21 July, 2013 – Vulnerability of the large East Antarctic Ice Sheet to rising temperatures

 Background: Concentrations of the greenhouse gas carbon dioxide have surpassed a level of 400pm in May 2013, a concentration that has not been observed in the earth’s atmosphere for a long, long time. To find similarly high concentrations, we have to go beyond times scales that we, or our ancestors, can directly study and observe. Instead we have to use the geological past and go all the way back to a time called the Pliocene epoch (2.6-5.3 million years ago). The Pliocene is the most recent time in the geological past when atmospheric carbon dioxide levels were similar to today. We can hence use it as a natural laboratory to study mechanisms and consequences of climate change. An interesting observation from studying the Pliocene is that global temperatures during the warmer parts of this epoch were 2-4°C warmer than today, in the range of temperatures predicted for the end of this century, and that global sea level was around 20 m higher than today. These conditions are considered natural consequences of the climate system, equilibrating at higher atmospheric carbon dioxide levels.  
Picture of the vast East Antarctic Ice Sheet from our expedition. Credit: Saiko Sugisaki.

What we did: My PhD student Carys Cook was the lead author on a study recently published in the journal Nature Geoscience, co-authored by many of our colleagues from IODP Expedition 318. For her work, Carys took the Pliocene sediments from one of our drillcores (Site U1361), and analysed the mud in it for its chemical composition. Any rock that is eroded from the continent carries a typical chemical fingerprint. In marine sediments we can use this chemical fingerprint of the continentally-derived mud to trace back where on the continent the material was derived from (i.e. the ‘provenance’ of the material ). Doing so for the Pliocene section at IODP Site U1361 we found that during particularly warm intervals a signature was obtained, which is today not exposed on the proximal Antarctic margin, but is instead hidden deep under the ice sheet. This in turn means that significant retreat of the ice sheet must have happened to allow Pliocene erosion and deposition of this particular bedrock type. Combining these insights with previously published results from ice sheet models, we could constrain that repeated retreat of the East Antarctic ice sheet during the Pliocene must have contributed at least between 3 and 10 m of global sea level, on top of the contributions from Greenland and West Antarctica.

Pliocene mud from the seafloor off Antarctica. Photo credit: Sandra Passchier.

 Why does this matter? The reason why this matters is that our new drill core was derived from a unique location, right in from of the Wilkes Subglacial Basin. The Wilkes Subglacial Basin is one of a number of basins underneath the vast East Antarctic Ice Sheet, where the ice is sitting on bedrock below sea level. The significance of the sub-ice topography shown below comes from the fact that so-called ‘marine-based’ sections of an ice sheet are in direct contact with the ocean (e.g. they do not rest on bedrock elevated well above sea level). This means that when temperatures rise, the ice is not only affected by the temperatures in the air, but additionally by the warmer ocean temperatures, which facilitates melting. Processes like this can today be observed for the smaller West Antarctic ice sheet. Our data however show, that also the subglacial basins of the East Antarctic ice sheet have responded to warming temperatures and elevated carbon dioxide levels in the Pliocene, and hence may be quite vulnerable to future warming.

Left: Topography of the Antarctic continent if all ice would be removed. Right: Thickness of the Antarctic ice.

What’s next? The next big step for Carys is to defend her PhD thesis later this summer. After this, both of us are going to tackle the question how fast the East Antarctic ice sheet may have reacted to changing environmental conditions in the Pliocene. We will also turn our attention to study the smaller West Antarctic ice sheet to figure out under which envrionmental conditions in the past it became unstable. This happened presumable more recently than during the Pliocene, but we do not really know when and how.

Carys at work.
To learn more about our work, check out the News and Views article in Nature Geoscience, written by our colleague Claus-Dieter Hillenbrand from the British Antarctic Survey, the news item in Science Magazine, and the Imperial College Press Release. Coverage in the press include among others articles in National Geographic, NBC, Daily Mail, and CS Monitor.

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