Scientists on Wednesday reported yet another reason why the world should curtail global warming: It would likely reduce half the current projected sea level rise from ice melting this century.
In a study that averaged the results of hundreds of computer simulations by research teams around the world, scientists said limiting warming to 1.5 degrees Celsius could reduce sea level rise from melting glaciers and the vast ice sheets of Greenland and the United States Antarctica could reduce from about 10 inches to about 10 inches by five by 2100.
This warming, equivalent to 2.7 degrees Fahrenheit, is the stricter of two goals set in the 2015 Paris Agreement to combat climate change.
But the world has warmed 1.1 degrees Celsius since 1900 and is not on track to hit the 1.5 degree target or even the higher Paris target of 2 degrees Celsius.
In a second study published in the same journal, Nature, a separate group of scientists studying only Antarctica found that those goals had been exceeded and a warming of 3 degrees Celsius was achieved – which is the world is roughly on track given current emissions reduction commitments – could trigger an abrupt increase in the smelting rate around 2060, and by the end of the century it would be ten times faster than it is today.
Together, the two studies are the latest best predictions for one of the key effects of climate change: rising oceans causing more flooding, forcing costly coastal infrastructure overhauls, and potentially creating millions of climate refugees.
The studies are also the most recent, highlighting both the benefits of reducing warming by drastically reducing emissions of carbon dioxide and other heat scavenging gases, and the dangers of ignoring them.
“You can conclude from both studies that mitigation and emission reduction are important in raising sea levels,” said Peter U. Clark, a geologist at Oregon State University who studies sea levels but was not involved in either study.
The melting of the ice currently accounts for about half of the rise in sea level; Almost all of the rest is due to the expansion of the warming seawater. A special report released in 2019 by the United Nations Intergovernmental Panel on Climate Change found that total sea level rise by 2100 could be anywhere from one foot to more than three feet, depending on global warming.
It is difficult to model how land ice will behave as temperatures rise, especially for the huge, thick ice sheets in Greenland and Antarctica. These studies show that the models are improving, said Dr. Clark, although uncertainties remain.
These uncertainties are especially great when it comes to Antarctica, which, if completely melted over many centuries, would raise the oceans several hundred feet. While the first research team, led by Tamsin Edwards, a climate researcher at King’s College, London, found that the potential ice contribution to sea level rise could be halved, researchers also examined a smaller group of the simulations in which Antarctica was more sensitive to it Warming.
In analyzing these more pessimistic results – but the kind that might be useful to policymakers and others who need to plan to protect costly ports, power plants, or other infrastructure – the researchers found that even if the world succeeds, the stricter To achieve the Paris goal, the contribution The rise of the Antarctic ice to sea level could be much higher.
Dr. Edwards said instead of reducing the total ice contribution to sea level rise from 10 inches to 5 inches by 2100, the more pessimistic results suggested the contribution could be 16 inches. And even if warming were limited to 1.5 degrees, there would be a 5 percent chance that melting ice would add nearly two feet to the oceans.
Overall, the results of the first study, which is expected to be included in the next Intergovernmental Panel on Climate Change assessment report, expected next year, are similar to previous reports. This is an indication that the uncertainties decrease as the models improve.
“It’s not that there are brand new numbers wildly higher or wildly lower for sea level rise,” she said of the work. “It’s much more about being able to unpack what our insecurity is and where it comes from.”
The second, Antarctic-focused study was the work of a team led by Robert M. DeConto of the University of Massachusetts and David Pollard of Pennsylvania State University. The two led a 2016 study that sparked intense debate within the expert community dealing with ice and sea levels because it included another mechanism by which Antarctica could cause sea level rise relatively quickly.
Under this mechanism, known as sea ice cliff instability, warmer temperatures over Antarctica would result in more surface ice melting. The water produced would intensify the breaking in the ice and could eventually lead to the rapid disintegration of thick ice shelves if icebergs break off into the water. That would accelerate the flow of the inner ice to the ocean.
Most sea level experts still do not include the mechanism in their models. In the Dr. Edwards-led study did not use any models.
Dr. DeConto said his new study builds on previous work. “It represents all of the improvements we’ve made in trying to computer model these complex systems,” he said. Regarding the instability of the ice cliffs, “We still think this is a process to be reckoned with,” he added.
If the warming is limited to 2 degrees Celsius, the ice sheet could change itself, according to Dr. DeConto continues to behave as it does now.
“When we jump into a world that reaches 3 degrees by 2100, things really start to change,” he said.
A key difference between this work and the 2016 study is when these changes occur. The new study shows that they will start later, around 2060 or 2070, when the surface melting in Antarctica increases to the point where the instability of the ice cliffs sets in.
And, as with all predictions about the impact of melting ice on sea level rise, melting will continue long after the end of the century. In conditions where greenhouse gas emissions remain high, “things are really starting to increase in the next century,” said Dr. DeConto.
Daniel M. Gilford, who contributed to the study as a postdoctoral fellow at Rutgers University, said the process would be essentially irreversible once the Antarctic ice shelves became unstable. Even if drastic steps were taken – such as efforts to remove carbon dioxide from the atmosphere – warming would remain at levels where enough new ice could not build up to replace what is lost.
This reinforces the idea that urgent measures to reduce emissions are now needed.
“There is no ‘Get out of Melting Free’ card,” said Dr. Gilford, who now works for the research organization Climate Central. “You need to reduce greenhouse gases now and in the near future if we are to avoid these long-term effects.”