Understanding the Earth’s response to past periods of greenhouse gas driven climate change can provide unique insight into our climate future including the extent to which climate sensitivity will change due to the interaction of processes that have not operated or have operated differently in our recent glacial state. Estimates of long-term equilibrium climate sensitivity based on the geologic record and averaged over tens to hundreds of millennia converge on 3 to 6°C. Even higher estimates (up to 7 to 10°C per doubling of CO2) of climate sensitivity during the most recent periods of global warming, the Paleocene-Eocene Thermal Maximum and middle Pliocene, suggest that current projections may underestimate future temperatures. Thus, constraining the equilibrium climate sensitivity for past periods of elevated CO2 and at time-scales that incorporate the interplay of both short- and long-term feedbacks is a critical element of better understanding the response of our global climate system to net radiative forcing. This will require scaled-up efforts by the scientific community in the development and calibration of proxies of paleoatmospheric CO2 contents and paleotemperatures given current uncertainties associated with existing mineral- and plant-based proxies.
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