Sea-Level Rise From Melting Land Ice May Double If Paris Pact Fails
(Bloomberg) -- Current national climate goals are so weak that even with them, melting glaciers and ice sheets are on track to raise sea levels twice as fast as they would if countries fulfilled their pledges under the Paris Agreement, says a new study published today in the journal Nature.
If nations meet only the targets in place today, land-based ice may contribute 25 centimeters to global oceans by 2100. If they fully uphold the international climate treaty, which calls for limiting warming to between 1.5° Celsius to 2°C, that estimate falls to 13cm.
Eighty-four scientists from 15 countries took part in the study, assembling hundreds of simulations of melting land ice from 19 far-flung regions, including Greenland and Antarctica, into a single global projection. Their analysis is consistent with the growing scientific consensus about where the world is headed. A better future is within reach, the authors conclude, but only if countries raise their ambitions. Melting from Greenland would fall off by 70% and melting from glaciers by 50%, for instance, if the Paris Agreement goals are met.
As meltwater flows away from glaciers, it contributes to the swelling tides that are already lapping against cities, beaches, and coasts worldwide. “Coastal flooding will still increase, but less severely if we manage to limit warming to 1.5ºC,” said Tamsin Edwards, a climate researcher at King’s College London and the study’s lead author.
Current global pledges to cut emissions and energy use are projected to lead to more than 3°C of warming by 2100, Edwards said. In lower probability scenarios, that estimate goes as high as 5°C, which would be utterly catastrophic. Considerable melting is already underway. The ocean has risen about 9 inches since 1880, about half of which is from water expanding as it warms, while a quarter each comes from the Greenland and Antarctica ice sheets and the land-based glaciers elsewhere. Sea-level rise since the early 1990s now accounts for a third of the historic total, a period that saw an acceleration in the rate of change.
Scientists’ models for projecting climate change have held up over the years. Recent work, including this paper, attempts to bring greater precision to mathematical visions of the future. To do so requires capturing all manner of simultaneous, complex Earth system processes that can either cancel each other or reinforce each other or both—no simple task for researchers.
“Greenland is really sensitive to atmospheric changes, and so basically in a warmer world, you get more melting along the surface of the ice sheet,” said Sophie Nowicki, a professor at the University of Buffalo and a co-author of the Nature study. “In Antarctica, it’s very complex because a warmer world could mean more snowfall, but it could also mean more melt at the side of the ice sheet.”
Warmer air holds more water vapor, which leads to more snowfall and eventually ice. But warmer water also eats away at ice sheets from beneath, accelerating melt and instability. Scientists can’t say for sure yet which process will dominate in the decades ahead, and different modeling groups produce different projections. The most dire of them project that ice sheet melt there will contribute 42cm to sea-level rise this century.
Antarctica is under assault from above, too. Ozone depletion and greenhouse gas pollution are strengthening polar winds, according to a paper published last week by researchers at the University of East Anglia in the U.K. That’s increasing melt rates on Larsen C, an ice shelf located on the Antarctic peninsula that has the continent’s highest surface melt rate.
In the current issue of Nature, another, smaller team of scientists led by Robert DeConto, a director of the School of Earth & Sustainability at the University of Massachusetts Amherst, wrote up the results of an Antarctic ice sheet model not included in the larger study. DeConto’s group found that if warming is limited to between 1.5ºC and 2ºC, Antarctic ice loss will continue at a pace similar to today’s throughout this century. Under a 3°C warming scenario—the path the world may currently be on, the researchers say—their model showed an abrupt jump in the pace of ice loss after 2060, contributing about 0.5cm annually to the rise of sea levels globally by 2100. Sea levels currently rise at an average rate of about 0.36cm per year. Edwards said that this paper’s results are mostly consistent with her study looking at multiple models.
On a 3°C pathway, “resulting ice loss would be irreversible on multi-century timescales, even if atmospheric temperatures return to preindustrial-like values,” DeConto and co-authors wrote. Warming of 1.5°C to 2°C “would have much less impact on low-lying coastlines, islands, and population centers, pointing to the importance of ambitious mitigation.”
©2021 Bloomberg L.P.