NEW YORK/POTSDAM: A new study of the Antarctic Ice Sheet concludes that a sustained level of global warming at four degrees Celsius above pre-industrial levels will produce an irreversible rise in global sea levels by over six metres.

According to report authors from the Potsdam Institute for Climate Impact Research (PIK), Potsdam University and New York’s Columbia University, if governments give up the Paris Agreement the world’s coastal cities will disappear.

The team said it deliberately avoided putting a time frame on when the cities would be flooded and instead spent one million hours of computing time assessing the critical warming levels at which parts of the Antarctic Ice Sheet become unstable.

"In the end, it is our burning of coal and oil that determines ongoing and future greenhouse-gas emissions and therefore, if and when critical temperature thresholds in Antarctica are crossed,” explained Ricarda Winkelmann, PIK researcher and one of the study’s authors. “And even if the ice loss happens on long time scales, the respective carbon dioxide levels can already be reached in the near future. We decide now whether we manage to halt the warming.

“So Antarctica's fate really lies in our hands – and with it that of our cities and cultural sites across the globe, from Rio de Janeiro's Copacabana to Sydney's Opera House. Thus, this study really is another exclamation mark behind the importance of the Paris Climate Accord: Keep global warming below two degrees."

Antarctica holds more than half of Earth's fresh water, frozen in a vast ice-sheet nearly five kilometres thick. As the surrounding ocean water and atmosphere warm due to human greenhouse gas emissions, the white cap on the South Pole loses mass and eventually becomes unstable.

“Because of its sheer magnitude, Antarctica's potential for sea-level contribution is enormous: We find that already at two degrees of warming, melting and the accelerated ice flow into the ocean will, eventually, entail 2.5 metres of global sea level rise just from Antarctica alone. At four degrees it will be 6.5 metres and at six degrees almost 12 metres if these temperature levels would be sustained long enough," continued Winkelmann.

The frozen continent is roughly 34 million years old. According to Anders Levermann, co-author and researcher at PIK and Columbia University, once the ice is melted it won’t freeze back to a pre-melt state due to the physical phenomenon of “hysteresis” - or self-enforcing mechanisms in the ice sheets' behaviour under warming conditions.

Co-author Torsten Albrecht explained: "In West Antarctica for instance, the main driver of ice loss is warm ocean water leading to higher melting underneath the ice shelves, which in turn can destabilize the grounded ice sheet. That makes glaciers the size of Florida slide into the ocean. Once temperatures cross the threshold of six degrees above pre-industrial levels, effects from the ice surface become more dominant: As the gigantic mountains of ice slowly sink to lower heights where the air is warmer, this leads to more melt at the ice surface – just as we observe in Greenland."

"If we give up the Paris Agreement, we give up Hamburg, Tokyo and New York," added Levermann.

‘The hysteresis of the Antarctic Ice Sheet’ published in Nature by authors Julius Garbe, Torsten Albrecht, Anders Levermann, Jonathan F. Donges and Ricarda Winkelmann (09-2020).