Increased sea-level sensitivity to CO2 forcing across the Middle Pleistocene Transition from ice-albedo and ice-volume nonlinearities

Abstract

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 analyse 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 hierarchical Bayesian method, and the inferred relationships are used to evaluate CO2 levels over the past 2 My in relation to sea level. Early-Pleistocene model CO2 averages 246 ppm (244 ppm - 249 ppm 95% c.i.) across 2-1 Ma and indicates that sea level was less sensitive to radiative forcing than in the late Pleistocene, consistent with foregoing δ11B-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 ~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 My can be explained on the basis of nonlinearities associated with ice-albedo feedbacks and ice-sheet geometry that are consistently present across this interval.

Uniformitarian prediction of early-Pleistocene atmospheric CO2

“…Estimates of Pleistocene CO 2 conditional on sea level are obtained using the inverse model that is detailed in Ref. [25] and briefly summarized here. The relationship among orbital variations, atmospheric CO 2 , and sea-level is represented using a zonally-averaged energy-balance balance model that is paired with an ice sheet following a plastic rheology.…”

“…In a previous study [25], we used a Bayesian approach to constrain a model of the relationships among orbital variations, CO 2 , and sea-level. The model includes a zonally-averaged representation of an ice sheet that grows and retreats in response to orbital variations, CO 2 forcing, and meridional heat flux.…”

“…Here we extend predictions of atmospheric CO 2 concentrations over the last 2 Ma under a similar assumption of consistent physical relationships between CO 2 and climate over time and test this assumption against existing observations. Our principal approach is to use a recently-developed Bayesian paleoclimate model [25] to infer CO 2 values conditional on past sea level. An ensemble of seven different CO 2 histories are inferred from an equal number of sea-level reconstructions.…”

Uniformitarian Prediction of Early‐Pleistocene Atmospheric CO2

“…Estimates of Pleistocene CO 2 conditional on sea level are obtained using the inverse model that is detailed in Liautaud and Huybers (2021) and briefly summarized here. The relationship among orbital variations, atmospheric CO 2 , and sea-level is represented using a zonally-averaged energy-balance balance model that is paired with an ice sheet following a plastic rheology.…”

“…In a previous study (Liautaud & Huybers, 2021), we used a Bayesian approach to constrain a model of the relationships among orbital variations, CO 2 , and sea‐level. The model includes a zonally‐averaged representation of an ice sheet that grows and retreats in response to orbital variations, CO 2 forcing, and meridional heat flux.…”