Utrecht University: 45 million years of Antarctic temperature change- lessons for the future

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An international team of scientists used molecular fossils and machine learning to build the first charts of Antarctic Ocean temperatures over the past 45 million years, offering important insights into the mechanisms driving temperature changes and into the future of the Antarctic ice sheet and sea level changes. The results suggest we are nearing a tipping point where ocean warming caused by atmospheric CO2 will cause a major rise in sea levels because of melting ice sheets.

The team was able to reconstruct the temperatures of the ocean when ice sheets grew and shrank during that period. The future loss of ice sheets and the retreat of glaciers in the Antarctic is critically important as melting ice in the region could cause sea levels to rise by several metres.

Fossil molecules
The study, published on September 15 in Nature Geoscience, was led by scientists from Victoria University of Wellington (NZ), GNS Science (NZ), and Birmingham University (UK). The team analysed molecular fossils from a compilation of outcrops and ocean core sediment samples taken during several ocean drilling projects. By studying these samples, scientists can draw conclusions about the temperature of the ocean when those fossil molecules deposited. “We have obtained a 45 million years Antarctic Ocean temperatures record, which offers a robust overview of the variability of these temperatures and how they relate to changes in the amount of ice and the topography of Antarctica. Our results pave the way for improved estimates of future trends” explain the co-authors Francesca Sangiorgi and Frida Hoem from the Department of Earth Sciences, Utrecht University.

Threshold
The trends of ocean temperatures, atmospheric CO2, and the amount of ice in Antarctica mostly co-vary through the last 45 million years. If a retreat is generally amplified by warmth, one surprising finding was that surface ocean cooling did not always correspond to increases in Antarctic ice. Specifically, for a period of about 1 million years in the late Oligocene (25 to 24 million years ago). “We concluded that tectonic subsidence of West Antarctica favoured the influx of still relatively warm ocean waters in the Ross Sea region precluding the advance of a marine-based ice sheet as we know it now. Only 1 million year later, when the ocean cooled enough, and at low (below 400 ppm) atmospheric CO2 concentrations, the ice sheet could expand” says Sangiorgi. “Our results support the hypothesis of a threshold response to atmospheric CO2 below which Antarctica marine ice sheet grows and above which ocean warming exacerbates their retreat.” The researchers concluded that marine ice sheets cannot persist when atmospheric CO2 exceeds this 400 ppm value, a threshold of atmospheric CO2 that we surpassed in 2013.

Long-term stability
This study highlights the importance of climatic and non-climatic drivers on the long-term stability of the Antarctic ice sheets and the fundamental role of ocean temperatures, and offers a lesson for the future. An increasingly warm ocean, caused by rising atmospheric CO2, will cause catastrophic melting of those portions of the Antarctic ice sheets that are marine-based, and lie below sea level. Marine-based ice sheets alone can contribute 20 m sea level rise upon complete melting. “We can see that ice in Antarctica is currently changing – not least with the loss of some ice-shelves and cracks appearing recently in the Thwaites Glacier, one of the largest glaciers in the region. This new study of Earth’s past is one of the clearest indications yet that humans continue to produce CO2 levels for which we can expect major ice loss at the Antarctic margins and global sea-level rise over the coming decades and centuries.”

The work was funded and facilitated by the International Ocean Discovery Program, Antarctica New Zealand, The Royal Society Te Apārangi Marsden Fund (NZ), The Natural Environment Research Council (UK), the US National Science Foundation award and the Dutch Research Council (NWO) polar programme. Support in kind was provided by the University of Birmingham, Utrecht University, Yale University and the Royal Netherlands Institute for Sea Research (NIOZ).

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