Long-Term Wetting and Drying Trends in Land Water Storage Derived From GRACE and CMIP5 Models

Coupled climate models participating in the CMIP5 (Coupled Model Intercomparison Project Phase 5) exhibit a large intermodel spread in the representation of long-term trends in soil moisture and snow in response to anthropogenic climate change. We evaluate long-term (January 1861 to December 2099) water storage trends from 21 CMIP5 models against observed trends in terrestrial water storage (TWS) obtained from 14 years (April 2002 to August 2016) of the GRACE (Gravity Recovery And Climate Experiment) satellite mission. This is complicated due to the incomplete representation of TWS in CMIP5 models and interannual climate variability masking long-term trends in observations. We thus evaluate first the spread in projected trends among CMIP5 models and identify regions of broad model consensus. Second, we assess the extent to which these projected trends are already present during the historical period (January 1861 to August 2016) and thus potentially detectable in observational records available today. Third, we quantify the degree to which 14-year tendencies can be expected to represent long-term trends, finding that regional long-term trends start to emerge from interannual variations after just 14 years while stable global trend patterns are detectable after 30 years. We classify regions of strong model consensus into areas where (1) climate-related TWS changes are supported by the direction of GRACE trends, (2) mismatch of trends hints at possible model deficits, (3) the short observation time span and/or anthropogenic influences prevent reliable conclusions about long-term wetting or drying. We thereby demonstrate the value of satellite observations of water storage to further constrain the response of the terrestrial water cycle to climate change.