The research shows up to one third of Antarctica’s ice sheets melted during the Pliocene epoch around three million years ago, causing sea-levels to rise as much as 25 metres above present levels. Levels of carbon dioxide in the Earth’s atmosphere were similar to today’s levels and in response, temperature was two to three degrees Celsius warmer.
The study was led by Dr Georgia Grant, a recent Victoria University of Wellington PhD graduate now at GNS Science, and used a new method of analysing marine geological sediments to construct a global sea-level record.
Dr Grant developed the new method of determining the magnitude of sea-level change through analysing the size of particles moved by waves, as part of her PhD research. The method was applied to the geological archive from Whanganui Basin on the west coast of New Zealand’s North Island, which contains some of the best evidence anywhere in the world for global sea-level changes. Dr Grant was able to show that during the past warm period of the Pliocene about three million years ago, global sea levels regularly fluctuated between 5 to 25 metres.
The study, which was funded by the Royal Society Te Apārangi’s Marsden Fund, also involved Professor Tim Naish and Dr Gavin Dunbar from the University’s Antarctic Research Centre, as well as other scientists from GNS Science, Victoria University of Wellington and Waikato University, and from the Netherlands, the United States and Chile.
Professor Naish says, “This study has important implications for the stability of the Antarctic ice sheet and its potential to contribute to future sea-levels. If we do not keep our greenhouse gas emissions in line with the Paris Climate Agreement target of two degrees warming, then we may potentially lose not only the West Antarctic Ice Sheet, but also the vulnerable margins of the East Antarctic Ice Sheet.”
Dr Grant says of critical concern is that over 90 percent of the heat from global warming to date has gone into the ocean, and much of it into the Southern Ocean which surrounds the Antarctic ice sheet. One third of Antarctica’s ice sheet—equivalent to up to 20 metres sea-level rise—sits below sea-level and is vulnerable to widespread and catastrophic collapse from ocean heating. It melted in the past when atmospheric carbon dioxide levels were 400 parts per million (ppm), as they are today.
“Our new study supports the idea that a tipping point may be crossed, if global temperatures are allowed to rise more than two degrees, which could result in large parts of the Antarctic ice sheet being committed to melt-down over the coming centuries. It reinforces the importance of the Paris target,” says Dr Grant.
She says the study also has implications for computer-based ice sheet modelling.
“Our new sea-level estimates provide a target for testing the results from computer models and improving their ability to make accurate projections of the Antarctic contribution to global sea-level rise,” she says.
“This new research by Dr Grant and colleagues is consistent with model results that show long term ice sheet retreat under current carbon dioxide levels,” says Dr Nick Golledge, from the University’s Antarctic Research Centre. “The rate of sea-level change estimated from this study is also in line with our understanding of climate sensitivity and supports future predictions of one metre of sea level rise by 2100.”