Plant Physiology Increases the Magnitude and Spread of the Transient Climate Response to CO2 in CMIP6 Earth System Models

Increasing concentrations of CO2 in the atmosphere not only influence climate through CO2’s effect as a greenhouse gas but also through its impact on plants. Plants respond to atmospheric CO2 concentrations in several ways that can alter surface energy and water fluxes and thus surface climate, including changes in stomatal conductance, water use, and canopy leaf area. These plant physiological responses are already embedded in Earth system models, and a robust literature demonstrates that they can affect global-scale temperature. However, the physiological contribution to transient warming has yet to be assessed systematically in Earth system models. Here this gap is addressed using carbon cycle simulations from the 5th and 6th phases of the Coupled Model Intercomparison Project (CMIP) to isolate the radiative and physiological contributions to the transient climate response (TCR). In CMIP6 models, the physiological effect contributes 0.11°C (standard deviation: 0.10°C; range: 0.02 - 0.29°C) of warming to the TCR, corresponding to 5.2% of the full TCR (standard deviation: 4.1%; range: 1.4 - 13.9%). Moreover, variation in the physiological contribution to the TCR across models contributes disproportionately more to the inter-model spread of TCR estimates than it does to the mean. The largest contribution of plant physiology to CO2-forced warming – and the inter-model spread in warming – occurs over land, especially in forested regions.