Historical and future global burned area with changing climate and human demography

Abstract

•We evaluate historical and future burned area (BA) trends and their drivers•Recent BA has decreased in central South America and mesic African savannas•High-latitude warming, (sub)tropical drying, and human ignitions will increase future BA•Fire suppression near human settlements can offset large potential BA increases Wildfire is an important natural disturbance for many ecosystems, helping to shape biome distributions and controlling the carbon balance. Major changes in fire activity could also have a strong impact on human societies. Changes in fire activity are influenced both by climatic changes and by changes in human demography via, e.g., population growth and urbanization. We show that in recent decades, global burned area has actually decreased, especially in central South America and mesic African savannas. However, our future simulations indicate that future climate and demographic change will reverse this trend and that burned area is likely to increase due to accelerated high-latitude warming and tropical and subtropical drying and human ignitions. These projections will inform more detailed, local work to develop wildfire management strategies and to assess ecological responses to global change, and will contribute to the discussion of what constitutes a safe upper limit to global warming. Wildfires influence terrestrial carbon cycling and represent a safety risk, and yet a process-based understanding of their frequency and spatial distributions remains elusive. We combine satellite-based observations with an enhanced dynamic global vegetation model to make regionally resolved global assessments of burned area (BA) responses to changing climate, derived from 34 Earth system models and human demographics for 1860–2100. Limited by climate and socioeconomics, recent BA has decreased, especially in central South America and mesic African savannas. However, future simulations predict increasing BA due to changing climate, rapid population density growth, and urbanization. BA increases are especially notable at high latitudes, due to accelerated warming, and over the tropics and subtropics, due to drying and human ignitions. Conversely, rapid urbanization also limits BA via enhanced fire suppression in the immediate vicinity of settlements, offsetting the potential for dramatic future increases, depending on warming extent. Our analysis provides further insight into regional and global BA trends, highlighting the importance of including human demographic change in models for wildfire under changing climate. Wildfires influence terrestrial carbon cycling and represent a safety risk, and yet a process-based understanding of their frequency and spatial distributions remains elusive. We combine satellite-based observations with an enhanced dynamic global vegetation model to make regionally resolved global assessments of burned area (BA) responses to changing climate, derived from 34 Earth system models and human demographics for 1860–2100. Limited by climate and socioeconomics, recent BA has decreased, especially in central South America and mesic African savannas. However, future simulations predict increasing BA due to changing climate, rapid population density growth, and urbanization. BA increases are especially notable at high latitudes, due to accelerated warming, and over the tropics and subtropics, due to drying and human ignitions. Conversely, rapid urbanization also limits BA via enhanced fire suppression in the immediate vicinity of settlements, offsetting the potential for dramatic future increases, depending on warming extent. Our analysis provides further insight into regional and global BA trends, highlighting the importance of including human demographic change in models for wildfire under changing climate. Wildfire is a natural and inevitable feature of the environment in many terrestrial ecosystems and has a strong influence on biogeography, ecosystem functioning, and land-atmosphere carbon and energy fluxes.1Bowman D.M. Balch J.K. Artaxo P. Bond W.J. Carlson J.M. Cochrane M.A. D'Antonio C.M. Defries R.S. Doyle J.C. Harrison S.P. et al.Fire in the earth system.Science. 2009; 324: 481-484Crossref PubMed Scopus (1817) Google Scholar,2Veraverbeke S. Rogers B.M. Goulden M.L. Jandt R.R. Miller C.E. Wiggins E.B. Randerson J.T. Lightning as a major driver of recent large fire years in North American boreal forests.Nat. Clim. Chang. 2017; 7: 529-534Crossref Scopus (171) Google Scholar However, fire also potentially puts humans at risk from atmospheric pollutants3van der Werf G.R. Randerson J.T. Giglio L. van Leeuwen T.T. Chen Y. Rogers B.M. Mu M. van Marle M.J.E. Morton D.C. Collatz G.J. et al.Global fire emissions estimates during 1997–2016.Earth Syst. Sci. Data. 2017; 9: 697-720Crossref Scopus (747) Google Scholar and health and infrastructure hazards.1Bowman D.M. Balch J.K. Artaxo P. Bond W.J. Carlson J.M. Cochrane M.A. D'Antonio C.M. Defries R.S. Doyle J.C. Harrison S.P. et al.Fire in the earth system.Science. 2009; 324: 481-484Crossref PubMed Scopus (1817) Google Scholar,4Moritz M.A. Batllori E. Bradstock R.A. Gill A.M. Handmer J. Hessburg P.F. Leonard J. McCaffrey S. Odion D.C. Schoennagel T. et al.Learning to coexist with wildfire.Nature. 2014; 515: 58-66Crossref PubMed Scopus (531) Google Scholar Overall, global burned area (BA) has declined significantly (by 24.3% ± 8.8%) over the past 18 years5Andela N. Morton D.C. Giglio L. Chen Y. van der Werf G.R. Kasibhatla P.S. DeFries R.S. Collatz G.J. Hantson S. Kloster S. et al.A human-driven decline in global burned area.Science. 2017; 356: 1356-1362Crossref PubMed Scopus (431) Google Scholar and represents the net of differential regional responses. Indeed, despite overall decreases in fire activity, the incidence of major and catastrophic fire events has increased in many regions, with widespread media attention to fires in the Amazon, western North America, the Mediterranean, and Australia. Unfortunately, future fire trends remain uncertain, both at the regional scale and in terms of their spatial distribution;6Moritz M.A. Parisien M.A. Batllori E. Krawchuk M.A. Van Dorn J. Ganz D.J. Hayhoe K. Climate change and disruptions to global fire activity.Ecosphere. 2012; 3: 1-22Crossref Google Scholar indices of climatic fire risk and fire activity are confidently predicted to exacerbate in a warmer and often drier world,7Kloster S. Lasslop G. Historical and future fire occurrence (1850 to 2100) simulated in CMIP5 Earth System Models.Glob. 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Observed impacts of anthropogenic climate change on wildfire in California.Earth's Future. 2019; 7: 892-910Crossref Scopus (246) Google Scholar Seasonality of temperature and precipitation, related to latitude and to major atmospheric circulation features such as monsoons and orographic features, also plays a key role in wildfire dynamics via effects on fuel amount and seasonal fuel moisture, as in the example of seasonal high temperature and low precipitation in Australia.15Thomas P.B. Watson P.J. Bradstock R.A. Penman T.D. Price O.F. Modelling surface fine fuel dynamics across climate gradients in eucalypt forests of south-eastern Australia.Ecography. 2014; 37: 827-837Crossref Scopus (39) Google Scholar Wind speed (W; m s−1) plays a key role in fire spread, but on a global scale, its influence on BA is limited16Lasslop G. Hantson S. Kloster S. Influence of wind speed on the global variability of burned fraction: a global fire model’s perspective.Int. J. Wildland Fire. 2015; 24: 989-1000Crossref Scopus (16) Google Scholar (or at least wind-speed data are of insufficient quality to evaluate its effects at the global scale). Although warmer climate and drying fuel are projected to increase future BA across many regions,8Pechony O. Shindell D.T. Driving forces of global wildfires over the past millennium and the forthcoming century.Proc. Natl. Acad. Sci. U S A. 2010; 107: 19167-19170Crossref PubMed Scopus (459) Google Scholar and notably in some boreal areas,17Flannigan M.D. Logan K.A. Amiro B.D. Skinner W.R. Stocks B.J. Future area burned in Canada.Clim. Change. 2005; 72: 1-16Crossref Scopus (629) Google Scholar empirical analysis suggests that climatic conditions that should lead to frequent fires do not always do so10Knorr W. Arneth A. Jiang L. Demographic controls of future global fire risk.Nat. Clim. Chang. 2016; 6: 781-785Crossref Scopus (73) Google Scholar,18Marlon J.R. Bartlein P.J. Carcaillet C. Gavin D.G. Harrison S.P. Higuera P.E. Joos F. Power M.J. Prentice I.C. Climate and human influences on global biomass burning over the past two millennia.Nat. Geosci. 2008; 1: 697-702Crossref Scopus (561) Google Scholar, suggesting a role for other, non-climatic drivers as well. Beyond climatic conditions, humans have shaped fire regimes for thousands of years.1Bowman D.M. Balch J.K. Artaxo P. Bond W.J. Carlson J.M. Cochrane M.A. D'Antonio C.M. Defries R.S. Doyle J.C. Harrison S.P. et al.Fire in the earth system.Science. 2009; 324: 481-484Crossref PubMed Scopus (1817) Google Scholar,5Andela N. Morton D.C. Giglio L. Chen Y. van der Werf G.R. Kasibhatla P.S. DeFries R.S. Collatz G.J. Hantson S. Kloster S. et al.A human-driven decline in global burned area.Science. 2017; 356: 1356-1362Crossref PubMed Scopus (431) Google Scholar The most obvious direct anthropogenic impact is by ignition, since humans currently light most fires in tropical forests, savannas, and agricultural regions.12Archibald S. 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These are (1) population density (POP; persons km−2), and thus number of anthropogenic ignitions; (2) socioeconomic development, e.g., urbanization, described as the ratio of rural to total population (RUR) (higher rural population is a major source of pyrogenic activity with longer contact with flammable vegetation); and (3) combined fire suppression and management, a proxy being the distance to human settlements (cities) (DIS; km), which is also strongly dependent on urbanization.20Venevsky S. Thonicke K. Sitch S. Cramer W. Simulating fire regimes in human-dominated ecosystems: Iberian Peninsula case study.Glob. Chang. Biol. 2002; 8: 984-998Crossref Scopus (129) Google Scholar Details of the three variables POP, RUR, and DIS are given in experimental procedures, while how these human activities influence BA is described in Note S1. Continued global population growth could thus potentially increase anthropogenic ignitions21Bistinas I. Harrison S.P. Prentice I.C. Pereira J.M.C. Causal relationships versus emergent patterns in the global controls of fire frequency.Biogeosciences. 2014; 11: 5087-5101Crossref Scopus (84) Google Scholar or alternatively decrease ignitions and suppress fires if people concentrate in cities, converting wildlands to urban areas and decreasing rural anthropogenic pyrogenic activity.22Keeley J.E. Syphard A.D. Historical patterns of wildfire ignition sources in California ecosystems.Int. J. Wildland Fire. 2018; 27: 781-799Crossref Scopus (44) Google Scholar An important manifestation of urbanization in the coupling of wildfire and human activities is a rapid growth of the wildland-urban interface (WUI), leading to a shorter DIS. For instance, the land area of the WUI increased in the United States by 33% between 1990 and 2010, making it the fastest growing land cover type and resulting in a significant increase in wildfire risk.23Radeloff V.C. Helmers D.P. Kramer H.A. Mockrin M.H. Alexandre P.M. Bar-Massada A. Butsic V. Hawbaker T.J. 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Here we project global and regional trends in BA in response to simultaneous climate change and changing human demography using a modeling approach. To do this, we used the Lund-Potsdam-Jena dynamic global vegetation model (LPJ-DGVM)36Sitch S. Smith B. Prentice I.C. Arneth A. Bondeau A. Cramer W. Kaplan J.O. Levis S. Lucht W. Sykes M.T. et al.Evaluation of ecosystem dynamics, plant geography and terrestrial carbon cycling in the LPJ dynamic global vegetation model.Glob. Chang. Biol. 2003; 9: 161-185Crossref Scopus (2203) Google Scholar modified to include a process-based Socio-Economic and natural Vegetation ExpeRimental global fire model (SEVER-FIRE).26Venevsky S. Le Page Y. Pereira J.M.C. Wu C. Analysis fire patterns and drivers with a global SEVER-FIRE v1.0 model incorporated into dynamic global vegetation model and satellite and on-ground observations.Geosci. 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Terrestrial vegetation and water balance—hydrological evaluation of a dynamic global vegetation model.J. Hydrol. 2004; 286: 249-270Crossref Scopus (604) Google Scholar SEVER-FIRE provides a quantitative and spatially resolved global evaluation of recent historical and climate-change-driven BA trends for terrestrial ecosystems globally. SEVER-FIRE is a global fire model operating at a daily time step (here interpolated from monthly climate input within LPJ), derived from the first process-based large-scale Regional FIRe Model (Reg-FIRM).20Venevsky S. Thonicke K. Sitch S. Cramer W. Simulating fire regimes in human-dominated ecosystems: Iberian Peninsula case study.Glob. Chang. 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SEVER-FIRE simulates the number of fires, the BA, and fire-related vegetation mortality and carbon emissions, which can be further used by feeding back into a DGVM or an Earth system model (ESM). The model was extensively validated at regional (e.g., Spain, Canada, and Africa) and global scales using fire statistics and remote-sensing data for both number of fires and BA.26Venevsky S. Le Page Y. Pereira J.M.C. Wu C. Analysis fire patterns and drivers with a global SEVER-FIRE v1.0 model incorporated into dynamic global vegetation model and satellite and on-ground observations.Geosci. Model Dev. 2019; 12: 89-110Crossref Scopus (9) Google Scholar We first ran LPJ-DGVM–SEVER-FIRE (LPJ-SEVER), forced with observed climatology, over the 20th century to evaluate model performance in reproducing present-day trends in BA. We then coupled LPJ-SEVER to a computationally efficient climate emulator called Integrated Model Of Global Effects of climatic aNomalies (IMOGEN).40Huntingford C. 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Global temperature change, in turn, is modeled as a function of changing historical and future levels of atmospheric greenhouse gas (GHG) concentrations, and again calibrated against ESMs. IMOGEN can then be used for any set of CO2 concentration or emissions scenarios (the latter including climate-carbon cycle feedbacks), to generate climate forcing for the host vegetation model (e.g., LPJ-SEVER). In this framework, wildfire-induced changes in terrestrial carbon storage can feed back to climate itself via updated atmospheric CO2 concentration. The pattern scaling and the global warming response to rising GHGs are calibrated against 34 different ESMs (see experimental procedures) in the Coupled Model Intercomparison Project Phase 5 (CMIP5) ensemble. IMOGEN also maps from ESMs onto a common spatial grid of resolution 3.75° longitude × 2.5° latitude. Furthermore, IMOGEN predicts changes in climate, i.e., anomalies, and these are added to the University of East Anglia Climate Research Unit (CRU) climatology,41Harris I. Jones P.D. Osborn T.J. Lister D.H. Updated high-resolution grids of monthly climatic observations – the CRU TS3.10 Dataset.Int. J. Climatol. 2013; 34: 623-642Crossref Scopus (4455) Google Scholar thus also bias-correcting ESM offsets. IMOGEN takes in monthly data and gives monthly data to LPJ-SEVER, which disaggregates to daily steps (i.e., the coupled model was operated 34 times to emulate the same number of ESMs), but all on the common IMOGEN spatial resolution of 3.75° longitude × 2.5° latitude for the period 1860–2100. We performed 34 × 4 coupled model runs in the future under four different CO2-socioeconomic scenarios (i.e., 34 ESMs emulated × 4 scenario simulations; see experimental procedures). The four scenarios were based on four standard Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5) Representative Concentration Pathways (RCPs)42Moss R.H. Edmonds J.A. Hibbard K.A. Manning M.R. Rose S.K. van Vuuren D.P. Carter T.R. Emori S. Kainuma M. Kram T. et al.The next generation of scenarios for climate change research and assessment.Nature. 2010; 463: 747-756Crossref PubMed Scopus (4257) Google Scholar of potential scenarios of atmospheric GHG emissions in combination with three demographic Shared Socioeconomic Pathways (SSPs)43Riahi K. van Vuuren D.P. Kriegler E. Edmonds J. O'Neill B.C. Fujimori S. Bauer N. Calvin K. Dellink R. Fricko O. et al.The Shared Socioeconomic Pathways and their energy, land use, and greenhouse gas emissions implications: an overview.Glob. Environ. Change. 2017; 42: 153-168Crossref Scopus (1532) Google Scho