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

Abstract

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.

Amplified warming from physiological responses to carbon dioxide reduces the potential of vegetation for climate change mitigation

“…atmospheric aerosols, clouds, and land processes) 49 . We anticipate our findings to remain broadly valid with the CMIP6 models, although the magnitude of PHYinduced warming is lower for CMIP6 than CMIP5 ESMs 18 , especially in the northern high latitudes. We hope our analysis provides an incentive to others to undertake such analysis as all the calculations (BGC, PHY, and RAD) become available for that newer CMIP6 ensemble.…”

“…That study proposed that CO 2 -driven greening will enhance ET and thereby producing a net cooling effect. We build on that analysis by here additionally including the CO 2 -induced partial stomatal closure 17,18 . We find this inclusion overtakes the LAI influence on ET changes, resulting in large reductions in ET that will instead contribute to net warming as CO 2 rises.…”

“…Indeed, increasing atmospheric CO 2 induces simultaneously an enhancement of photosynthesis which often increases LAI, and a reduction in stomatal conductance [12][13][14] . Recent analyses that focus on these physiological feedbacks (PHY) imply that they may exacerbate future heat extremes 15,16 and amplify regional and global warming 17,18 . However, the contributing processes of PHY feedbacks that affect local warming changes remain highly uncertain and are rarely considered in the assessments of the mitigation capacity of the terrestrial ecosystem.…”