Climate state dependence of ice sheet variability

Cenozoic climate has evolved through stepwise quasi-equilibrium states in response to declining CO₂ concentration. As a result, terrestrial polar ice sheets developed in Antarctica ~35 million years ago describing relatively large glacial-interglacial changes, prior to an increasing marine-based ice sheet component by ~15 Ma with lower glacial-interglacial variability, before returning to large glacial-interglacial amplitudes in response to the intensification of Northern Hemisphere Ice Sheets (~2.7 Ma). While mean surface temperature scales linearly with the total concentration of carbon in the atmosphere, this is not the case for past variations in global mean sea-level whose amplitudes are climate-state (CO₂)-dependent. By examining past climate drivers (atmospheric CO₂) and the response of ice volume (sea level), polar ice sheets are seen to demonstrate vastly different sensitivities under changing climate states highlighted by the ‘100-kyr’ problem of non-linear ice sheet change.

In this study, a new independent global ice volume (sea-level) record (X-PlioSeaNZ: 3.3 – 1.7 Ma) is used to evaluate the deep-sea oxygen isotope proxy record (δ¹⁸O_benthic).  An empirical, power-law relationship emerges between δ¹⁸O_benthic and sea-level in contrast to long-standing linear δ¹⁸O_benthic calibrations. This relationship suggests relatively higher deep-ocean temperature contribution to δ¹⁸O_benthic signal and correspondingly lower global ice volume estimates under warmer past climates. It also demonstrates the need for variable ice volume-δ¹⁸O_benthic calibrations in response to the evolving bipolar ice sheet geographies over the last ~3 million years (Myr). Consequently, as the Earth system adjusts to 2-3°C of global warming over the coming decades and centuries, a lower paleo-ice sheet sensitivity (compared to the Last Glacial Maximum) is expected for ice sheet configurations where marine based ice sheets act as a buffer to terrestrial based ice sheets and brings geologic reconstructions into agreement with current projections for future sea-level rise.