Projected Changes to Hydroclimate Seasonality in the Continental United States
Scientists at the NASA Goddard Institute for Space Studies and the Center for Climate Systems Research in collaboration with colleagues from Lawrence Livermore National Laboratory and the Lamont-Doherty Earth Observatory analyzed more than 400 climate model simulations to characterize the projected changes in eight hydroclimate variables across seven regions of the continental United States (CONUS), along with shifts in the amplitude and phase of their annual cycles. They found that even in regions where no robust change in the annual mean is expected, coherent changes to the annual cycle are projected. Whether changes become detectable above the background noise of internal variability depends strongly on the future scenario considered: there are significant earlier shifts in the annual cycle in a high emissions scenario but these changes are largely avoided in the lowest-forcing scenario. The variables analyzed are the near-surface air temperature, evaporation, total precipitation, amount of precipitation that falls as snow, surface, and column-integrated total runoff, and surface and column-integrated total soil moisture.
Past studies identified human influences on historical global drought trends, on large-scale aridity trends, and the severity of individual regional droughts. In the absence of major emissions reductions, these trends will continue to intensify. Despite the established work to date, there is still substantial uncertainty in projecting and understanding future hydroclimate changes, particularly at the smaller regional scales that are critical for adaptation decisions. This study examines future regional changes to the hydroclimate of CONUS and reports changes to both annual averages and annual cycles projected by the models participating in the most recent phase of the Coupled Model Intercomparison Project (CMIP6). In the absence of mitigation efforts, the United States will need to adapt to multiple significant and interlinked changes in hydroclimate. Climate models project not only robust changes in multiple climate variables but significant changes to their annual cycles. Adaptation decisions therefore must take such seasonal shifts into account, and strategies predicated on existing annual cycles may not be optimal in the future.
State-of-the-art climate models project substantial changes to the climate of the continental United States (CONUS). Models project changes to the yearly average rainfall, soil moisture, and runoff in many regions. But crucially, they also project changes to the annual cycle, with peaks in evaporation, runoff, and soil moisture projected to shift earlier in the year even if the yearly averages of those quantities do not change. While different climate models disagree on the exact nature of many shifts, a large source of uncertainty is human behavior and policy choices, and many significant changes can be avoided if greenhouse gas emissions fall drastically in the future.