COS 83-3 - Understanding future wildfire is confounded by uncertainty in climate change and fire effects on fuel loading.

Abstract: Wildfire regimes are changing rapidly around the globe due to various interactions of human land use, management practices such as fire suppression, and climate change. Each of these works on one or both of fuels and fire weather. These complex interactions can result in varying responses of fire regimes to climate change, with not all regions following the simple maxim hotter and drier = increased and more severe wildfire. We will present a synthesis of prior simulation results and analysis of new results to investigate how uncertainty in ecosystem fire regime model prediction of fuel accumulation, particularly decomposition, may propagate to prediction of future fire regimes, and how those uncertainties interact with both fire exclusion and climate change. Climate change affects fire regimes at two relative time scales. Climate change resulting in hotter and drier conditions on the scale of 1-2 years is likely to increase fire area burned and result in more severe fire behavior. In the longer term (years to decades), however, climate change affects both fuel production (photosynthesis and net primary production) and fuel loss (decomposition). Climate change effects on fire weather also modify fuel loadings through potential increases in fire activity and wildfire self-limitation, whereas fuel accumulation related to fire suppression releases some ecosystem from fire-related fuel limitation. Climate change effects on fuels will operate by increasing or decreasing biological activity either through photosynthesis (fuel production) or decomposition (fuel loss). Model representation of climate change effects on decomposition are particularly uncertain. We investigated several integrated ecosystem wildfire regime models for decomposition model parameter and model structure sensitivity and their effects on projected fine fuel loading and associated wildfire area burned, as well as their interactions with fire suppression-related fuel accumulation. We discovered that uncertainties in decomposition model structures propagate to uncertainty in both fine fuel loading and associated simulated fire area burned. These uncertainties increased with climate-change related warming. Simulated effects of warming are also sensitive to seasonality of warming, and these interact with suppression-related fuel accumulation. Simulated fire area burned increased the most with warming and prior fire suppression. We require analysis of model uncertainty related to fuel accumulation in order to develop models that robustly integrate climate and fire effects on fuels over both short and long time scales, which is necessary to effectively project and understand future wildfire regimes under climate change.