Justin Mankin and co-authors have published a paper titled Blue Water Trade‐Offs With Vegetation in a CO2‐Enriched Climate in Geophysical Research Letters. The study uses a large ensemble of simulations to show that 42% of global vegetated land areas are projected to have “greening” in the form of additional vegetation growth at the same time as “drying” in the form of reduced soil moisture in a business‐as‐usual world. Simultaneous greening and drying is curious and suggests that future ecosystems—which could demand more water due to warmer and longer growing seasons and CO2 fertilization—siphon water that historically would have become the runoff that fills rivers and streams, termed “blue water.” The study shows that warming and changes in plant growth from CO2 creates an explicit water trade‐off in which future vegetation directly diminishes runoff relatively or absolutely for nearly half of global land areas. The results have important implications for future water availability, but also point to the crucial importance of resolving model uncertainties associated with terrestrial vegetation and its response to increasing CO2. A regional study on these effects in the western United States was also published last year.
