ARTICLE OPEN https://doi.org/10.1038/s43247-024-01228-7 Enhanced future vegetation growth with elevated carbon dioxide concentrations could increase fire activity 1234567890():,; Robert J. Allen 1 ✉, James Gomez 1, Larry W. Horowitz 2 & Elena Shevliakova 2 Many regions of the planet have experienced an increase in fire activity in recent decades. Although such increases are consistent with warming and drying under continued climate change, the driving mechanisms remain uncertain. Here, we investigate the effects of increasing atmospheric carbon dioxide concentrations on future fire activity using seven Earth system models. Centered on the time of carbon dioxide doubling, the multi-model mean percent change in fire carbon emissions is 66.4 ± 38.8% (versus 1850 carbon dioxide concentrations, under fixed 1850 land-use conditions). A substantial increase is associated with enhanced vegetation growth due to carbon dioxide biogeochemical impacts at 60.1 ± 46.9%. In contrast, carbon dioxide radiative impacts, including warming and drying, yield a negligible response of fire carbon emissions at 1.7 ± 9.4%. Although model representation of fire processes remains uncertain, our results show the importance of vegetation dynamics to future increases in fire activity under increasing carbon dioxide, with potentially important policy implications. 1 Department of Earth and Planetary Sciences, University of California Riverside, Riverside, CA 92521, USA. 2 NOAA/OAR Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA. ✉email: rjallen@ucr.edu COMMUNICATIONS EARTH & ENVIRONMENT | (2024)5:54 | https://doi.org/10.1038/s43247-024-01228-7 | www.nature.com/commsenv 1 ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-024-01228-7 F ire is an important Earth system process that alters ecosystem and atmospheric composition1–5. Over the last decade, several regions, such as the western US, have experienced an increase in the frequency and size of fires6 and a lengthening of the fire weather season7. Fires are also projected to increase in the coming years, with suggested drivers including intensified drought, more frequent heatwaves, and changes in fire suppression tactics8–13. More generally, anthropogenic climate change has been projected to enhance fire weather across most burnable global land area14, including the western US, Australia, and the Mediterranean9,15–19, as well as the Amazon under low climate mitigation scenarios20. In addition to enhanced fire weather, increasing atmospheric CO2 concentrations are associated with enhanced carbon uptake and storage by the terrestrial biosphere through the CO2 fertilization effect21,22. Several recent studies indicate the intensification of terrestrial biosphere activity23–27, including “greening" of the planet, much of which was attributed to the CO2 fertilization effect25,28,29. However, it is uncertain how this fertilization effect will influence fires. Higher CO2 fertilization has been estimated to increase fire occurrence through increasing fuel load30,31, but alternatively has been estimated to mitigate fire severity through increasing live fuel moisture content32. These responses can also depend on fire regimes. For example, in fuel-limited fire regimes, fire is more responsive to fuel-loading changes whereas in flammability-limited fire regimes, fire is more responsive to fuel moisture changes33. Furthermore, fire regimes may also shift from flammability- to fuel-limited or become increasingly fuellimited in response to climate change34. Here, we use seven state-of-the-art Earth system models (Methods) from the Coupled Model Intercomparison Project version 6 (CMIP6)35, all of which include representation of fire activity of varying complexity, to quantify the impact of an idealized increase of CO2 on fire carbon emissions ("fFire" variable from the CMIP6 database). Our goal is to assess how wildfire activity is projected to change under idealized increases in atmospheric CO2 in the current generation of models, and moreover, to assess the relative importance of physical climate impacts (e.g., warming and drying) relative to vegetation dynamics (e.g., CO2 fertilization effect and enhanced vegetation growth). Although the model spread is large, we find a robust increase in fFire in response to increasing CO2, largely due to biogeochemical mechanisms, i.e., the CO2 fertilization effect. Results Model evaluation. We first evaluate the ability of CMIP6 models to simulate fi