(Image: © Garbe et al.)
The short quick answer is that on our current path a new study reveals that much of the Ice in Antarctica will melt and will raise sea level by many meters. It will take centuries, but it is inevitable if we take no action. For the first time in 30 million years Antarctica could be ice free and many the the costal cities we are familiar with will be gone.
Let’s take a closer look at the study.
Study: The hysteresis of the Antarctic Ice Sheet
Published in Nature on Sept 23, 2020, the paper reveals the future Antarctica.
The warmer it gets, the faster Antarctica loses ice – and much of it will then be gone forever. Consequences for the world’s coastal cities and cultural heritage sites would be detrimental, from London to Mumbai, and from New York to Shanghai.
That’s what a team of researchers from the Potsdam Institute for Climate Impact Research, Potsdam University and New York’s Columbia University has found out in their new study on how much warming the Antarctic Ice Sheet can survive. In around one million hours of computation time, their unprecedentedly detailed simulations delineate where exactly and at which warming levels the ice would become unstable and eventually melt and drain into the ocean. They find a delicate concert of accelerating and moderating effects, but the main conclusion is that unmitigated climate change would have dire long-term consequences: If the global mean temperature level is sustained long enough at 4 degrees above pre-industrial levels, Antarctic melting alone could eventually raise global sea levels by more than six meters.
“Antarctica holds more than half of Earth’s fresh water, frozen in a vast ice-sheet which is nearly 5 kilometers 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 2 degrees of warming, melting and the accelerated ice flow into the ocean will, eventually, entail 2.5 meters of global sea level rise just from Antarctica alone. At 4 degrees, it will be 6.5 meters and at 6 degrees almost 12 meters if these temperature levels would be sustained long enough.”
Ricarda Winkelmann, researcher at the Potsdam Institute for Climate Impact Research (PIK) and University of Potsdam, and corresponding author of the study.
Long-term change: it’s not rapid, but it’s forever
The paper’s title refers to the complex physical phenomenon of hysteresis. In this case, that translates into irreversibility.
“Antarctica is basically our ultimate heritage from an earlier time in Earth’s history. It’s been around for roughly 34 million years. Now our simulations show that once it’s melted, it does not regrow to its initial state even if temperatures eventually sank again. Indeed, temperatures would have to go back to pre-industrial levels to allow its full recovery – a highly unlikely scenario. In other words: What we lose of Antarctica now, is lost forever.”
Anders Levermann, co-author and researcher at PIK and Columbia University
The reasons behind this irreversibility are self-enforcing mechanisms in the ice sheets’ behavior under warming conditions.
“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.”
Co-author Torsten Albrecht
The fate of New York, Tokyo, Hamburg is in our hands
Ice loss and melting have accelerated significantly over the last decades in Antarctica. The authors however have explicitly not addressed the question of time scale in their work, but rather assess 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. 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.”
Ricarda Winkelmann, researcher at the Potsdam Institute for Climate Impact Research (PIK) and University of Potsdam, and corresponding author of the study.
Levermann adds: … “If we give up the Paris Agreement, we give up Hamburg, Tokyo and New York.“
(Side note: the above has been sourced by materials provided by the POTSDAM INSTITUTE FOR CLIMATE IMPACT RESEARCH (PIK)
It is just a simulation of Antarctica Ice melt
I should note that the paper is the result of a modelling simulation. There are other factors also in play. The Conclusion section within the paper itself does make this clear …
Our analysis reveals a strong, multi-step hysteresis behaviour of the Antarctic Ice Sheet, confirming that in certain regions ice loss is in fact irreversible even if temperatures were to return to colder levels. We focus here solely on the ice-internal feedbacks; however, it should be noted that additional feedback mechanisms such as the ice–albedo feedback, as well as, for instance, sea-level changes due to the effect of self-gravitation or due to a large amount of meltwater added to the ocean, could further dampen or amplify the ice sheet’s response to sustained warming. One such positive feedback in the ice–ocean interaction, which might lead to warm waters being trapped below the sea surface, recently received renewed attention.
Another potential positive feedback process, termed the marine ice-cliff instability, has only lately been brought to wider attention Underpinned by observations of rapid ice-shelf and glacier retreat it has been suggested that hydrofracturing might lead to high coastal ice cliffs that become mechanically unstable at heights exceeding 90 m above the waterline, eventually causing the disintegration of marine ice sheets . However, this effect is still disputed in the simulations presented here.
Although our approach encompasses the dominant processes and feedbacks relevant for studying the long-term response of the Antarctic Ice Sheet, fully interactive ice–atmosphere–ocean simulations are required to project the Antarctic ice evolution under future emission trajectories. In reality, temperatures have been changing and will prob- ably continue to change at much faster rates than considered here. Our results should thus not be confused with sea-level projections—we deliberately choose small forcing rates here because this allows us to identify critical temperature thresholds decisive for the overall stability of the Antarctic Ice Sheet. Our results imply that, if the Paris Agreement to limit global warming to well below 2 °C above pre-industrial temperatures is not met, one or more critical thresholds might be sub- sequently crossed in Antarctica, committing us to long-term, possibly irreversible, sea-level rise of up to dozens of metres
Stability of Antarctica – Tweets
It is the cover story in Nature …