Future warming and drying during the summer fire season in the northern hemisphere predicted by CMIP6 models

In the northern hemisphere, strong warming trends are expected during the summer wildfire season through the remainder of the 21st century as a consequence of anthropogenic climate change. Precipitation, especially in boreal and arctic regions, is expected to increase in parallel. The dual impacts of warming and changing hydrology may have contrasting effects on fire weather. Here we explored the drivers of contemporary and future changes in vapor pressure deficit, a widely used proxy for dead fuel moisture status and wildfire activity, through analysis of gridded meteorological observations and simulations contributed to the 6th phase of the Climate Model Intercomparison Project. We find that the strong positive trend in vapor pressure deficit (VPD) observed from 1980 to 2020 has been driven primarily by increasing temperature, with a minor secondary contribution from decreases in relative humidity. CMIP6 models simulate the magnitude of observed VPD trends in many regions, and suggest that in the future, decreases in surface relative humidity will play an increasingly important role as a driver of summer VPD increases. In boreal and arctic regions, we find that strong positive trends in summer VPD will persist in many regions despite concurrent increases in precipitation. To explain this apparent paradox, we examined the structure of daily precipitation from SSP585 simulations. We find that the number of dry days will significantly increase during summer in many regions in the northern hemisphere by the end of the 21st century. Although heavy rain days will become frequent events in the Arctic and across Asian monsoon regions, contributing to increases in seasonal mean precipitation levels and the distribution precipitation intensity changes, there are fewer days with light or moderate rain. This appears to create longer and more intense periods of high VPD weather (in between rain events), increasing the vulnerability of ecosystems in many regions to wildfire.