Historical Changes in Surface Soil Moisture Over the Contiguous United States: An Assessment of CMIP6

The newly released simulations from the Sixth Phase of the Coupled Model Intercomparison Project (CMIP6) provide an opportunity to re‐examine long‐term soil moisture trends. In this study, we assessed the historical simulation (1850–2014) of surface soil moisture from 18 general circulation models that are part of CMIP6 and compared them with model simulations from CMIP5 and NLDAS‐2, and in situ observations over CONUS. Our results show that for CONUS as a whole, there is no significant long‐term trend in soil moisture, but significant drying trends exist over medium‐term, short‐term, and near‐present periods based on the CMIP6. Our comparison of CMIP6 to NLDAS‐2 (1980–2014) and in situ observations (2004–2014) demonstrates that it more accurately captures soil moisture trends in CONUS than CMIP5. CMIP6 shows a significant wetting trend in the Northwest, Midwest, Northeast, and Southeast. There is greater agreement amongst the individual models in CMIP6 than CMIP5, but intermodel variability still exists. Plain Language Summary Soil moisture is an important variable that affects both energy and water balance near land surface. Global climate models are commonly used to simulate historical and future soil moisture conditions. We analyzed soil moisture in the 0–10 cm soil layer from the newly released Sixth Phase of the Coupled Model Intercomparison Project (CMIP6) to assess long‐term soil moisture trends (1850–2014) the contiguous United States. Significant increases in soil moisture were found in the Midwest, Northeast, Southeast, and Northwest. However, when considering all of the contiguous United States, there is no trend in soil moisture. This differs from CMIP5 which showed a significant drying trend in the United States. We validated the CMIP6 results by comparing them with the NLDAS‐2 land‐surface model (1980–2014) and in situ soil moisture observations (2004–2014). The comparison shows that CMIP6 is more accurate than CMIP5. There is still substantial variation among the individual models that comprise CMIP6; however, there is greater consensus amongst the CMIP6 models than CMIP5. Key Points Sixth Phase of the Coupled Model Intercomparison Project (CMIP6) shows significant long‐term soil moisture trends in the northern half of CONUS and Southeast, but not for CONUS as a whole CMIP6 has less uncertainty and is better able to capture soil moisture trends than CMIP5 Substantial model uncertainty remains in CMIP6