Friday, May 7, 2021

Catastrophic Sea-Level Rise from Antarctic Melting Is Possible with Severe Global Warming

If Paris Agreement targets are not met, the collapse of melting Antarctic ice shelves – like the Wilkins Ice Shelf in 2009 – could cause catastrophic global sea level rise in the second half of the century. 

Image: NASA

The Antarctic ice sheet is much less likely to become unstable and cause dramatic sea-level rise in upcoming centuries if the world follows policies that keep global warming below a key 2015 Paris climate agreement target, according to a Rutgers coauthored study.

But if global warming exceeds the target – 2 degrees Celsius (3.6 degrees Fahrenheit) – the risk of ice shelves around the ice sheet’s perimeter melting would increase significantly, and their collapse would trigger rapid Antarctic melting. That would result in at least 0.07 inches of global average sea-level rise a year in 2060 and beyond, according to the study in the journal Nature.

That’s faster than the average rate of sea-level rise over the past 120 years and, in vulnerable coastal places like downtown Annapolis, Maryland, has led to a dramatic increase in days of extreme flooding.

Global warming of 3 degrees Celsius (5.4 degrees Fahrenheit) could lead to catastrophic sea-level rise from Antarctic melting – an increase of at least 0.2 inches per year globally after 2060, on average.

“Ice-sheet collapse is irreversible over thousands of years, and if the Antarctic ice sheet becomes unstable it could continue to retreat for centuries,” said coauthor Daniel M. Gilford, a post-doctoral associate in the Rutgers Earth System Science & Policy Lab led by coauthor Robert E. Kopp, a professor in the Department of Earth and Planetary Sciences within the School of Arts and Sciences at Rutgers University–New Brunswick. “That’s regardless of whether emissions mitigation strategies such as removing carbon dioxide from the atmosphere are employed.”

The Paris Agreement, achieved at a United Nations climate change conference, seeks to limit the negative impacts of global warming. Its goal is to keep the increase in global average temperature well below 2 degrees Celsius above pre-industrial levels, along with pursuing efforts to limit the increase to 1.5 degrees Celsius (2.7 degrees Fahrenheit). The signatories committed to eliminating global net carbon dioxide emissions in the second half of the 21st century.

Climate change from human activities is causing sea levels to rise, and projecting how Antarctica will contribute to this rise in a warmer climate is a difficult but critical challenge. How ice sheets might respond to warming is not well understood, and we don’t know what the ultimate global policy response to climate change will be. Greenland is losing ice at a faster rate than Antarctica, but Antarctica contains nearly eight times more ice above the ocean level, equivalent to 190 feet of global average sea-level rise, the study notes.

The study explored how Antarctica might change over the next century and beyond, depending on whether the temperature targets in the Paris Agreement are met or exceeded. To better understand how the ice sheet might respond, scientists trained a state-of-the-art ice-sheet model with modern satellite observations, paleoclimate data and a machine learning technique. They used the model to explore the likelihood of rapid ice-sheet retreat and the western Antarctic ice-sheet’s collapse under different global greenhouse gas emissions policies.

Current international policies are likely to lead to about 3 degrees Celsius of warming, which could thin Antarctica’s protective ice shelves and trigger rapid ice-sheet retreat between 2050 and 2100. Under this scenario, geoengineering strategies such as removing carbon dioxide from the atmosphere and sequestering (or storing) it would fail to prevent the worst of Antarctica’s contributions to global sea-level rise.

“These results demonstrate the possibility that unstoppable, catastrophic sea level rise from Antarctica will be triggered if Paris Agreement temperature targets are exceeded,” the study says.

Gilford said “it’s critical to be proactive in mitigating climate change now through active international participation in reducing greenhouse gas emissions and by continuing to ratchet down proposed policies to meet the ambitious Paris Agreement targets.”

Rutgers coauthors include Erica Ashe, a post-doctoral scientist in the Rutgers Earth System Science & Policy Lab. Scientists at the University of Massachusetts Amherst, Pennsylvania State University, University of California Irvine, University of Bristol, McGill University, Woods Hole Oceanographic Institution and University of Wisconsin-Madison contributed to the study.

Contacts and sources:
Rutgers University

Publication: The Paris Climate Agreement and future sea-level rise from Antarctica.
DeConto, R.M., Pollard, D., Alley, R.B. et al. Nature, 2021 DOI: 10.1038/s41586-021-03427-0

Antarctica Remains the Wild Card for Sea-Level Rise Estimates Through 2100

A massive collaborative research project covered in the journal Nature this week offers projections to the year 2100 of future sea-level rise from all sources of land ice, offering the most complete projections created to date.

The front of Getz Ice Shelf, Antarctica. 
Credit: Jeremy Harbeck, NASA Icebridge

“This work synthesizes improvements over the last decade in climate models, ice sheet and glacier models, and estimates of future greenhouse gas emissions,” said Stephen Price, one of the Los Alamos scientists on the project. “More than 85 researchers from various disciplines, including our team at Los Alamos National Laboratory, produced sea-level rise projections based on the most recent computer models developed within the scientific community and updated scenarios of future greenhouse gas emissions,” said Price.

The estimates show that limiting global warming to 1.5 degrees C above pre-industrial temperatures would cut projected 21st century sea-level rise from land ice in half, relative to currently pledged emissions reductions. For example, the paper notes that, when looking at all land ice sources, the median projection of cumulative rise in sea level by the year 2100 decreases from approximately 25 cm to approximately 13 cm when emissions are limited.

The term “land ice” includes mountain glaciers such as those in Alaska, Europe, high-mountain Asia, etc.; ice caps including those of Iceland and the Canadian Arctic; and ice sheets in Greenland and Antarctica.
Continental wild card

Interestingly, Price points out, Antarctica continues to be the wild card. “Future changes to Antarctica remain highly uncertain,” he said. “Because of this, our high-end estimates for sea-level rise from land ice are more than twice as large as the ‘most likely’ estimate.” This is largely due to substantial uncertainty in how strongly warm ocean waters erode floating parts of the ice sheet from beneath.

Apart from that uncertainty, the bulk of the Antarctic sea-level rise projections do not show a strong sensitivity to different emissions scenarios, but a small number of projections result in an up to five-fold increase in sea-level contribution, Price said. Indeed, improving DOE’s ability to accurately simulate Southern Hemisphere climate and Antarctic ice sheet evolution has been a focus of Los Alamos efforts for more than a decade.

Present day Antarctic ice flow as simulated by the MALI ice sheet model. Interior grey-to-white areas indicate slow flowing regions while warmer colored areas indicate faster flowing regions (dark red ~10 meters/day). Large orange-to-red regions near the ice sheet margins are floating ice shelves. Fine lines indicate the path of ice flow from the interior, through outlet glaciers and ice streams, and into fringing ice shelves.
Credit: jointly developed by Los Alamos and Sandia national laboratories, and simulation image by John Patchett of LANL. 

Contacts and sources:
DOE/Los Alamos National Laboratory

Publication: Projected land ice contributions to twenty-first-century sea level rise.
Tamsin L. Edwards, Sophie Nowicki, Ben Marzeion, Regine Hock, Heiko Goelzer, Hélène Seroussi, Nicolas C. Jourdain, Donald A. Slater, Fiona E. Turner, Christopher J. Smith, Christine M. McKenna, Erika Simon, Ayako Abe-Ouchi, Jonathan M. Gregory, Eric Larour, William H. Lipscomb, Antony J. Payne, Andrew Shepherd, Cécile Agosta, Patrick Alexander, Torsten Albrecht, Brian Anderson, Xylar Asay-Davis, Andy Aschwanden, Alice Barthel, Andrew Bliss, Reinhard Calov, Christopher Chambers, Nicolas Champollion, Youngmin Choi, Richard Cullather, Joshua Cuzzone, Christophe Dumas, Denis Felikson, Xavier Fettweis, Koji Fujita, Benjamin K. Galton-Fenzi, Rupert Gladstone, Nicholas R. Golledge, Ralf Greve, Tore Hattermann, Matthew J. Hoffman, Angelika Humbert, Matthias Huss, Philippe Huybrechts, Walter Immerzeel, Thomas Kleiner, Philip Kraaijenbrink, Sébastien Le clec’h, Victoria Lee, Gunter R. Leguy, Christopher M. Little, Daniel P. Lowry, Jan-Hendrik Malles, Daniel F. Martin, Fabien Maussion, Mathieu Morlighem, James F. O’Neill, Isabel Nias, Frank Pattyn, Tyler Pelle, Stephen F. Price, Aurélien Quiquet, Valentina Radić, Ronja Reese, David R. Rounce, Martin Rückamp, Akiko Sakai, Courtney Shafer, Nicole-Jeanne Schlegel, Sarah Shannon, Robin S. Smith, Fiammetta Straneo, Sainan Sun, Lev Tarasov, Luke D. Trusel, Jonas Van Breedam, Roderik van de Wal, Michiel van den Broeke, Ricarda Winkelmann, Harry Zekollari, Chen Zhao, Tong Zhang, Thomas Zwinger. Nature, 2021; 593 (7857): 74 DOI: 10.1038/s41586-021-03302-y