Antarctica during past warm periods

During the last 250 years, atmospheric CO2 concentrations have risen well above preindustrial levels (~280 ppm) and exceeded even the highest interglacial CO2 levels of the last few hundred thousand (!) years. Because we are rapidly shifting to a climate state that is historically unprecedented, past warm periods are crucial analogs to understand the global environmental changes that are occurring now and in the future. To find a past period that might resemble future conditions, is therefore necessary to look back in time millions of years. Studying these past warm periods in ‘deep time’ improves our estimates of the rate and amount of global sea level rise we project for the coming century. My research reconstructs past ice sheet extent and contributions to global sea level during these past warm periods.

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Atmospheric CO2 concentrations and surface temperature anomalies reconstructed for the past 60 million years and into the future (modified from Burke et al., 2018)

Antarctic climate and ice-sheet evolution in the mid-Miocene

Although the warm mid-Miocene is a key future analog, it occurred so long ago that geologic data are sparse and important factors such as CO2 and mountain uplift remain relatively unconstrained. Researchers can run models, but without geologic data to check the models against, it’s difficult to choose which simulation is correct. Conversely, researchers can extrapolate from geologic data, but such data points offer only local snapshots, not a wider climatic context. Therefore, I use both models and data together! I ran a suite of model simulations of the Antarctic ice sheet using a variety of different possible mid-Miocene CO2 concentrations and mountain uplift scenarios. Model simulations were then benchmarked using geologic records of paleoclimate, such as past temperature, vegetation, and glacial proximity. These data-constrained model runs allowed us to make inferences about which CO2 and tectonic model scenarios satisfy the known geologic constraints, and thereby extrapolate continent-wide glacial conditions.

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Suite of model snapshots representing different CO2 and tectonic scenarios (Transantarctic Mountain – TAM – elevation adjustments). Warm ‘interglacial’ simulations on the upper rows have a warm astronomical configuration, and ‘glacial’ simulations on the bottom row have a cold astronomical configuration.

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Collecting paleoclimatic data to benchmark the model simulations in the Friis Hills, McMurdo Dry Valleys (2016 field camp; photo credit: Richard Levy)

Ruthie with colleague Hannah Chorley (Friis Hills, McMurdo Dry Valleys, 2016)

Reconciling dynamic marine ice sheet fluctuations with deep freeze in the McMurdo Dry Valleys

This project integrated geologic data and numerical models to address a long-standing debate about the stability of the Antarctic ice sheet during past warm periods. Marine data (drill core data and seismic surveys) show marine-based ice sheet fluctuations since the mid-Miocene, suggesting that the Antarctic ice sheet is sensitive to relatively small temperature fluctuations and was smaller during past warm periods. Conversely, well-preserved landforms on ice-free land (in the McMurdo Dry Valleys) require continuous cold-desert conditions across the same time period, prompting the interpretation that a stable Antarctic ice sheet has persisted across multiple past warm periods and therefore may be less susceptible to future climate warming. I ran climate model simulations over the Dry Valleys during past warm conditions, and showed that high-elevation regions of the Dry Valleys can remain freezing despite open ocean (no ice) in the nearby Ross Sea. Therefore, Dry Valleys landforms do not necessarily require ‘stable’ Antarctic ice sheet behavior: in model simulations conducted under past warm conditions, persistently cold temperatures in the Dry Valleys can occur even when the marine-based West Antarctic Ice Sheet has collapsed.

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Most cold-desert preserved landforms in the McMurdo Dry Valleys (MDVs) are found at high elevation, generally overlapping with modeled 'cold zones' from 280 ppm and 460 ppm climate model simulations. In these model-based 'cold zones', even the warmest summer temperatures remain well below freezing despite a receded or collapsed West Antarctic Ice Sheet (WAIS). The modern WAIS configuration is shown in the inset map.

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Glacial cyclicity of the West Antarctic Ice Sheet during the Pliocene

In addition to the mid-Miocene warm period, I also study Antarctic Ice Sheet dynamics during the warm Pliocene. My research investigates Antarctica’s contribution to global sea level across Pliocene glacial/interglacial cycles. This modeling work is part of a large scientific collaboration stemming from IODP Expedition 379 to the Amundsen Sea; I am using ice sheet models to reconstruct orbital-scale ice-sheet evolution, using ocean temperature reconstructions from the IODP drill core data to force model simulations across millions of years.

Learn more about the IODP 379 expedition from onboard expedition scientists and staff: