Guest essay by Eric Worrall
According to a new study, increased transpiration from CO2 engorged plants will return more water vapour to the atmosphere, reducing runoff available to fill reservoirs.
Mid-latitude freshwater availability reduced by projected vegetation responses to climate change
AbstractRead more: https://www.nature.com/articles/s41561-019-0480-x
Plants are expected to generate more global-scale runoff under increasing atmospheric carbon dioxide concentrations through their influence on surface resistance to evapotranspiration. Recent studies using Earth System Models from phase 5 of the Coupled Model Intercomparison Project ostensibly reaffirm this result, further suggesting that plants will ameliorate the dire reductions in water availability projected by other studies that use aridity metrics. Here we complicate this narrative by analysing the change in precipitation partitioning to plants, runoff and storage in multiple Earth system models under both high carbon dioxide concentrations and warming. We show that projected plant responses directly reduce future runoff across vast swaths of North America, Europe and Asia because bulk canopy water demands increase with additional vegetation growth and longer and warmer growing seasons. These runoff declines occur despite increased surface resistance to evapotranspiration and vegetation total water use efficiency, even in regions with increasing or unchanging precipitation. We demonstrate that constraining the large uncertainty in the multimodel ensemble with regional-scale observations of evapotranspiration partitioning strengthens these results. We conclude that terrestrial vegetation plays a large and unresolved role in shaping future regional freshwater availability, one that will not ubiquitously ameliorate future warming-driven surface drying.
Sadly the full study is paywalled, but I think we get the idea; Plants are bad. The study suggests anything which increases transpiration, like anthropogenic CO2 emissions or presumably other changes which lead to increased transpiration, like planting trees, increases stress on the global supply of fresh water.
Of course, there are a few contradictory studies which suggest rainfall decreases if trees are cleared, studies which suggest plant transpiration plays an important positive role in the water cycle, but these studies are based on observations rather than climate projections.