Reconstructing the evolution of ice sheets, sea level, and atmospheric CO2 during the past 3.6 million years

Abstract

Abstract. Understanding the evolution of, and the interactions between, ice sheets and the global climate over geological time is important for being able to constrain earth system sensitivity. However, direct observational evidence of past CO2 concentrations only exists for the past 800\u2009000 years. Records of benthic δ18O date back millions of years, but contain signals from both land ice volume and ocean temperature. In recent years, inverse forward modelling has been developed as a method to disentangle these two signals, resulting in mutually consistent reconstructions of ice volume, temperature and CO2. We use this approach to force a hybrid ice-sheet – climate model with a benthic δ18O stack, reconstructing the evolution of the ice sheets, global mean sea level and atmospheric CO2 during the late Pliocene and the Pleistocene, from 3.6 million years (Myr) ago to the present day. During the warmer-than-present climates of the Late Pliocene, reconstructed CO2 varies widely, from 320–440\u2009ppmv for warm periods such as Marine Isotope Stage (MIS) KM5c, to 235–250\u2009ppmv for the MIS M2 glacial excursion. Sea level is relatively stable during this period, with a high stand of 6–14\u2009m, and a drop of 12–26\u2009m during MIS M2. Both CO2 and sea level are within the wide ranges of values covered by available proxy data for this period. Our results for the Pleistocene agree well with the ice-core CO2 record, as well as with different available sea-level proxy data. During the early Pleistocene, 2.6–1.2\u2009Myr ago, we simulate 40\u2009kyr glacial cycles, with interglacial CO2 decreasing from 280–300\u2009ppmv at the beginning of the Pleistocene, to 250–280\u2009ppmv just before the Mid-Pleistocene Transition (MPT). Peak glacial CO2 decreases from 220–250\u2009ppmv to 205–225\u2009ppmv during this period. After the MPT, when the glacial cycles change from 40\u2009kyr to 80/120\u2009kyr cyclicity, the glacial-interglacial contrast increases, with interglacial CO2 varying between 250–320\u2009ppmv, and peak glacial values decreasing to 170–210\u2009ppmv.