CMIP6 summertime warm bias in central CONUS due to the relative neglect of transpiration from local vegetation

Authors: DONG, J. - US Department Of Agriculture (USDA); LEI, F. - Mississippi State University; Crow, Wade

Submitted to: Nature Communications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/22/2021
Publication Date: 1/17/2022
Citation: Dong, J., Lei, F., Crow, W.T. 2022. CMIP6 summertime warm bias in central CONUS due to the relative neglect of transpiration from local vegetation. Nature Communications. 13:336. https://doi.org/10.1038/s41467-021-27938-6.
DOI: https://doi.org/10.1038/s41467-021-27938-6

Interpretive Summary: Climate change is expected to significantly impact agricultural productivity in the central United States. Accurately projecting the severity of such impacts for coming decades is a key goal of earth system models (ESMs). However, historical ESM simulations have a general tendency to overestimate summertime daily maximum near-surface air temperature in the central United States. Past efforts to remove this bias have focused on errors in the atmospheric component of ESMs. However, this paper demonstrates that such efforts are misplaced and illustrates that the bias instead originates in the land surface component of these models. In particular, ESMs overestimate the importance of bare soil evaporation relative to vegetation transpiration (i.e., plant root uptake of soil water). This ensures they excessively reduce summertime evapotranspiration (ET) in response to normal seasonal drying of the soil surface. Since ET is the most efficient means of cooling near-surface air temperature, an excessive reduction of summertime ET translates into a warm temperature bias in the models. This insight will be used to improve our ability to project and mitigate the agricultural impact of climate change in the central United States.

Technical Abstract: Earth system models (ESMs) from the Coupled Model Intercomparison Project Phase 6 (CMIP6) experiment exhibit a well-known summertime warm bias in midlatitude land regions – most notably in the central continental United States (CUS). This bias has generally been attributed to systematic atmospheric errors in the representation of precipitation and clouds. However, based on validated datasets and off-line modeling, we find that the CUS summertime warm bias is instead dominated by the incorrect partitioning of total surface evapotranspiration (ET) into its canopy transpiration and soil evaporation components. Specifically, CMIP6 ESMs do not effectively use available rootzone soil moisture for summertime transpiration and instead rely excessively on shallow soil and canopy-intercepted water storage to supply ET. As such, expected summertime precipitation deficits in CUS introduce a negative ET bias and positive temperature bias. This tendency potentially biases CMIP6 projections of regional water stress and summer temperature variability under elevated CO2 conditions. As such, the improved representation of transpiration/ET partitioning should be prioritized in efforts to reduce long-standing ESM regional biases.