The Impact of CO2-Driven Vegetation Changes on the Future of Flash Drought in the Northern Hemisphere

Vegetation plays a crucial role in soil moisture regulation and the development of rapid-onset droughts known as flash droughts. We use climate model experiments with the Community Earth System Model (CESM2) to examine how the vegetation response to rising CO₂ impacts projections of future flash drought in the Northern Hemisphere mid-latitudes. By isolating the influences of CO₂ fertilization and CO₂ stomatal conductance effects from CO₂ radiative forcing, we find that: (a) CO₂-induced changes to plant characteristics are of sufficient magnitude to modify flash drought characteristics, (b) CO₂ fertilization effects counteract the CO₂ stomatal conductance effects on projected flash drought occurrence, and (c) the combined influence of the vegetation response to rising CO₂ can either amplify or counteract CO₂ radiative-driven flash drought changes depending on location. In water-limited regions such as the western United States, the Mediterranean Basin, the Middle East, and west/central Asia where CO₂ fertilization dominates and surface vegetation strongly controls water availability, elevated leaf area offsets reductions in stomatal conductance and transpiration, increasing the likelihood of future flash droughts. Vegetation-driven increases in flash drought in these areas are generally aligned in sign with projected increases due to radiative forcing. Conversely, in more energy limited regions such as western Canada, East Asia, and parts of Europe, preserved soil moisture from reduced stomatal conductance and transpiration suppresses flash droughts despite increased leaf area from CO₂ fertilization. These reductions in flash drought from vegetation counteract radiative-driven increases. This study elucidates physical processes underlying projected flash drought development, improving predictive capabilities and mitigation strategies.