An emergent constraint exposes widespread underestimation of drought impacts by Earth System Models

Abstract

The response of vegetation canopy conductance (gc) to changes in moisture availability (ϒgc) during drought is a major source of uncertainty in climate projections. Representing ϒgc accurately in Earth System Models (ESMs) is particularly problematic because no regional scale gc observations exist with which to evaluate it. Here, we overcome this challenge by deriving an emergent constraint on ϒgc across ESMs from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). We leverage an ensemble of satellite, reanalysis and station-based estimates of surface temperatures, which are physically and statistically linked to ϒgc due to the local cooling effect of gc. We find that models systemically underestimate ϒgc by ~50%, particularly in semi-arid grasslands, croplands, and savannas. Based on the mediating effect of gc on carbon, water and energy fluxes through land-atmosphere interactions, the underestimation of modeled ϒgc in these regions contributes to biases in temperature, transpiration and gross primary production. Our results provide a novel benchmark to improve model representation of vegetation dynamics and land-atmosphere feedbacks in these regions, thus improving forecasting ability of climate extremes under future climate change scenarios.