Abstract. Understanding the evolution of, and the interactions between, ice sheets and
the global climate over geological timescales is important for being able to
project their future evolution. However, direct observational evidence of past
CO2 concentrations, and the implied radiative forcing, only exists
for the past 800 000 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 ppmv for warm periods to
235–250 ppmv for the early glacial excursion ∼3.3 million years
ago. Sea level is relatively stable during this period, with maxima of
6–14 m and minima of 12–26 m during glacial episodes. 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. For the Early Pleistocene,
2.6–1.2 Myr ago, we simulate 40 kyr glacial cycles, with
interglacial CO2 decreasing from 280–300 ppmv at the
beginning of the Pleistocene to 250–280 ppmv just before the
Mid-Pleistocene Transition (MPT). Peak glacial CO2 decreases from
220–250 to 205–225 ppmv during this period. After the MPT, when the
glacial cycles change from 40 to 80 120 kyr cyclicity, the
glacial–interglacial contrast increases, with interglacial CO2
varying between 250–320 ppmv and peak glacial values decreasing to
170–210 ppmv.
The Current State and 125 Kyr History of Permafrost in the Kara Sea Shelf: Modeling Constraints
