Atmospheric Carbon Dioxide and Climate Sensitivity in a Warmer World

Anthropogenic C emissions to the atmosphere have tripled over the past two decades. If left unabated, atmospheric CO₂ by 2100 could reach levels of the warm greenhouse climate of more than 34 million years ago.  Once our fossil fuel reserves as depleted and if carbon sequestration efforts remain at present-day levels, atmospheric CO₂ could ultimately reach as high as 2000 ppmV. How the Earth’s global surface temperature will respond to such greenhouse-gas external forcing hinges on our understanding of climate sensitivity – generally defined as the change in surface temperature per doubling of atmospheric CO₂. Climate models, the primary tool for estimating climate sensitivity, however, yield climate sensitivity estimates that span from 1.5 to 11°C, with IPCC-type model estimates in the lower part of the range.  Much of the uncertainty in this large range reflects the temporal hierarchy and complexity of short-term (decades to centuries) and long-term (thousands to tens of thousands of years) feedbacks. Longer-term physical and biochemical feedbacks, not typically considered by climate models, in particular may become increasingly more relevant on human timescales with continued global warming.

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.