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The transient climate response (TCR), defined to be the warming in near-surface air temperature after 70years 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.

Plain Language Summary The transient climate response (TCR) is defined to be the warming after 70years of a 1% per year increase in atmospheric CO2. It is one of the important metrics in climate science because it plays a key role in determining how much warming we will experience in the future. Previous work has found that TCR inferred from observed warming over the twentieth century tends to be lower than TCR in climate models. This has been used by suggest that climate models are overpredicting future warming. We use a large number of climate model runs to investigate the methodology of this comparison. We find that TCR estimated from the twentieth century simulations may indeed be much lower than the model's true TCR. This arises from biases in the methodology of estimating TCR from twentieth century warming, as well as biases in the construction of the observational temperature data sets. We therefore find no evidence that models are overestimating TCR.