Model Consensus Projections of US Regional Hydroclimates Under Greenhouse Warming
Livermore Lab scientists have investigated ensemble-averages of multi-model simulations of hydroclimate under three progressively more severe future greenhouse emissions scenarios. They computed projections of future summer hydrological quantities averaged over three U.S. Southern Tier regions: the climatologically arid Southwest (SW), semi-arid Southern Great Plains (SGP), and humid Southeast (SE). The researchers identified projected general increases in continental drying, but with a severity that varies by region: the SW and SGP are likely to suffer the greatest consequences relative to present conditions, and the SE the least.
The prospect of a future general aridification of the U.S. Southern Tier portends substantial disruption of physical and societal infrastructure in these regions. The high relative risk for the SGP, a major agricultural region, is of particular concern.
LLNL scientists have investigated ensemble-mean (“consensus”) values of some 15 CMIP5 multi-model simulations of 1976-2005 summer U.S. regional hydroclimates. The researchers also analyzed projected consensus estimates of 2070-2099 hydroclimatic changes under the “Representative Concentration Pathways” 2.6, 4.5, and 8.5 greenhouse scenarios, respectively. Uncertainties in the consensus estimates were reduced by area-averaging hydrological quantities over 10x10-degree SW, SGP, and SE regions of the U.S. Because the number of samples in these regional areas varied according to model grid spacing, the consensus estimates also involved a weighting of the CMIP5 projections that was proportional to the horizontal resolution of each model. Examples of the analyzed regional hydrological quantities included precipitation P, evaporation and runoff ratios E/P and Q/P, and a dryness index DI, defined as the ratio Rnet/P, where Rnet is the net surface radiative heating. Even when projected P amounts do not decrease, the greenhouse-induced increase in Rnet drives a robust increase in aridification of the U.S. southern-tier. The severity of this projected drying varies with region, being proportionately greatest in the SW and SGP (relative to current conditions), and least in the SE. The projected general aridification of the U.S. southern tier should be viewed with some caution, however, since it may partly result from shortcomings in the CMIP5 climate models’ treatment of land processes (especially vegetation effects), or in the specified RCP greenhouse scenarios, which do not fully account for prospective future land-use changes.