AMHERST, Mass. – Authors of a new study that combine a well-established sea-level rise projection framework plus a model of Antarctic ice-sheet instability suggest in a paper released today that scientists won't be able to determine until the 2060s which of two different sea-level rise scenarios is most likely to occur.
Coastal communities should therefore have flexible contingency plans for a range of outcomes by the year 2100, say lead author Robert Kopp at Rutgers University and co-author Robert DeConto at the University of Massachusetts Amherst.
The authors say that one of the mechanisms they modeled, marine ice sheet instability, has been studied for decades, but another, marine ice cliff instability, has only recently been considered as an important contributor to future sea level change.
They point out that based on measuring large-scale phenomena like global sea level and Antarctic mass changes, it is not yet clear which of two scenarios will eventually unfold. The authors believe their study, which appears today in the journal Earth's Future, is the first to link global and local sea-level rise projections with simulations of two major mechanisms by which climate change can affect the vast Antarctic ice sheet.
DeConto, who with David Pollard at Penn State developed the ice sheet computer model used in the study, says, "The widespread loss of Antarctic ice shelves, driven by a warming ocean or warming atmosphere, could spell disaster for our coastlines, and there is sound geological evidence that supports what the models are telling us."
It's possible, they add, that a process called hydrofracturing, which may have helped to break up the Larsen B ice shelf on the Antarctic Peninsula in 2002, could leave much of the Antarctic coast with 300-foot cliffs of ice exposed to the open ocean and subject to collapse. The interaction between hydrofracturing and ice-cliff collapse could drive global sea level much higher than projected in the Intergovernmental Panel on Climate Change (IPCC)'s 2013 assessment report and in a 2014 study led by Kopp.
DeConto says, "We're making progress, but we still don't know exactly when these processes might kick in, and how fast sea level might rise if they do. The ice shelves are the key. They hold back the flow of Antarctic ice toward the ocean, so we don't want to lose them. The problem is, they don't last very long when they are sitting in warm water or if they are covered with summer meltwater, so keeping global temperatures in check is critical."
The Earth faces a range of possible outcomes with climate change. At the less severe end, 2 feet of global-average sea-level rise by 2100 would submerge land that's currently home to about 100 million people. Toward the high end, 6 feet of rise would swamp the current homes of more than 150 million. Either scenario would have drastic effects in New England and on other coastal states.
Kopp says, "There's a lot of ambiguity in post-2050 projections of sea-level rise and we may have to live with that for a while. We could end up with 8 feet of sea level-rise in 2100, but we're not likely to have clear evidence for that by 2050."
He and DeConto say that lower sea-level rise would be much more likely if the world meets the 2015 Paris Agreement goal of zero net greenhouse gas emissions in the next 50 years, the study shows. They also say cientists may also become able to distinguish between different scenarios sooner by studying the physics of local ice-sheet changes and refining reconstructions of changes during warm periods in geological history.
Kopp notes, "You should plan for 2050, while also considering what options to follow under more extreme scenarios after 2050. If we end up in a world with 2 or 2.5 meters (6.6 to 8 feet) of global sea level rise in 2100, that's a lot to adapt to. That necessitates taking a flexible approach, where possible: building for the half foot to 1.3 feet of sea-level rise that are likely by 2050, while plotting out options that will depend on what we learn in the next few decades and how sea level rises beyond that."