Summary:
Allowing global temperatures to rise above 2°C compared to pre-industrial significantly increases the risk of triggering unstoppable Antarctic ice sheet retreat, destabilization and multi-meter sea level rise. If current Paris Agreement commitments, resulting in nearly 3°C of warming by 2100 are not made more ambitious, this threshold of no return will be reached already in 2060; resulting in a sharp jump in the pace of Antarctic ice loss, contributing 0.5 cm or 5 mm of sea-level rise per year by 2100 from Antarctica alone. (By comparison, today’s rate of SLR from all sources – land glaciers, both ice sheets and thermal expansion — is around 4 mm/year.)
These conclusions are from an extensive Nature paper released today. Once begun, the study found that ice loss from Antarctica and associated sea-level rise continues for centuries, leading to as much as 10 meters sea-level rise from Antarctica by 2300 if today’s high emissions levels continue unabated.
With current Paris commitments (which would result in between 2.5-3°C of warming), rates of sea-level rise are slowed, but remains irreversible once 2°C is breached around mid-century. Antarctic ice loss continues – even with CO2 levels rapidly returning to preindustrial with geo-engineered carbon dioxide reductions. The resulting temperature decrease was not sufficient to halt massive ice sheet loss, due in part to ocean warming that persisted even once atmospheric temperatures went down, preventing re-formation of buttressing ice shelves.
Authors urge policymakers to understand the importance of keeping temperatures firmly in check with the Paris Agreement targets of 1.5-2°C, without overshoot; and to raise their ambition to urgently reduce emissions by 2030 in line with this necessary temperature limit.
Main Messages:
The Antarctic Ice Sheet is more sensitive to warming, with greater potential for catastrophic global impacts from sea-level rise than Greenland.
Antarctica holds almost 8 times more potential sea-level rise than Greenland, with enough ice to raise global sea levels by 58 meters (190 ft). The configuration of the two polar ice sheets also differs, with most of Antarctica’s ice sheet terminating directly in the ocean and held back by ice shelves: floating platforms extending out from the continental ice. These ice shelves buttress and provide stability to the entire Antarctic ice sheet, but are extremely sensitive to atmospheric and ocean warming. Once ice shelves start retreating as temperatures rise, several mechanisms further accelerate their destabilization, eventually leading to complete ice shelf collapse. With the loss of protective ice shelves, the ice sheet accelerates its speed and discharge into the surrounding ocean. In addition, a third of the Antarctic ice sheet rests on bedrock that is actually below sea level, in some regions by 1000 meters or more, sloping downwards from the ice sheet edge. As ice at the margins disappears, warming water can then flow downhill underneath the ice sheet, eating at it from below and further accelerating its loss into the ocean.
Following low emissions scenarios is the only solution to drastically reduce impacts on low-lying coastlines and populations.
Under the 1.5°C and 2°C scenarios, assuming no overshoot of these temperatures, and that these temperatures then remain constant, ice loss from Antarctica would continue at a pace similar to today’s throughout the 21st century, and likely at the same rate for many centuries beyond. This slower loss would contribute about 8-9 cm additional global sea-level rise from Antarctica by 2100. Paleo-climatic records indicate multi-meter sea-level rise even at constant temperatures of 1.5° and 2°C, of around 6-9 meters and 11-20 meters, respectively; but at rates over thousands of years, allowing far more time for adaptation. However, the study records a large acceleration in ice loss at 3°C above pre-industrial, triggered by the destabilization of the entire West Antarctic Ice Sheet. Sea-level rise would then reach 0.5cm/yr (5 mm/yr) from Antarctica alone, compared with 4mm from all sources today; so total sea-level rise from Antarctica, Greenland, land glaciers and thermal expansion might exceed 1 cm (10mm) per year before the end of the century. With emissions continuing at today’s high levels however, in so-called “business as usual” scenarios, Antarctica could contribute more than 1 m of sea level rise by 2125, with rates exceeding 6 cm (600 mm) per year by 2150; and totalling nearly 10 meters by 2300 from Antarctica alone.
Delaying mitigation and overshooting 1.5°C would have major and irreversible consequences.
Every decade lost to delayed geo-engineered carbon dioxide removal (CDR – negative emissions) has substantial long-term consequences on sea-level rise from Antarctica. If CDR were to start at any point later than 2060 and beyond 2°C of warming, Antarctic ice sheet loss still continues for centuries, despite the return to lower temperatures as CO2 levels come down. Once the ice shelves collapse, and marine-based portions of the Antarctic ice sheet become destabilized above 2°C, extensive ice sheet loss becomes unstoppable. Rapid cooling and return to pre-industrial temperatures will not be able to reverse this situation (and note, that the level of modelled CDR is higher than what most experts consider technologically feasible at this time). Overshooting 1.5° or 2°C , and relying on CDR to bring down temperatures after mid-century will therefore not avert catastrophic long-term sea-level rise from Antarctica. Instead, emissions must decrease significantly from today’s commitments in order to prevent global temperatures rising more than 2°C, and preferably held below 1.5°C.
Negative feedbacks will not significantly slow the pace of ice loss.
With increased melting and calving, Antarctica will release large amounts of freshwater into the Southern Ocean. While some have speculated this might slow warming, by this cold freshwater cooling both the air and surrounding ocean, this study found it would slow the pace of Antarctic ice loss only after 2125, well after the 2060 threshold for irreversible loss. Other postulated negative “slowing” feedbacks associated with dynamics such as expanding crevasses on the surface of ice shelves, or a rebounding bedrock as the ice sheet melts have minimal impacts on the rate of ice loss over at least the next 200 years. In short, these negative Antarctic feedbacks would kick in long after the point of no return has passed. The only known and reliable means to halt or at least slow ice sheet loss is emissions reductions at levels that will hold temperatures consistently within the 2°C Paris Agreement limit.
https://www.nature.com/articles/s41586-021-03427-0
By Amy Imdieke, Global Outreach Director, and Pam Pearson, Director of ICCI.
Published May. 4, 2021 Updated Jul. 12, 2022 3:24 pm
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