The impacts of climate, land use, and demography on fires during the 21st century simulated by CLM-CN

Biogeosciences, 9, 509–525, 2012 www.biogeosciences.net/9/509/2012/ doi:10.5194/bg-9-509-2012 © Author(s) 2012. CC Attribution 3.0 License. Biogeosciences The impacts of climate, land use, and demography on fires during the 21st century simulated by CLM-CN S. Kloster1 , N. M. Mahowald2 , J. T. Randerson3 , and P. J. Lawrence4 1 Land in the Earth System, Max Planck Institute for Meteorology, Hamburg, Germany 2 Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA 3 Department of Earth System Science, University of California, Irvine, CA, USA 4 National Center for Atmospheric Research, Boulder, CO, USA Correspondence to: S. Kloster () Received: 15 August 2011 – Published in Biogeosciences Discuss.: 28 September 2011 Revised: 20 December 2011 – Accepted: 3 January 2012 – Published: 26 January 2012 Abstract. Landscape fires during the 21st century are expected to change in response to multiple agents of global change. Important controlling factors include climate controls on the length and intensity of the fire season, fuel availability, and fire management, which are already anthropogenically perturbed today and are predicted to change further in the future. An improved understanding of future fires will contribute to an improved ability to project future anthropogenic climate change, as changes in fire activity will in turn impact climate. In the present study we used a coupled-carbon-fire model to investigate how changes in climate, demography, and land use may alter fire emissions. We used climate projections following the SRES A1B scenario from two different climate models (ECHAM5/MPI-OM and CCSM) and changes in population. Land use and harvest rates were prescribed according to the RCP 45 scenario. In response to the combined effect of all these drivers, our model estimated, depending on our choice of climate projection, an increase in future (2075– 2099) fire carbon emissions by 17 and 62 % compared to present day (1985–2009). The largest increase in fire emissions was predicted for Southern Hemisphere South America for both climate projections. For Northern Hemisphere Africa, a region that contributed significantly to the global total fire carbon emissions, the response varied between a decrease and an increase depending on the climate projection. We disentangled the contribution of the single forcing factors to the overall response by conducting an additional set of simulations in which each factor was individually held constant at pre-industrial levels. The two different projections of future climate change evaluated in this study led to increases in global fire carbon emissions by 22 % (CCSM) and 66 % (ECHAM5/MPI-OM). The RCP 45 projection of harvest and land use led to a decrease in fire carbon emissions by −5 %. The RCP 26 and RCP 60 harvest and landuse projections caused decreases around −20 %. Changes in human ignition led to an increase of 20 %. When we also included changes in fire management efforts to suppress fires in densely populated areas, global fire carbon emission decreased by −6 % in response to changes in population density. We concluded from this study that changes in fire emissions in the future are controlled by multiple interacting factors. Although changes in climate led to an increase in future fire emissions this could be globally counterbalanced by coupled changes in land use, harvest, and demography. 1 Introduction Contemporary landscape fires emit about 1.6 to 2.8 Pg C yr−1 into the atmosphere (van der Werf et al., 2010). This equals around 20–30 % of present day fossil fuel burning emissions (Boden et al., 2009). Changes in fires and subsequent changes in ecosystem carbon stocks can therefore have considerable impacts on atmospheric greenhouse gas concentrations and future climate change. In addition, increases in fires represent a severe hazard to human health and ecosystem services, which in many areas will require the development and implementation of new adaptation strategies (Bowman et al., 2009). From paleorecords we know that climate, and particularly rapid climate change, plays an important role in determining fire activity (Marlon et al., 2009). Observations from more recent decades clearly show a link between changing climate Published by Copernicus Publications on behalf of the European Geosciences Union. 510 S. Kloster et al.: Fires during the 21st century and fire activity (Soja et al., 2007). For example, in the western United States higher temperatures increased the duration and intensity of wildfires since the 1980s (Westerling et al., 2006). Human-induced climate change has had a detectable influence on the area burned by forest fire in Canada over recent decades (Gillett et al., 2004). Accelerated carbon losses for the last several decades were reported for Alaskan forests and peatlands (Turetsky et al., 2011). Fires depend on fuel type, fuel moisture, and fuel availability. In addition, fires require an ignition source which can be either of natural (lightning) or anthropogenic origin (Thonicke et al., 2001; Arora and Boer, 2005). Climate impacts fires directly by modulating fuel moisture and indirectly through the climate control of fuel availability (Flannigan et al., 2009). Fuel availability, for example, depends on the rate of plant growth and litter decomposition that are sensitive to climate change. Fuel availability is controlled by fire as well, which consumes biomass and lowers fuel loads. As such fires and fuel availability are coupled via a negative feedback loop that is likely to influence the way fires will change in the future. Fires are directly anthropogenically controlled through human caused ignition and fire management efforts, set in place to suppress fires in places where properties are at risk (Bowman et al., 2009). Land use and wood harvest rates impact fires indirectly by controlling fuel loads and fuel connectivity (Marlon et al., 2009). Consequently, fires depend on social and economic drivers of land use and on demographic trends. Here we investigated how fires may change during the 21st century. We used a global land carbon model that interactively simulated landscape fires, with fires responding to climate, land use and demographic driving variables. The model accounted for fires controlled by fuel moisture, fuel availability, and the abundance of ignition sources (Kloster et al., 2010). We examined how future changes in climate, land use, and demography were likely to influence trajectories of fire emissions during the 21st century. We also considered indirect controls, including for example climate-induced changes in fuel. In the following method section we will introduce the fire model that was evaluated for the 20th century in an earlier work (Kloster et al., 2010) and our simulation design. In the results section we discuss the simulated future fire emissions and the contribution of the single forcing factors (climate, land use, and demography) to the ove (...truncated)