Increased Sea Level Sensitivity to CO2 Forcing across the Middle Pleistocene Transition from Ice-Albedo and Ice-Volume Nonlinearities

Proxy reconstructions indicate that sea level responded more sensitively to CO2 radiative forcing in the late Pleistocene than in the early Pleistocene, a transition that was proposed to arise from changes in ice-sheet dynamics. In this study we analyze the links between sea level, orbital variations, and CO2 using an energy-balance model having a simple ice sheet. Model parameters, including for age models, are inferred over the late Pleistocene using a Bayesian method, and the inferred relationships are used to evaluate CO2 levels over the past 2 million years in relation to sea level. Early Pleistocene model CO2 averages 244 ppm (241-246 ppm 95% confidence interval) across 2 to 1 million years ago and indicates that sea level was less sensitive to radiative forcing than in the late Pleistocene, consistent with foregoing delta B-11-derived estimates. Weaker early Pleistocene sea level sensitivity originates from a weaker ice-albedo feedback and the fact that smaller ice sheets are thinner, absent changes over time in model equations or parameters. An alternative scenario involving thin and expansive early Pleistocene ice sheets, in accord with some lines of geologic evidence, implies 15-ppm-lower average CO2 or similar to 10-15-m-higher average sea level during the early Pleistocene relative to the original scenario. Our results do not rule out dynamical transitions during the middle Pleistocene, but indicate that variations in the sea level response to CO2 forcing over the past 2 million years can be explained on the basis of nonlinearities associated with ice-albedo feedbacks and ice-sheet geometry that are consistently present across this interval.

Automated summary

Study analyzes the sensitivity of sea level to CO2 changes during different ice sheet dynamics in late Pleistocene and early Pleistocene, indicating that the sea level response to radiative forcing was less sensitive in the early Pleistocene compared to the late Pleistocene.