Environmental Science and Engineering Seminar

Determining the modern climate's sensitivity to greenhouse-gas forcing has been a central challenge for over 40 years. To constrain the notoriously uncertain upper bound of climate sensitivity, we must look to the "natural experiments" in Earth's past. Recent advances in climate reconstruction provide new constraints on the spatial patterns of past temperature change, which play a leading role in Earth's climate sensitivity. In Part I, we will investigate the cold Last Glacial Maximum and the warm Pliocene. By combining recent reconstructions with atmospheric general circulation models, we will show why cloud feedbacks strongly amplify the temperature changes in Earth's past and why this represents a major update to our understanding of modern climate sensitivity. In Part II, we will turn to the recent past (1850–2023) and investigate the unresolved uncertainty in feedbacks over the historical record. We will introduce a new "coupled" reconstruction that synthesizes observational and dynamical constraints across the atmosphere and ocean. Using this reconstruction, we will provide a dynamically consistent perspective on historical feedbacks, variability, and trends. Finally, we will come back to the future and consider implications for climate sensitivity and 21st-century warming.