Paleoclimate pattern effects help constrain climate sensitivity and 21st-century warming

Abstract: Paleoclimates provide examples of past climate change that inform estimates of modern warming from greenhouse-gas emissions, known as Earth's climate sensitivity. However, differences between past and present climate change must be accounted for when inferring climate sensitivity from paleoclimate evidence. The closest paleoclimate analog to near-term warming from greenhouse-gas emissions is the Pliocene (5.3-2.6 Ma), a warm epoch with atmospheric CO2 concentrations similar to today. Recent reconstructions indicate the Pliocene was 1°C warmer than previously thought, implying higher climate sensitivity, which is also supported by recent reconstructions showing more cooling from reduced CO2 at the Last Glacial Maximum (LGM; 19-23 thousand years ago). However, large-scale patterns of paleoclimate temperature change differ strongly from modern projections. Climate feedbacks and sensitivity depend on temperature patterns, and such "pattern effects" must be accounted for when using paleoclimates to constrain modern climate sensitivity. Here we combine data-assimilation reconstructions with atmospheric general circulation models to show Earth's climate is more sensitive to Pliocene forcing than modern CO2 forcing. Pliocene ice sheets, topography, and vegetation alter patterns of ocean warming and excite destabilizing cloud feedbacks, and LGM feedbacks are similarly amplified by the North American ice sheets. Accounting for paleoclimate pattern effects produces a best estimate (median) for modern climate sensitivity of 2.8°C and 66% confidence interval of 2.4-3.4°C (90% CI: 2.1-4.0°C), substantially reducing uncertainty in projections of 21st-century warming. Significance statement: Climate sensitivity's uncertain upper bound determines the worst-case projections of global warming. Recent paleoclimate reconstructions suggest high sensitivity of 5°C per CO2 doubling. However, by analyzing spatial patterns of Pliocene warming—the closest analog to near-term warming—we show that ice sheets and topography amplified past warming through regional impacts on oceans and clouds. Similarly, the Last Glacial Maximum's cooling was amplified by ocean and cloud responses to massive ice sheets. Because these amplifying feedbacks are associated with non-CO2 forcings unique to paleoclimates, the upper bound on modern warming from doubling CO2 is reduced by 1°C, constraining climate sensitivity to 2.1–4.0°C (90% confidence). Thus paleoclimate evidence revises climate sensitivity's upper bound and 21st-century warming projections.