HISTORIC CO2 LEVEL WARNING

Historic CO2 level warning. Present CO2 levels caused 20-metre-sea-level rise in the past. Around three million years ago, during the Pliocene epoch, up to one third of Antarctica’s ice sheets melted, causing sea-levels to rise 20 metres. Levels of carbon dioxide in the Earth’s atmosphere were similar to today’s levels and in response, temperature increased by two to three degrees Celsius. These events from the geological past confirm the recent IPCC report on the oceans and the frozen parts of the planet. The findings were published on October 2nd 2019 in Nature by an international group of researchers, among them Dr. Paolo Stocchi from the Royal Netherlands Institute for Sea Research (NIOZ).

“These facts from the earth’s archives have 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 underwater parts of the East Antarctic Ice Sheet,” says Dr Stocchi.

Since the Industrial Revolution, 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 bathes the Antarctic ice sheet. One third of Antarctica’s ice sheet—its volume is equivalent to up to 20 metres global 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 ppm, as they are today. “If such amounts of ice were to melt in Antarctica over the next centuries, the reduced ice sheet will no longer attract as much water as now, and the sea level around Antarctica can even drop, while a large part of the ‘released’ water will move elsewhere. Due to the special role that gravity plays in the distribution of water over the globe, 30% can be added to the global average sea level rise in the northern hemisphere,” says Stocchi.

The publication in Nature is consistent with model results that show long term ice sheet retreat under current carbon dioxide levels. The rate of sea-level change estimated from this study also supports future predictions of one metre of sea level rise by 2100. Co-author Stocchi says the study also has implications for computer-based ice sheet modelling: “Our findings provide a historical point of reference for testing the results from computer models and improving their ability to make accurate projections of the Antarctic contribution to global sea-level rise.”

New methods to construct past sea-level changes

Dr Georgia Grant, first author, developed a 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 at the Antarctic Research Centre of Victoria University of Wellington (ARC/VUW, New Zealand). The method was applied to the geological archive from Wanganui Basin on the west coast of New Zealand’s North Island, which is one of the best places in the world to observe past global sea-level changes. Dr Stocchi computed how the Greenland and Antarctic Ice Sheets responded to warmer climates and modelled the response of global sea level under the influence of deformations of the Earth’s crust and gravitational and rotational changes.

The study is the result of an on-going international and multidisciplinary cooperation among the NIOZ Sea Level Research Centre, ARC/VUW (New Zealand), as well as other scientists from New Zealand, and from the United States and Chile.

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