No apparent state-dependency of equilibrium climate sensitivity between the Pleistocene glacial and interglacial climate states.

Abstract

Quantifying climate sensitivity is essential for future climate projections, yet it varies with major Earth system changes. We present a glacial CO₂ reconstruction using paleosols from the Chinese Loess Plateau, covering 2580 to 800 thousand years ago. A stepwise decline in glacial CO₂ levels from ~300 ppm to <200 ppm is observed. Our paleosol-based CO₂ estimates support the key role of atmospheric CO₂ in driving major climate transitions during the Pleistocene, such as the long-term global cooling and the amplification of the glacial cycles. Based on compiled glacial and interglacial CO₂ records, Earth system sensitivity, defined as the global temperature change for a doubling of CO₂ once the whole Earth system has reached equilibrium, is estimated to be ~6.2-7.4 K (3.2-12.0 K, 95% confidence). Equilibrium climate sensitivity, after accounting for the different efficacy between ice-sheet and CO₂ forcing and other slow feedbacks, is estimated to be 3.3 K (2.1-6.3 K, 95% confidence) and 3.7 K (1.7-6.3 K, 95% confidence), respectively. The lack of a significant difference between these values suggests no apparent state-dependency of climate sensitivity between glacial and interglacial climate states.