The transient climate response (TCR) is one way to measure the sensitivity of the Earth’s climate to an increase in anthropogenic greenhouse gases. The TCR, defined to be the warming after 70 years of a 1% per year increase in CO2, cannot be directly measured, but it can be estimated using observed temperatures over the 20th century and model-derived estimates of radiative forcing. Using a 100-member ensemble of a fully coupled global climate model (MPI-ESM1.1), we find that internal variability contributes significant uncertainty in observational estimates of TCR, with TCR values from the historical ensemble ranging from 1.34 to 1.94 K, compared to the model’s true TCR of 1.81 K. This spread can be attributed entirely to internal variability. A majority of the variability observed in the TCR is due to heat transport in the oceans — ensemble members that transport more heat to the deep ocean have lower TCRs. Another key factor is variability in sea ice, which regulates the amount of solar energy that is reflected back to space. Together, these two factors explain most of the variability observed in the TCR. Due to the observed effects of internal variability, estimates of the TCR from the historical record could deviate significantly from the climate system’s true value.
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