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Abstract The transient climate response (TCR), defined to be the warming in near‐surface air temperature after 70 years of a 1% per year increase in CO2, can be estimated from observed warming over the nineteenth and twentieth centuries. Such analyses yield lower values than TCR estimated from global climate models (GCMs). This disagreement has been used to suggest that GCMs' climate may be too sensitive to increases in CO2. Here we critically evaluate the methodology of the comparison using a large ensemble of a fully coupled GCM simulating the historical period, 1850–2005. We find that TCR estimated from model simulations of the historical period can be much lower than the model's true TCR, replicating the disagreement seen between observations and GCM estimates of TCR. This suggests that the disagreement could be explained entirely by the methodology of the comparison and undercuts the suggestions that GCMs overestimate TCR.
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Plant Physiology Increases the Magnitude and Spread of the Transient Climate Response to CO2 in CMIP6 Earth System Models
Abstract Increasing concentrations of CO 2 in the atmosphere influence climate both through CO 2 ’s role as a greenhouse gas and through its impact on plants. Plants respond to atmospheric CO 2 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 most 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 phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP) to isolate the radiative and physiological contributions to the transient climate response (TCR), which is defined as the change in globally averaged near-surface air temperature during the 20-yr window centered on the time of CO 2 doubling relative to preindustrial CO 2 concentrations. In CMIP6 models, the physiological effect contributes 0.12°C ( σ : 0.09°C; range: 0.02°–0.29°C) of warming to the TCR, corresponding to 6.1% of the full TCR ( σ : 3.8%; range: 1.4%–13.9%). Moreover, variation in the physiological contribution to the TCR across models contributes disproportionately more to the intermodel spread of TCR estimates than it does to the mean. The largest contribution of plant physiology to CO 2 -forced warming—and the intermodel spread in warming—occurs over land, especially in forested regions.
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- Award ID(s):
- 1752796
- PAR ID:
- 10230750
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 33
- Issue:
- 19
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- 8561 to 8578
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JCLI-D-20-0078.1
