Projected Hydroclimate Changes Driven by Carbon Dioxide Trends and
Vegetation Modeling in CMIP6
Abstract
Past and projected changes in global hydroclimate in Earth system models
have been examined. The Budyko framework that relates the partitioning
of precipitation into evaporation to a location’s aridity has been
modified to account for the effect of interannual terrestrial water
storage and compared to traditional methods. The new formulation better
fits climate model data over most of the globe. Old and new formulations
are used to quantify changes in the spatial patterns of hydroclimate
based locally on year-to-year variations water and energy cycle
variables. Focus is on multi-model median responses to changing climate.
The changes in hydroclimate from preindustrial to recent historical
(1965-2014) conditions often have different patterns and characteristics
than changes due only to increasing CO2. For simulations
with gradually increasing CO2, differing model
treatments of vegetation are found specifically to have categorically
different impacts on hydroclimate, particularly altering the
relationship between aridity and the fraction of precipitation
contributing to evaporation in models that predict vegetation changes.
Models that predict vegetation phenology have consistently different
responses to increasing CO2 than models that do not.
Dynamic vegetation models show more widespread but less consistent
differences than other models, perhaps reflecting their less mature
state. Nevertheless, there is clearly sensitivity to vegetation that
illustrates the importance of including the representation of biospheric
shifts in Earth system models.