aridity will be mainly due to the strong increase in greenhouse gas emissions and responsible for global change (Pachauri and Reisinger 2007 ; Thomson et al. 2011 ; Meinshausen et al. 2011 ; Riahi et al. 2011 ). In addition, we have to consider that fires have not always been the sole result of arid periods. Indeed, while dry and warm climate promoted fires between 11 and 7 ky cal BP, humans have been the main driver after 5 ky cal BP with slash-and-burn activities for agro-pastoral activities (Vannière et al. 2016 ; Lestienne et al. 2020b ). Beside this, differences in vegetation composition must also be considered. Indeed, the early Holocene vegetation in Corsica was mostly composed by pinewoods with low diversity (Reille 1992 ; Reille et al. 1999 ; Lestienne et al. 2020b ), while the current vegetation is more diversified, with grasslands and low shrubby formations at high elevation, and with oakwood and sclerophyll vegetation in lower elevation (PNRC 1983; Reille et al. 1997 , 1999 ). However, climatic changes will impact significantly the future vegetation dynamics by changing the dominant species and by conducing to a possible desertification (Gao and Giorgi 2008 ; Anav et al. 2011 ) while during the early Holocene, the strong seasonality favoured vegetation growing during wet springs. DGVM models based on these changes estimate that the fire danger (combining climate fire hazard and environment vulnerability) could increase from 2 to 4% and the amount of carbon burned could be 25% higher than today (based on the RCP6.0 scenario) (Anav et al. 2011 ; Dupuy et al. 2020 ). In the early Holocene, the dry and warm climatic conditions have promoted fires (Vannière et al. 2008 , 2011 ), which have in turn opened the landscape and promoted more diversified vegetation with the development of shrublands in spite of pinewoods (Beffa et al. 2016 ; Lestienne et al. 2020b ). Nowadays, fire ignitions are mostly due to humans activities (i.e. accidental or intentional ignitions) (Curt et al. 2016 ; Costafreda-Aumedes et al. 2018 ). Associated to the warmer and dryer climate, these conditions will strongly contribute to increase wildfire frequency and intensity in the Mediterranean region, and likely the occurrence of large fires and megafires (Mouillot et al. 2002 ; Pausas 2004 ; Giannakopoulos et al. 2005 ; Tedim et al. 2013 ; Batllori et al. 2013 ; Lahaye et al. 2018 ; Lestienne et al. 2020b ). In terms of burned area, the increase would reach 15–140% by the end of the 21th century, depending upon the RCP scenario (Amatulli et al. 2013 ; Dupuy et al. 2020 ). 4.3. Advantages and limits of the method Few studies have tried to predict the future fire hazard or fire danger in the Mediterranean Basin. Most of them used a very short historical reference period (Varela et al. 2019 ) and just one period for the future (Faggian 2018 ; Varela et al. 2019 ). The most recent and complete study has been done by Fargeon et al. ( 2020 ) at high spatial resolution (8 km). However, the historical depth is short (i.e. 20 years) as compared to more than 30 years in our study. Moreover, the MDC is an efficient and really simple index to assess the fire hazard (Bürger 2013 ; Lestienne et al. 2020a ). The few inputs needed (only precipitation and maximum temperature) make it adaptable for a wide temporal range (Wagner et al. 1987 ; Girardin and Wotton 2009 ). However, the quality of the MDC signal (and so the quality of the FSL estimation too) strongly depends on the climate models used. Five climate models, among the most used (Watanabe et al. 2010 , 2011 ; Chylek et al. 2011 ; YiMin et al. 2018 ), have been used to computed the fire hazard forecasts, giving confidence to our results (Shiogama et al. 2007 ; Ahlström et al. 2012 ). Such confidence is further reinforced by the strong similarities between our results and those from other studies (Giorgi et al. 1992 ; Pachauri and Reisinger 2007 ; Cuttelod et al. 2009 ; Planton et al. 2012 ). Past climate simulations over several millennia from full climate models are scarce. Fortunately, HadCM3BL-M1, being an intermediate complexity model version from the full Hadley Center Model is one of those that covered the Holocene and beyond (ref Valdes 2017). Its efficiency has previously been compared with full complexity climate models for the Canadian boreal region with satisfaction (Hély et al. 2010 ). These Holocene climate simulations were also used to assess the role of climate and vegetation changes on boreal forest fire size during the Holocene (Hély et al. 2020 ). However, we still face to a strong lack of knowledge and we urgently need to have access to other climate simulations for the Holocene (snapshot periods as for the HadCM3BL-M1 version, or ideally transient (i.e. continuous) simulations). 5. Conclusion In this study, we highlighted the increase in fire hazard intensity (MDC) and duration (FSL) for the next decades (at least until 2100). Even though the MDC will not reach the maximum Holocene values, the FSL will reach (for RCP4.5) or even exceed (for RCP8.5) the maximum Holocene FSL from 2050. These increases are mainly attributed to the global warming and to the increase in summer aridity that goes with in the Mediterranean region. Early Holocene fires have been driven by climate due to high solar insolation, whereas from 6 ky cal BP, fires have been driven by human activities (pastoral activities, slash and burn cultivation…). Currently, and for the first time in the Holocene, we are confronted to an increase in both drought and human influences, and this combination will probably promote frequent and extreme fires. Such increase, associated with the increase in wildland/urban interface due to the urban sprawl could threat the population, houses and structures. Moreover, it could also lead to a biodiversity loss. In France, fire policy established after the devastating wildfires of 1990 has been very effective under normal weather conditions. However, this capacity could be undermined under more extreme weather condition, and the devastating fires of 2003, 2016 or 2019 have already showed that our capacity to control fires is limited. For the next 30 years, the game seems to be played already because both studied RCP scenarii showed the same fire hazard trends. However, our current policies will also influence the climate response at a longer time-scale and we need to reconsider our political choices to avoid catastrophic consequences in term of ecology and/or economy (e.g. extremes wildfires close to the cities, biodiversity losses, etc.). Declarations Funding: This research was funded by Région Bourgogne Franche-Comté through Chrono-environnement laboratory, the MSHE Ledoux and the projects ONOMAD, QMedFire and ENVILEG led by BV. This study was also supported by the CNRS PaléoMEx-MISTRALS program. This study is part of the PAGES-GPWG activities. Competing Interests: The authors declare no conflict of interest. Availability of data and materials: Data can be made available on request. Authors' contributions: Conceptualization: CH and ML; methodology, validation, investigation, resources, writing—review and editing and visualization: ML, BV, TC, IJB and CH; formal analysis and data curation: ML and CH; writing—original draft preparation: ML; supervision, CH and BV; project administration and funding acquisition: BV. Ethical Approval: Not applicable. Consent to participate: Not applicable. Consent to Publish : All authors consent to publish in the journal of “Climatic Change”. References Aguado I, Chuvieco E, Martin P, Salas J (2003) Assessment of forest fire danger conditions in southern Spain from NOAA images and meteorological indices. International Journal of Remote Sensing 24:1653–1668 Ahlström A, Schurgers G, Arneth A, Smith B (2012) Robustness and uncertainty in terrestrial ecosystem carbon response to CMIP5 climate change projections. 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Climatic Change 109:33. https://doi.org/10.1007/s10584-011-0149-y Sá ACL, Benali A, Fernandes PM, et al (2017) Evaluating fire growth simulations using satellite active fire data. Remote Sensing of Environment 190:302–317. https://doi.org/10.1016/j.rse.2016.12.023 Shiogama H, Nozawa T, Emori S (2007) Robustness of climate change signals in near term predictions up to the year 2030: Changes in the frequency of temperature extremes. Geophysical Research Letters 34: Tedim F, Remelgado R, Borges C, et al (2013) Exploring the occurrence of mega-fires in Portugal. Forest Ecology and Management 294:86–96 Thomson AM, Calvin KV, Smith SJ, et al (2011) RCP4.5: a pathway for stabilization of radiative forcing by 2100. Climatic Change 109:77. https://doi.org/10.1007/s10584-011-0151-4 Valdes PJ, Armstrong E, Badger MPS, et al (2017) The BRIDGE HadCM3 family of climate models: HadCM3@Bristol v1.0. Geoscientific Model Development 10:3715–3743. https://doi.org/10.5194/gmd-10-3715-2017 Vannière B, Blarquez O, Rius D, et al (2016) 7000-year human legacy of elevation-dependent European fire regimes. Quaternary Science Reviews 132:206–212. https://doi.org/10.1016/j.quascirev.2015.11.012 Vannière B, Colombaroli D, Chapron E, et al (2008) Climate versus human-driven fire regimes in Mediterranean landscapes: the Holocene record of Lago dell’Accesa (Tuscany, Italy). Quaternary Science Reviews 27:1181–1196 Vannière B, Power MJ, Roberts N, et al (2011) Circum-Mediterranean fire activity and climate changes during the mid-Holocene environmental transition (8500-2500 cal. BP). The Holocene 21:53–73. https://doi.org/10.1177/0959683610384164 Varela V, Vlachogiannis D, Sfetsos A, et al (2019) Projection of forest fire danger due to climate change in the french mediterranean region. Sustainability 11:4284 Vogiatzakis IN, Mannion AM, Sarris D (2016) Mediterranean island biodiversity and climate change: the last 10,000 years and the future. Biodiversity and conservation 25:2597–2627 Wagner CEV, Forest P, Station E, et al (1987) Development and Structure of the Canadian Forest FireWeather Index System. Can For Serv, Forestry Tech Rep Watanabe M, Suzuki T, O’ishi R, et al (2010) Improved Climate Simulation by MIROC5: Mean States, Variability, and Climate Sensitivity. J Climate 23:6312–6335. https://doi.org/10.1175/2010JCLI3679.1 Watanabe S, Hajima T, Sudo K, et al (2011) MIROC-ESM 2010: Model description and basic results of CMIP5-20c3m experiments. Geoscientific Model Development 4:845 Westerling A, Hidalgo H, Cayan D, Swetnam T (2006) Warming and earlier spring increase western US forest wildfire activity. Science 313:940–943. https://doi.org/10.1126/science.1130691 YiMin TZBYY, Bo LWZLL, WenMin WPLZG, et al (2018) The FGOALS climate system model as a modeling tool for supporting climate sciences: An overview. Earth and Planetary Physics 2 Supplementary Files 04LESTIENNEetalSUPMAT.docx Cite Share Download PDF Status: Under Review Version 1 posted Reviewers invited by journal 01 Mar, 2021 Reviews received at journal 01 Mar, 2021 Editor assigned by journal 02 Feb, 2021 First submitted to journal 25 Jan, 2021 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. 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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-164555","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":14573608,"identity":"896e236c-1dde-490b-be11-8701b58598f1","order_by":0,"name":"Marion Lestienne","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4UlEQVRIiWNgGAWjYBACPhCRACLYGx8e4CFGCxsDA2MDWAvPYQMStIBZEsnEamFvf/7gQcWdPH7JxwwH3jDY2RscYH78Aa8WnjOGDQlnnhVLzk5mODiHIZnZ4ACbgQFeLRI5jA2JbYcTN9zOP3CYhwGoHui8BLxa5J8/bEj8dzhx/83DDCAtPCAtB/DbwmDYkNgAtEWCGaxFAqgFGiA4/ZJjOCPh2OHEGWdAfjFINpA8zGaMTwcDP/vxBx9/1BxO7G8/zPjgTYWdPd/xZvwhhgZAQcVMgvpRMApGwSgYBdgBANwtSoE5aIabAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-7229-2300","institution":"CNRS UMR6249: Chrono-environnement","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Marion","middleName":"","lastName":"Lestienne","suffix":""},{"id":14573609,"identity":"aaa8c36b-ec35-4535-a0d8-fb604871821f","order_by":1,"name":"Boris Vannière","email":"","orcid":"","institution":"CNRS UMR6249: Chrono-environnement","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Boris","middleName":"","lastName":"Vannière","suffix":""},{"id":14573610,"identity":"a1978417-ae7d-47d9-b2fc-ecd970f6816d","order_by":2,"name":"Thomas Curt","email":"","orcid":"","institution":"INRAE RECOVER, Université Aix Marseille","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Thomas","middleName":"","lastName":"Curt","suffix":""},{"id":14573611,"identity":"6a20f59e-04f4-4c5d-877f-303ad9b76eac","order_by":3,"name":"Isabelle Jouffroy-Bapicot","email":"","orcid":"","institution":"CNRS UMR6249: Chrono-environnement","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Isabelle","middleName":"","lastName":"Jouffroy-Bapicot","suffix":""},{"id":14573612,"identity":"a408a554-e59e-4abb-9bfa-9db02ea5f752","order_by":4,"name":"Christelle Hély","email":"","orcid":"","institution":"Institut des sciences de l'évolution: Institut des sciences de l'evolution de Montpellier","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Christelle","middleName":"","lastName":"Hély","suffix":""}],"badges":[],"createdAt":"2021-01-28 04:05:59","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-164555/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-164555/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":6730229,"identity":"91da44ae-8b54-4d34-8423-db3a5c6bc916","added_by":"auto","created_at":"2021-03-08 23:55:43","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":212314,"visible":true,"origin":"","legend":"Geographical context of Corsica, a French European island and changes in rainfall around the Mediterranean Basin between 1971-2010 and the comparison period 1902-2010 (NOAA, 2011). Figure adapted from NOAA/Earth System Research Laboratory.","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-164555/v1/679e09a97cc1f53866b30b5e.jpg"},{"id":6729574,"identity":"fde53a60-ded3-4ff1-93af-f3c0ccce775a","added_by":"auto","created_at":"2021-03-08 23:49:43","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":135764,"visible":true,"origin":"","legend":"Synthesis of the climatic datasets extraction methodology.","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-164555/v1/620929c881bb2442b0893f25.jpg"},{"id":6729577,"identity":"c60cee1b-76ab-4d14-866e-b85381102eff","added_by":"auto","created_at":"2021-03-08 23:49:43","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":93327,"visible":true,"origin":"","legend":"MDC (Monthly Drought Code) and FSL (Fire Season Length) changes in Corsica during the Holocene and future decades. All computations were performed on the 5 ERA-interim pixels delimitating Corsica. MDC computation was performed from May to October, and the FSL subsequently estimated based on the 300-unit MDC threshold (see section 2.3). References to fires and human intensity of activities at the figure’s bottom come from a previous study based on microcharcoal, pollen and non-pollen palynomorphs analysis (Arborean/non-arborean pollen ratio for land openness; crop, ruderal pollen and dung fungal spores percentages for agropastoral activities) from the Bastani Lake in Corsica (Lestienne et al., 2020b). Past insolation has been calculated using the “palinsol” R package (Crucifix 2016) which includes data from Berger and Loutre (1991).","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-164555/v1/767eede8e85682d0efcb3d73.jpg"},{"id":6729897,"identity":"9fe65684-8c3f-440f-b323-a3bfb73f6ecf","added_by":"auto","created_at":"2021-03-08 23:52:43","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":106091,"visible":true,"origin":"","legend":"MDC (Monthly Drought Code) distributions for each fire-season month over the next decades computed using the IPCC CMIP5 RCP scenarios. Red lines represent the 1979-2016 MDC distribution (modern reference period) from ERA-interim dataset with the plain line being the median and the dotted lines being the first and third quartiles values. Blue line represents the maximum MDC Holocene value estimated for Corsica over the last 11000 years BP using the HadCM3BL-M1 model. ","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-164555/v1/764f5a02c6964c4fe73d2094.jpg"},{"id":13676058,"identity":"1fa5fee3-1b77-4dc7-b1eb-daca9b40453f","added_by":"auto","created_at":"2021-09-17 11:29:22","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":690672,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-164555/v1/42246dbb-9b50-4b18-a0b7-38f577c7ab3a.pdf"},{"id":6729899,"identity":"095e5890-65ee-4c9d-aca8-bcbe224d9db2","added_by":"auto","created_at":"2021-03-08 23:52:43","extension":"docx","order_by":7,"title":"","display":"","copyAsset":false,"role":"supplement","size":2138702,"visible":true,"origin":"","legend":"","description":"","filename":"04LESTIENNEetalSUPMAT.docx","url":"https://assets-eu.researchsquare.com/files/rs-164555/v1/5f5795ef4722f411b304fba7.docx"}],"financialInterests":"","formattedTitle":"Integrating past variability in climate-driven Mediterranean fire hazard assessments for 2020-2100","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eOver the past decades, a surge in the number of large and uncontrolled wildfires has occurred on all terrestrial ecosystems, and this can be observed irrespectively of national fire-fighting capacities or management tactics (Westerling et al. \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e; Lohman et al. \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e; Forsyth and Van Wilgen \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e; Bowman et al. \u003cspan class=\"CitationRef\"\u003e2009\u003c/span\u003e). The global warming predicted by climate models for the 21st century and beyond may amplify this trend, and threat most ecosystems worldwide (Giannakopoulos et al. \u003cspan class=\"CitationRef\"\u003e2005\u003c/span\u003e; Pachauri and Reisinger \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e; Varela et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eLocated on a latitudinal gradient at the interface between temperate Europe and subarid North-Africa within the Mediterranean basin, the French island of Corsica has a climate characterized by a seasonal alternation between a warm and dry summer season with a marked drought period, and a cool season with high rainfall (Pausas \u003cspan class=\"CitationRef\"\u003e2004\u003c/span\u003e; S\u0026aacute; et al. \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e). This, together with the traditional use of pastoral fire, makes Corsica a fire hotspot (Keeley \u003cspan class=\"CitationRef\"\u003e2009\u003c/span\u003e; Leys et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e). Along the last decades (1973\u0026ndash;2019), 2 369 fires larger than 10 ha have occurred in Corsica and 305 063 ha (~\u0026thinsp;67 779 ha.yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) have been burned (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://www.promethee.com/\u003c/span\u003e\u003c/span\u003e). Corsica harbours a high biodiversity, many endemic species (M\u0026eacute;dail and Verlaque \u003cspan class=\"CitationRef\"\u003e1997\u003c/span\u003e; M\u0026eacute;dail \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e), and preserved Mediterranean forest ecosystems (Medail and Quezel \u003cspan class=\"CitationRef\"\u003e1997\u003c/span\u003e; Vogiatzakis et al. \u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e). However, this renders Corsica vulnerable to climate and disturbance-regime changes as shown its past environmental history and the predictions for the future (Giannakopoulos et al. \u003cspan class=\"CitationRef\"\u003e2005\u003c/span\u003e; Giorgi \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e; Vogiatzakis et al. \u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e; Varela et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e; Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020b\u003c/span\u003e; Fargeon et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). Recent studies have suggested that the fire danger could be three times higher when summer temperature anomaly exceeds\u0026thinsp;+\u0026thinsp;2\u0026deg;C (Fargeon et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e) and that the fire danger could be doubled in Corsica (Varela et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eIn this context, it is of importance to assess the future climate-related fire hazard in Corsica for the current century (2020\u0026ndash;2100). To do this, we use 5 climate models with two RCP scenarios (4.5 and 8.5, respectively) and we compare these results to recent studies (Varela et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e; Fargeon et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). Then, we compare the future trends in fire activity to those for the entire Holocene period based on our previous works (Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020a\u003c/span\u003e). Indeed, values of the past fire hazard provide a valuable historical baseline and give valuable insights about thresholds and shifts in ecosystems responses related to past fire regime changes.\u003c/p\u003e\n\u003cp\u003eSeveral climate-related indexes of fire-danger have been tested for the Mediterranean region among which the Fire Weather Index (FWI) (Wagner et al. \u003cspan class=\"CitationRef\"\u003e1987\u003c/span\u003e) and its sub-indexes, which all have overall showed their efficiency on current climate conditions (Aguado et al. \u003cspan class=\"CitationRef\"\u003e2003\u003c/span\u003e; Moriondo et al. \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e; Carvalho et al. \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e; Varela et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e; Fargeon et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). However, most of these fire-danger indexes require daily weather forecast data to be computed, which can limit their use for past assessment and future prediction. The Monthly Drought Code (MDC), whose computation is derived from the computation of the Drought Code, i.e. a sub-index of the FWI, has also been created first in Canada (Girardin and Wotton \u003cspan class=\"CitationRef\"\u003e2009\u003c/span\u003e) and recently tested over Corsica (Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020a\u003c/span\u003e). Its computation uses monthly means of maximum temperature and monthly precipitation, therefore providing an assessment of climate-related fire hazard without the constraint of daily data requirement.\u003c/p\u003e"},{"header":"2. Materials And Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003cp\u003e\u003cstrong\u003e2.1. \u003cspan class=\"SmallCaps\"\u003eStudy area\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCorsica is a mountainous French island located in the western Mediterranean basin (Fig.\u0026nbsp;1). In spite of high mountains, there are no glaciers and no permanent snow on the island (Conchon \u003cspan class=\"CitationRef\"\u003e1986\u003c/span\u003e). During the early Holocene (i.e. 11,000 years ago), the vegetation was mainly composed by pinewood and heather. During the Neolithic (around 6000 years ago), a significant change occurred in land cover with the expansion of oak forests, which have dominated the island during most of the Holocene thereafter (Reille \u003cspan class=\"CitationRef\"\u003e1992\u003c/span\u003e; Reille et al. \u003cspan class=\"CitationRef\"\u003e1999\u003c/span\u003e; Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020b\u003c/span\u003e). Nowadays, the ecological value of the region, the high population and settlement density on the island and the abandonment of traditional land uses (i.e. the subsequent fuel accumulation) represent important stakes that, combined with climate warming (Fig.\u0026nbsp;1) (NOAA 2011), induce an exponential increase in fire risk (Keeley \u003cspan class=\"CitationRef\"\u003e2009\u003c/span\u003e; Leys et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e) in this fire-prone region (Appendix A).\u003c/p\u003e\n\u003cp\u003e\u003cspan class=\"BoldItalic\"\u003eFigure 1 - Geographical context of Corsica, a French European island and changes in rainfall around the Mediterranean Basin between 1971\u0026ndash;2010 and the comparison period 1902\u0026ndash;2010 (NOAA, 2011). Figure adapted from NOAA/Earth System Research Laboratory.\u003c/span\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003cp\u003e\u003cstrong\u003e2.2. \u003cspan class=\"SmallCaps\"\u003eClimate datasets for climate changes scenarios\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eRefer to Fig.\u0026nbsp;2 for a summary diagram.\u003c/p\u003e\n\u003cp\u003e\u003cspan class=\"BoldItalic\"\u003eFigure 2 - Synthesis of the climatic datasets extraction methodology.\u003c/span\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCurrent climate dataset\u003c/strong\u003e was extracted from ERA-Interim (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.ecmwf.int/\u003c/span\u003e\u003c/span\u003e). It is a daily global atmospheric reanalysis continuously updated in real time and allowing to obtain daily temperature and precipitation data since 1979. The spatial resolution is approximately 60 x 80 km (Corsica being covered by a total of five pixels) (Dee et al. \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePaleoclimate simulated datasets\u003c/strong\u003e were extracted from simulations provided by HadCM3BL-M1, this model being a variant of the fully complex Hadley Centre climate model HadCM3, usually involved in IPCC assessment reports, and belonging to the HadCM3 family of climate models (Valdes et al. \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e). Based on orbital forcing parameters and some other prescribed variables for the ocean compartment, this intermediate-complexity model version performed a snapshot equilibrium simulation of the atmospheric compartment per millennium, from which climate normals (monthly means) for main climate variables (\u003cem\u003ei.e.\u003c/em\u003e air temperature and precipitation used here) are available. We therefore used these datasets of climate normals for the last eleven millennia (centered at changeovers 11, 10, \u0026hellip;, 2, and 1 kilo years calibrated Before Present i.e. before 1950 \u0026ndash; abbreviated thereafter ky cal BP), as well as for the control run representative of the pre-industrial period (AD 1750, considered as equivalent to 0 BP). Then, temperature and precipitation anomalies (\u003cem\u003ei.e\u003c/em\u003e. difference and rate of change for temperature and precipitation, respectively) have been computed and downscaled for each millennium changeover based on Ramstein et al. (\u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e). Using an inverse-distance weighting approach on the four closest HadCM3BL-M1 pixels from those covering Corsica, we downscaled these anomalies by applying them to the current period climate normal that we computed from the ERA-Interim dataset to reconstruct past climate for each Holocene millennium changeover at the whole island scale (Ramstein et al. \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e; H\u0026eacute;ly et al. \u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e). As Holocene climate changes were mainly due to insolation changes, we also retrieved insolation values for the entire Holocene period using the \u0026ldquo;palinsol\u0026rdquo; R package (Crucifix \u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e) which includes data from Berger and Loutre (\u003cspan class=\"CitationRef\"\u003e1991\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFuture climate datasets\u003c/strong\u003e are monthly means projections from simulations of five CMIP5 coupled models (Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e) (Pachauri et al. 2015) under two RCP scenarios (4.5 and 8.5, respectively). In a preliminary step (Appendix B), we first compared them to the overall range of historical variability (1979\u0026ndash;2016) in order to test their representativeness and to take into account the variability induced by models (Fargeon et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eSummary of the climate models used in this study (based on the IPCC Fifth Assessment Report AR5 (Pachauri et al., 2015).\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003cth rowspan=\"2\" align=\"left\"\u003e\n\u003cp\u003eModel\u003c/p\u003e\n\u003c/th\u003e\n\u003cth colspan=\"2\" align=\"left\"\u003e\n\u003cp\u003eResolution\u003c/p\u003e\n\u003c/th\u003e\n\u003cth rowspan=\"2\" align=\"left\"\u003e\n\u003cp\u003eReferences\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eAtmospheric\u003c/strong\u003e\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOceanic\u003c/strong\u003e\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003ePast\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eHadCM3BL-M1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3,75\u0026deg; x 2,5\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3,75\u0026deg; x 2,5\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eValdes et al. (\u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd rowspan=\"5\" align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eFuture\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eFGOALS_S2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1,65\u0026deg; x 2,8\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1\u0026deg; x 1\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eBao et al. (2013)\u003c/em\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCanESM2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e2,8\u0026deg; x 2,8\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1\u0026deg; x 1,4\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eChylek et al. (\u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eMIROC_ESM_CHEM\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e2,8\u0026deg; x 2,8\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1\u0026deg; x 1,4\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eWatanabe et al. (\u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eMIROC5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1,4\u0026deg; x 1,4\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0,5\u0026deg; x 1,5\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eWatanabe et al. (\u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eMRI_CGM3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1,1\u0026deg; x 1,1\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0,5\u0026deg; x 1\u0026deg;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eYukimoto et al. (2012)\u003c/em\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n\u003cp\u003e\u003cstrong\u003e2.3. \u003cspan class=\"SmallCaps\"\u003eClimate fire hazard assessment\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe MDC (Girardin and Wotton, \u003cspan class=\"CitationRef\"\u003e2009\u003c/span\u003e) is unitless and captures the moisture content of deep and compact organic layers. As for the DC, the MDC indicates the effects of seasonal drought on forest fuels and the probability of smouldering in deep duff layers and in large logs, but they differ in their computation requirements. Although these indices were initiated in Canada, we showed in a previous study that the DC was efficient to discriminate fire days and no-fire days in Corsica for the period 1979\u0026ndash;2016 and that the MDC was an efficient tool to reconstruct the Holocene climate-related fire hazard (Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020a\u003c/span\u003e). The great advantage of the DC and the MDC is that they only require precipitation and temperature data (Wagner et al. \u003cspan class=\"CitationRef\"\u003e1987\u003c/span\u003e). Moreover, while the DC needs daily data, the MDC has been created to be directly computed from monthly means of precipitation and maximum temperatures, which makes it very powerful for simulated past and future datasets (Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020a\u003c/span\u003e). The MDC has been computed at each millennium changeover for the entire Holocene (from 11 to 0 K years cal. BP) in order to compare with the coeval charcoal-inferred fire history reconstructed from lacustrine sediments by Lestienne et al. (\u003cspan class=\"CitationRef\"\u003e2020a\u003c/span\u003e). The 280-unit threshold in the DC appears meaningful to characterize extreme drought and a high probability of fire hazard in boreal forests (Girardin and Wotton \u003cspan class=\"CitationRef\"\u003e2009\u003c/span\u003e). In Corsica, we previously showed that this DC threshold is about 300 units (Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020a\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.4. \u003cspan class=\"SmallCaps\"\u003eFire-Season Length assessment\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n\u003cp\u003eBased on the Holocene analysis of MDC values, and according to the method originally developed by H\u0026eacute;ly et al. (\u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e) for the DC and adapted to the MDC (Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020a\u003c/span\u003e), the threshold value of 300 units was used to calculate the Fire-Season Length (FSL) for the past 11 millennia and for the future decades until 2100. Basically, for all months with MDC values above the 300-unit threshold, we considered the full month length (30 or 31 days) as part of the FSL. To define the starting (and ending) month of the FSL and to add the related number of starting (ending) days (\u003cem\u003ei.e.\u003c/em\u003e \u0026lt;30 or 31 for each month), we used a basic linear interpolation between months, reporting a MDC value lower than the 300-unit MDC, and the following (preceding) month with above-threshold MDC value (Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020a\u003c/span\u003e). On the expected starting (ending) month, we associated the lower-than-threshold MDC value to the first (last) day and the above-threshold value to the last (first) day of this month to perform the computation.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n\u003cp\u003e\u003cstrong\u003e3.1. \u003cspan class=\"SmallCaps\"\u003eestimation of future fire hazard in a multi-millennial context\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn terms of fire hazard, the results showed that, by the end of the 21st century, the MDC will increase by 7.5% and up to 21% in Corsica, respectively for the RCP4.5 and RCP8.5 scenarii as compared to the 1979\u0026ndash;2016 period reference (Fig.\u0026nbsp;3, Appendix C). Such future fire hazard values will be similar to those estimated for 7 ky cal BP, but they will not exceed the maximum Holocene MDC values. The duration of periods with high fire hazard (FSL), will increase higher (up to 25% i.e. 35 days for RCP4.5 and 32% i.e. 51 days for RCP8.5 by the end of the 21st century), and will exceed the Holocene maximum FSL value after 2040 from both RCP scenarii. Indeed, after 2040, FSL will stabilize around 130 days per a year (i.e. more than 4 months) for the RCP4.5 scenario, while it will exceed 150 days (i.e. more than 5 months) with the RCP8.5 scenario.\u003c/p\u003e\n\u003cp\u003e\u003cspan class=\"BoldItalic\"\u003eFigure 3 - MDC (Monthly Drought Code) and FSL (Fire Season Length) changes in Corsica during the Holocene and future decades. All computations were performed on the 5 ERA-interim pixels delimitating Corsica. MDC computation was performed from May to October, and the FSL subsequently estimated based on the 300-unit MDC threshold (see Sect.\u0026nbsp;2.3). References to fires and human intensity of activities at the figure\u0026rsquo;s bottom come from a previous study based on microcharcoal, pollen and non-pollen palynomorphs analysis (Arborean/non-arborean pollen ratio for land openness; crop, ruderal pollen and dung fungal spores percentages for agropastoral activities) from the Bastani Lake in Corsica (\u003c/span\u003eLestienne et al., \u003cspan class=\"CitationRef\"\u003e2020b\u003c/span\u003e). \u003cspan class=\"BoldItalic\"\u003ePast insolation has been calculated using the \u0026ldquo;palinsol\u0026rdquo; R package (\u003c/span\u003eCrucifix \u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e\u003cspan class=\"BoldItalic\"\u003e) which includes data from\u003c/span\u003e Berger and Loutre (\u003cspan class=\"CitationRef\"\u003e1991\u003c/span\u003e\u003cspan class=\"BoldItalic\"\u003e).\u003c/span\u003e\u003c/p\u003e\n\u003cp\u003eFor comparison, the Holocene estimation of MDC and FSL values highlighted five periods (Fig.\u0026nbsp;3). Between 11 to 9 ky cal BP, the fire hazard index was relatively high, with MDC and FSL values above 600 units and 100 days, respectively. Afterwards, i.e. between 9 and 7 ky cal BP, the FSL values remained high with a peak of 126 days at 9 ky cal BP, while MDC values progressively decreased below 500 units. After 7 ky cal BP, FSL felt below 80 days, while MDC continued to decrease until 5 ky cal BP. After 5 ky cal BP, low fire hazard indexes lasted 4 millennia, characterized by minima values: below 400 units for MDC and between 54 et 77 days for FSL. Finally, between 1 and 0 ky cal BP the fire hazard switched back toward increasing trend due to both a slight increase in the MDC values, passing above 400 units, and a sharp increase in FSL values above 100 days, attesting change in fire-related climate conditions.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n\u003cp\u003e\u003cstrong\u003e3.2. \u003cspan class=\"SmallCaps\"\u003eIntra-season variability\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe fact that the 21st century FSL curve follows also an increasing but amplified trend compared to the MDC curve (Fig.\u0026nbsp;3) is related to the different trends of monthly drought intensity, especially after 2050. Indeed, beyond the overall increase in monthly values of fire hazard intensity (MDC) for both RCP scenarii, fire-season months can be splitted in three groups (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e). The early fire-season (i.e. May and June) will have a higher fire hazard intensity than the Holocene maximum for both RCP scenarii, while it will be within (respectively higher than) the range of the reference period variability for the RCP4.5 (respectively RCP8.5 after 2050). Therefore, such drought intensity will induce at least a similar or an earlier onset of the fire-season (Appendix C). For the mid fire-season (i.e. July and August), only the RCP8.5 scenario will induce higher fire hazard intensity than the Holocene maximum after 2050, while both RCP scenarii will induce higher fire hazard intensity after 2050 than during the reference period. Such increase in mid fire-season hazard intensity would indirectly lead to an extension of the FSL if humidity of late summer and fall months would not compensate enough for the drought. This is the trend shown in the climate forecast (Appendix B), and that explains at least partially the extended fire-season end (Appendix C). Indeed, the late fire-season (i.e. September and October) is also extended, up to an extra month (i.e. November) towards the end of the 21st century in both RCP scenarii (Appendix C) due to on going temperature increase and still no extra rainfall (Appendix B).\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e \u003cspan class=\"BoldItalic\"\u003e- MDC (Monthly Drought Code) distributions for each fire-season month over the next decades computed using the IPCC CMIP5 RCP scenarios. Red lines represent the 1979\u0026ndash;2016 MDC distribution (modern reference period) from ERA-interim dataset with the plain line being the median and the dotted lines being the first and third quartiles values. Blue line represents the maximum MDC Holocene value estimated for Corsica over the last 11000 years BP using the HadCM3BL-M1 model.\u003c/span\u003e\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n\u003cp\u003e\u003cstrong\u003e4.1. \u003cspan class=\"SmallCaps\"\u003efire hazard changes for the next decades\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe showed that the MDC and the FSL will increase until \u003cem\u003eca.\u003c/em\u003e 2050 for both RCP scenarii. Then, these increases will slow down or stabilize with the RCP4.5 scenario, while they will continue their ascents with the RCP8.5 scenario.\u003c/p\u003e\n\u003cp\u003eSuch overall trends agree with those found from previous studies that used the FWI or one of its sub-indexes to analyse future changes in fire hazard in the Mediterranean basin. For instance, Moriondo et al. (\u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e), dealing with future trend in FSL in the Mediterranean basin but based on previous generation of IPCC scenarii showed that FSL will increase by \u003cem\u003eca.\u003c/em\u003e 40 and \u003cem\u003eca.\u003c/em\u003e 32 days for the scenario SRES A2 (close to RCP8.5) and the scenario SRES B2 (no RCP equivalent), respectively. These FSL values are in line with ours (from +\u0026thinsp;48 days or 50% to +\u0026thinsp;24 days or 25% for RCP8.5 and RCP4.5, respectively) despite different IPCC scenario generations, different current reference periods and extents (1961\u0026ndash;1990 versus 1979\u0026ndash;2016 for this study), different temporal projections (only one period for the future (2071\u0026ndash;2100) for their study versus all decades until 2100 in this study), and differences in the mode of computation of FSL. Moreover, Moriondo et al. (\u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e) worked on six Mediterranean countries (i.e. Portugal, Spain, Greece, Italy, France, and Balkan), while ours focuses on Corsica, geographically located at the centre of the Mediterranean region covered in Moriondo et al. (\u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e). Our approach uses the most updated IPCC scenarii and the best temporal resolution allows better highlighting of changes in the next decades.\u003c/p\u003e\n\u003cp\u003eThe review of Giorgi and Lionello (\u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e), based on an ensemble of global and regional climate change simulations, showed that the next decades (2071\u0026ndash;2100) will be characterized by a decrease in precipitation and an increase in temperature, and that these changes will mostly occur in summer. This is in line with our results showing a drought increase over the next decades, especially in summer (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, Appendix B). The causes for this large summer drying signal have been previously investigated by Rowell and Jones (2006), who examined four possible and non-exclusive mechanisms: i) the low spring soil moisture conditions leading to reduced summer convection; ii) the large land-sea contrast in warming condition leading to reduced relative humidity and precipitation over the continent; iii) the positive summer soil moisture precipitation feedback; iv) the remote influences (e.g. descending motions induced by the strengthening of the Asian monsoon). According to these authors, these changes will be similar among the RCP scenarii until 2050 and will therefore depend on the political and economic choices after 2050 (Pachauri and Reisinger \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e; Cuttelod et al. \u003cspan class=\"CitationRef\"\u003e2009\u003c/span\u003e; Planton et al. \u003cspan class=\"CitationRef\"\u003e2012\u003c/span\u003e). Such changes have been reported in most projections from both global and regional models and most IPCC scenarii (Kittel et al. \u003cspan class=\"CitationRef\"\u003e1997\u003c/span\u003e; Giorgi and Francisco \u003cspan class=\"CitationRef\"\u003e2000\u003c/span\u003e; Giorgi et al. \u003cspan class=\"CitationRef\"\u003e2001\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eMore recent studies (Faggian \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e; Varela et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e; Fargeon et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e), using the same two RCP scenarii as ours, but based on the FWI (alone or in combination with ISI and DC indexes) showed that the fire danger will increase from 24 to 67% depending on the RCP scenario. It is worth noting that ISI and FWI require more inputs (i.e. wind speed and relative humidity) and therefore likely more uncertainties, and that authors did not systematically discriminated changes in fire danger intensity from changes in the fire-season length.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n\u003cp\u003e\u003cstrong\u003e4.2. \u003cspan class=\"SmallCaps\"\u003eHow much future fire hazard will depart from past?\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBy comparing our Anthropocene future predictions with the Holocene values, we can see that the FSL will exceed the maximum values at Holocene from 2040, while MDC will reach in 2100 values never observed since 6 k years cal. BP. From these results, we can expect an increase in fire occurrences in the next decades, but the climatic conditions and the vegetation will differ in the future fire-prone periods as compared to the first half of the Holocene period and could also affect the fire occurrence. Indeed, the current insolation is lower than in the early Holocene (11\u0026thinsp;\u0026minus;\u0026thinsp;7 k years cal. BP), which corresponded to the maximum Holocene insolation (Berger and Loutre \u003cspan class=\"CitationRef\"\u003e1991\u003c/span\u003e) as shown in Fig.\u0026nbsp;3, especially for the months of June, July and August at 45\u0026deg;N latitude (H\u0026eacute;ly et al. \u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e; Renssen et al. \u003cspan class=\"CitationRef\"\u003e2012\u003c/span\u003e). This high insolation in early Holocene induced a strong seasonality, with the highest temperature and lowest precipitation in summer, and inversely for winter as reconstructed by Dormoy et al. (\u003cspan class=\"CitationRef\"\u003e2009\u003c/span\u003e) from Mediterranean marine pollen records. Here we showed that the FSL will increase in the future, due to both the increase in temperature as a response of increasing concentrations in Greenhouse Gas and the decrease in precipitation as a response of atmospheric dynamics over the Mediterranean region that will occur for every month (Appendix C), with most aridity in summer months. This difference in seasonality origin between early Holocene with high insolation and the 21st century with low insolation but high concentrations of Greenhouse Gas could explain the differences between the early-Holocene and the future fire hazard predictions in terms of intensity (MDC) and duration (FSL). Indeed, the MDC calculation needs monthly maximal temperatures and monthly precipitations data (Girardin and Wotton \u003cspan class=\"CitationRef\"\u003e2009\u003c/span\u003e). Because the early Holocene summer conditions was warmer and dryer, the MDC was high, but the strong seasonality with cooler and wetter conditions in both spring and autumn induced relatively short FSL. In the future, the climatic conditions in summer will not reach the maximal Holocene values, but the global aridification occurring for every month will extend the FSL much longer. Contrary to the early Holocene when insolation was the main climate driver, the global increase in aridity will be mainly due to the strong increase in greenhouse gas emissions and responsible for global change (Pachauri and Reisinger \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e; Thomson et al. \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e; Meinshausen et al. \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e; Riahi et al. \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eIn addition, we have to consider that fires have not always been the sole result of arid periods. Indeed, while dry and warm climate promoted fires between 11 and 7 ky cal BP, humans have been the main driver after 5 ky cal BP with slash-and-burn activities for agro-pastoral activities (Vanni\u0026egrave;re et al. \u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e; Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020b\u003c/span\u003e). Beside this, differences in vegetation composition must also be considered. Indeed, the early Holocene vegetation in Corsica was mostly composed by pinewoods with low diversity (Reille \u003cspan class=\"CitationRef\"\u003e1992\u003c/span\u003e; Reille et al. \u003cspan class=\"CitationRef\"\u003e1999\u003c/span\u003e; Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020b\u003c/span\u003e), while the current vegetation is more diversified, with grasslands and low shrubby formations at high elevation, and with oakwood and sclerophyll vegetation in lower elevation (PNRC 1983; Reille et al. \u003cspan class=\"CitationRef\"\u003e1997\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e1999\u003c/span\u003e). However, climatic changes will impact significantly the future vegetation dynamics by changing the dominant species and by conducing to a possible desertification (Gao and Giorgi \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e; Anav et al. \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e) while during the early Holocene, the strong seasonality favoured vegetation growing during wet springs. DGVM models based on these changes estimate that the fire danger (combining climate fire hazard and environment vulnerability) could increase from 2 to 4% and the amount of carbon burned could be 25% higher than today (based on the RCP6.0 scenario) (Anav et al. \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e; Dupuy et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eIn the early Holocene, the dry and warm climatic conditions have promoted fires (Vanni\u0026egrave;re et al. \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e), which have in turn opened the landscape and promoted more diversified vegetation with the development of shrublands in spite of pinewoods (Beffa et al. \u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e; Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020b\u003c/span\u003e). Nowadays, fire ignitions are mostly due to humans activities (i.e. accidental or intentional ignitions) (Curt et al. \u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e; Costafreda-Aumedes et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e). Associated to the warmer and dryer climate, these conditions will strongly contribute to increase wildfire frequency and intensity in the Mediterranean region, and likely the occurrence of large fires and megafires (Mouillot et al. \u003cspan class=\"CitationRef\"\u003e2002\u003c/span\u003e; Pausas \u003cspan class=\"CitationRef\"\u003e2004\u003c/span\u003e; Giannakopoulos et al. \u003cspan class=\"CitationRef\"\u003e2005\u003c/span\u003e; Tedim et al. \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e; Batllori et al. \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e; Lahaye et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e; Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020b\u003c/span\u003e). In terms of burned area, the increase would reach 15\u0026ndash;140% by the end of the 21th century, depending upon the RCP scenario (Amatulli et al. \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e; Dupuy et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n\u003cp\u003e\u003cstrong\u003e4.3. \u003cspan class=\"SmallCaps\"\u003eAdvantages and limits of the method\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFew studies have tried to predict the future fire hazard or fire danger in the Mediterranean Basin. Most of them used a very short historical reference period (Varela et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e) and just one period for the future (Faggian \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e; Varela et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e). The most recent and complete study has been done by Fargeon et al. (\u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e) at high spatial resolution (8 km). However, the historical depth is short (i.e. 20 years) as compared to more than 30 years in our study.\u003c/p\u003e\n\u003cp\u003eMoreover, the MDC is an efficient and really simple index to assess the fire hazard (B\u0026uuml;rger \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e; Lestienne et al. \u003cspan class=\"CitationRef\"\u003e2020a\u003c/span\u003e). The few inputs needed (only precipitation and maximum temperature) make it adaptable for a wide temporal range (Wagner et al. \u003cspan class=\"CitationRef\"\u003e1987\u003c/span\u003e; Girardin and Wotton \u003cspan class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eHowever, the quality of the MDC signal (and so the quality of the FSL estimation too) strongly depends on the climate models used. Five climate models, among the most used (Watanabe et al. \u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e; Chylek et al. \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e; YiMin et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e), have been used to computed the fire hazard forecasts, giving confidence to our results (Shiogama et al. \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e; Ahlstr\u0026ouml;m et al. \u003cspan class=\"CitationRef\"\u003e2012\u003c/span\u003e). Such confidence is further reinforced by the strong similarities between our results and those from other studies (Giorgi et al. \u003cspan class=\"CitationRef\"\u003e1992\u003c/span\u003e; Pachauri and Reisinger \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e; Cuttelod et al. \u003cspan class=\"CitationRef\"\u003e2009\u003c/span\u003e; Planton et al. \u003cspan class=\"CitationRef\"\u003e2012\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003ePast climate simulations over several millennia from full climate models are scarce. Fortunately, HadCM3BL-M1, being an intermediate complexity model version from the full Hadley Center Model is one of those that covered the Holocene and beyond (ref Valdes 2017). Its efficiency has previously been compared with full complexity climate models for the Canadian boreal region with satisfaction (H\u0026eacute;ly et al. \u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e). These Holocene climate simulations were also used to assess the role of climate and vegetation changes on boreal forest fire size during the Holocene (H\u0026eacute;ly et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). However, we still face to a strong lack of knowledge and we urgently need to have access to other climate simulations for the Holocene (snapshot periods as for the HadCM3BL-M1 version, or ideally transient (i.e. continuous) simulations).\u003c/p\u003e\n\u003c/div\u003e"},{"header":"5. Conclusion","content":" \u003cp\u003eIn this study, we highlighted the increase in fire hazard intensity (MDC) and duration (FSL) for the next decades (at least until 2100). Even though the MDC will not reach the maximum Holocene values, the FSL will reach (for RCP4.5) or even exceed (for RCP8.5) the maximum Holocene FSL from 2050. These increases are mainly attributed to the global warming and to the increase in summer aridity that goes with in the Mediterranean region.\u003c/p\u003e \u003cp\u003eEarly Holocene fires have been driven by climate due to high solar insolation, whereas from 6 ky cal BP, fires have been driven by human activities (pastoral activities, slash and burn cultivation\u0026hellip;). Currently, and for the first time in the Holocene, we are confronted to an increase in both drought and human influences, and this combination will probably promote frequent and extreme fires. Such increase, associated with the increase in wildland/urban interface due to the urban sprawl could threat the population, houses and structures. Moreover, it could also lead to a biodiversity loss. In France, fire policy established after the devastating wildfires of 1990 has been very effective under normal weather conditions. However, this capacity could be undermined under more extreme weather condition, and the devastating fires of 2003, 2016 or 2019 have already showed that our capacity to control fires is limited.\u003c/p\u003e \u003cp\u003eFor the next 30 years, the game seems to be played already because both studied RCP scenarii showed the same fire hazard trends. However, our current policies will also influence the climate response at a longer time-scale and we need to reconsider our political choices to avoid catastrophic consequences in term of ecology and/or economy (e.g. extremes wildfires close to the cities, biodiversity losses, etc.).\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e This research was funded by R\u0026eacute;gion Bourgogne Franche-Comt\u0026eacute; through Chrono-environnement laboratory, the MSHE Ledoux and the projects ONOMAD, QMedFire and ENVILEG led by BV. This study was also supported by the CNRS Pal\u0026eacute;oMEx-MISTRALS program. This study is part of the PAGES-GPWG activities.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests: \u003c/strong\u003eThe authors declare no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials:\u0026nbsp; \u003c/strong\u003eData can be made available on request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors' contributions: \u003c/strong\u003eConceptualization: CH and ML; methodology, validation, investigation, resources, writing\u0026mdash;review and editing and visualization: ML, BV, TC, IJB \u0026nbsp;and CH; formal analysis and data curation: ML and CH; writing\u0026mdash;original draft preparation: ML; supervision, CH and BV; project administration and funding acquisition: BV.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval:\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to participate:\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Publish\u003c/strong\u003e: All authors consent to publish in the journal of \u0026ldquo;Climatic Change\u0026rdquo;.\u003c/p\u003e"},{"header":"References","content":"\u003cp\u003eAguado I, Chuvieco E, Martin P, Salas J (2003) Assessment of forest fire danger conditions in southern Spain from NOAA images and meteorological indices. 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Earth and Planetary Physics 2\u003c/p\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":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"climatic-change","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"clim","sideBox":"Learn more about [Climatic Change](https://www.springer.com/journal/10584)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/clim/default.aspx","title":"Climatic Change","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"paleofires, fire season, Monthly Drought Code (MDC), modelling, Holocene, Anthropocene","lastPublishedDoi":"10.21203/rs.3.rs-164555/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-164555/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn the Mediterranean basin, Corsica (French island) harbours among the best-preserved Mediterranean forest ecosystems and its high biodiversity could be threatened by the climate and disturbance-regime changes due to the global warming. This study aims i) to estimate the future climate-related fire hazard in Corsica for the current century (2020\u0026ndash;2100) based on two RCP scenarios (RCP4.5 and RCP8.5), and ii) to compare the predicted trends with the entire Holocene period for which fire hazard has previously been assessed. An ensemble of future climate simulations from two IPCC RCP scenarios has been used to compute the Monthly Drought Code (MDC) and the Fire Season Length (FSL) and to assess the level of fire hazard assessment. Here, we show that the MDC and the FSL would both strongly increase over the next decades due to the combined effect of temperature increase and precipitation decrease in the Corsica region. Moreover, the maximum Holocene FLS (7000 to 9000 years ago), will be reached (and even exceeded depending upon the scenario) after 2040. For the first time in the Holocene, we may be confronted to an increase in the number of fire-prone months driven by climate combined with many human-caused ignitions. This combination should increase the burned area from 15\u0026ndash;140%. For the next 30 years, the game seems to be already played as both RCP scenarios resulted in similar increase in fire hazard intensity and duration. It is thus mandatory to reconsider fire-management and fire-prevention policy to mitigate the future fire risk, and its catastrophic consequences for ecosystems, population, and economy.\u003c/p\u003e","manuscriptTitle":"Integrating past variability in climate-driven Mediterranean fire hazard assessments for 2020-2100","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2021-03-08 23:49:41","doi":"10.21203/rs.3.rs-164555/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewersInvited","content":"","date":"2021-03-02T00:00:00+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2021-03-02T00:00:00+00:00","index":0,"fulltext":""},{"type":"editorAssigned","content":"","date":"2021-02-03T00:00:00+00:00","index":"","fulltext":""},{"type":"submitted","content":"Climatic Change","date":"2021-01-25T09:21:35+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"climatic-change","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"clim","sideBox":"Learn more about [Climatic Change](https://www.springer.com/journal/10584)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/clim/default.aspx","title":"Climatic Change","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"fe1a9a35-9a5d-400a-8159-c9b3e7163c5a","owner":[],"postedDate":"March 8th, 2021","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":2835213,"name":"Climatology"},{"id":2835214,"name":"Climate Analysis and Modeling"}],"tags":[],"updatedAt":"2021-10-28T02:18:07+00:00","versionOfRecord":[],"versionCreatedAt":"2021-03-08 23:49:41","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-164555","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-164555","identity":"rs-164555","version":["v1"]},"buildId":"-HB7Z8yhvgn0wM9Nzuekk","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}