Ancient Antarctica reveals a ’one–two punch’ behind ice sheet collapse

Rising ocean temperatures may play more of a role in melting ice sheets than global warming — with the worst effects experienced on the other side of the globe.

A University of Leicester geoscientist has contributed to a new paper in Nature Geoscience, ‘Spatially variable response of Antarctica’s ice sheets to orbital forcing during the Pliocene’, that explores the complicated dynamics.

The study’s results directly address one of the main goals of the International Ocean Discovery Program (IODP) Expedition 374: to identify the sensitivity of the Antarctic ice sheet to Earth’s orbital configuration under a variety of climate boundary conditions. 

The research considers the Antarctic ice sheet during the late Pliocene epoch, from 3.6 to 2.6 million years ago. From 3.2 to 2.8 million years ago, the global average temperatures were around 2 to 3° Celsius higher than pre-industrial values, in line with the “middle of the road” scenario for climate change, in which temperatures are expected to rise around 2.7°C by 2100.

Binghamton University Associate Professor of Earth Sciences Molly Patterson is the first author. “Thus, Pliocene records are considered to be useful analogues for understanding what a future with this level of warming might be like,” Patterson explained.

Climate forcing refers to any external factor that causes a change in Earth’s energy balance —incoming versus outgoing heat — and ultimately leads to warming or cooling in the Earth system.

Non-human factors that can affect this energy balance include tectonic changes, volcanic eruptions and shifts in the sun’s energy output, such as sunspot cycles that happen every 11 years. Another factor is “orbital forcing,” or changes in Earth’s orbit around the sun; this has typically driven glacial and interglacial cycles, which have lasted around 100,000 years — at least for the last 800,000 years or so.

The non-human factors that affect the Earth’s climate occur on different time scales, Patterson said.

“Here we are using geological archives to test how these important components of the climate system respond naturally to warmer climates,” she said.

Antarctica is primarily divided into two sectors: West Antarctica, where the ice sheet sits in the ocean, and East Antarctica, where the ice sheet primarily sits on land. During the warm periods of the Pliocene, large parts of West Antarctica and some low-lying areas of East Antarctica experienced significant ice-melt, contributing to a 1-3 m rise in global sea levels.

One of the study’s main conclusions: Under warming conditions similar to the Pliocene, the part of West Antarctica located adjacent to the Pacific Ocean will see its ice disappear at a faster rate. Over the long term, however, both oceanic and atmospheric warming will contribute to rising global sea levels.

“In other words, it’s a one–two punch on the system with a consequence of raising sea levels globally,” Patterson said.

Dr Tim van Peer from the University of Leicester School of Geography, Geology and the Environment contributed to the findings after participating on the research vessel JOIDES Resolution during IODP Expedition 374 to the Ross Sea. His primary duties were to co-determine the age of sediments, which showed that the East and West Antarctic ice sheets have two different heartbeats. The West Antarctic heartbeat is much faster than the East Antarctic heartbeat, because it is closer to the ocean, therefore melting quicker than the East Antarctic ice sheet.

Dr van Peer said: “Our study emphasises that the West Antarctic Ice Sheet is sensitive to ocean warming. These geological archives capture past ice sheet variability and we use them to test the accuracy of climate models projecting future melting scenarios.”

What you may not realize: Because of gravitational effects similar to ocean tides, the loss of ice in the Southern Hemisphere actually has a greater impact on coastlines in the Northern Hemisphere. Conversely, when ice sheets lose mass in the Northern Hemisphere, Southern Hemisphere coastlines are affected more.

With that in mind, New York or London would be more affected by rising sea levels from the loss of Antarctic ice than a similar rise from melting ice sheets in Greenland, Patterson pointed out.

Geological archives and modelling experiments provide the long-term context needed to evaluate current changes and help scientists identify the mechanisms that drive the climate system. Ultimately, this research may help us formulate more accurate predictions about our climate change future.