From Observation to Attribution: nudged atmospheric circulation simulations to attribute burned area trends

Wildfires are dynamic components of the Earth system, responding to both natural climate variability, ecological dynamics and human activities. While global burnt area (BA) has declined in recent decades, regional fire severity has increased, impacting ecosystems and infrastructure. Observational data, including satellite-derived products, enable monitoring of wildfire activity and the quantification of BA. However, they cannot fully separate the effects of anthropogenic climate forcing from internal climate variability. Earth system models offer a way to systematically investigate these drivers, providing critical insights into the causes of changing fire patterns. 

Using a storyline approach, we separate the thermodynamic and dynamic components of climate change, largely driven by human-induced forcing, and assess their impacts on BA. This allows us to attribute wildfire responses to external forcing versus internal variability. 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 the representation of large-scale circulation patterns. 

We present a first evaluation of wildfire characteristics in the nudged CESM2 simulations by comparing simulated output with observational data. Specifically, we compare the simulations with observational data from GFED5 (Chen et al., 2023), to investigate mean values and trends in BA, fire season length and fire weather indices for the period 2001–2020. Further, we present a first assessment of historical BA trends, highlighting how circulation-driven variability and thermodynamic changes can be separated across regions.

This evaluation provides a foundation for future studies using nudged CESM2 simulations to represent key wildfire characteristics and enables attribution studies that disentangle the relative roles of changes in climate and land use between pre-industrial and historical periods.