From Observation to Attribution: Evaluating Nudged-Circulation CESM2 Burnt Area Simulations

Wildfires are dynamic components of the Earth system, responding to both natural variability and human activities. Although global burnt area (BA) has declined in recent decades, regional increases in fire severity have been observed, with significant impacts on ecosystems and human infrastructure. A wide range of observational data, including satellite-derived products, is available for monitoring and detecting wildfires and quantifying BA. While these datasets provide essential information on past and present fire activity, Earth system models allow a systematic investigation into the causal drivers of wildfire, and specifically its responses to anthropogenic forcing. However, the relative contributions of anthropogenic climate forcing and internal climate variability to wildfire activity often remain unclear, and those causal effects cannot be disentangled in observational datasets alone. 

Using a storyline approach, we separate the thermodynamic and dynamic components of climate change, largely driven by human-induced forcing, and assess how these components affect BA. This allows attribution of wildfire responses to external forcing versus internal variability. Therefore, we analyze nudged circulation simulations from the Community Earth System Model Version 2 (CESM2; Danabasoglu et al., 2020) under different anthropogenic forcing scenarios. The pre-industrial simulation is based on a CO₂ concentration of 282 ppm, whereas the historical simulation uses time-varying historical CO₂ concentrations. Both simulations are nudged to horizontal winds from the ERA5 reanalysis, ensuring that large-scale circulation patterns are represented. 

Here we present a first evaluation of wildfire characteristics in the CESM2 nudged simulations by comparing simulated output with observational data. Specifically, we compare mean values and trends in BA, fire season length, and the Fire Weather Index, as well as its individual components with GFED5 (Chen et al., 2023) for the period 2001–2020. To illustrate the potential of the framework, we present a case study of BA trends, highlighting how circulation-driven variability and thermodynamic changes can be separated across regions.

This evaluation provides a foundation for subsequent studies, using the nudged CESM2 simulations to represent key wildfire characteristics. The analysis forms a basis for attribution studies that disentangle the relative roles of changes in climate and land use in shaping differences between pre-industrial and historical scenarios.