A team of scientists said Tuesday that one of North America's largest interconnected glaciers is melting twice as fast as it did before 2010, a move they called a “hugely worrying” sign that land ice in many areas could be disappearing much faster than previously thought.
Researchers estimate that the Juneau Ice Sheet, which spans the coastal mountains of Alaska and British Columbia, lost 1.4 cubic miles of ice per year between 2010 and 2020. That’s a dramatic acceleration from the previous few decades and even more rapid than in the mid-20th century or earlier, the scientists said. Overall, the ice sheet has lost a quarter of its volume since the late 18th century, part of a period of glacial expansion known as the Little Ice Age.
Bethan Davis, a glaciologist at Newcastle University in the U.K. and lead author of the study, said that as society continues to pump more and more of the planet-warming carbon dioxide into the atmosphere, many areas could pass a tipping point where glaciers will melt at a rapid rate.
“The more carbon we can take out of it, the more hope we have of keeping these wonderful icebergs,” Dr. Davis said. “The more carbon we put in, the greater the risk that it will be irreversible and completely removed.”
The fate of Alaska’s ice is of enormous global importance. No other region on Earth is projected to have a greater impact on global sea level rise this century from melting glaciers.
The Juneau Icefield covers 1,500 square miles of rugged terrain north of the state capital of Juneau. The area has become warmer and wetter over the past half-century, causing glaciers to melt longer and have less snow to replenish them.
The ice sheet contains 1,050 glaciers, at least as of 2019.
To reconstruct how the ice has evolved over the past two and a half centuries, Dr. Davis and her colleagues combined decades of ice measurements with information from satellite images, aerial photography, maps and surveys. They looked to studies of tree rings and peat to understand past environments. They also went out onto the ice to confirm what they saw from satellites.
The changes they discovered were far-reaching.
Scientists found that between 1770 and 2019, all of the ice sheet glaciers retreated. More than 100 glaciers disappeared completely. As the glaciers melted and the water pooled, nearly 50 new lakes were formed.
Scientists also found that the rate of volume loss in the ice sheet slowed somewhat in the mid-20th century, then accelerated after 1979, and then accelerated even further after 2005.
Scientists say this acceleration may be related to how the whiteness of the ice, or albedo, as glaciologists call it, affects melting, and vice versa. With less snow, more rock and boulders are exposed in the ice. This dark surface absorbs more solar radiation, causing the surrounding ice to thin more quickly. Tourism and wildfires also deposit soot and dust on the glacier surface, further accelerating the melting rate.
Another factor, Dr. Davis and her colleagues say, is that as the ice sheet thins, more of its surface area lies at lower elevations. This exposes a large, flat surface to warmer air, causing it to thin more quickly.
Scientists have known that glacier melt is affected by these kinds of self-reinforcing feedbacks, said Martin Trooper, a physicist at the University of Alaska Fairbanks who was not involved in the new study. But models of glacier change overall still don’t fully incorporate this physical complexity, Davis said. “If you want to know how these ice sheets are going to behave, the physics have to be realistic,” she said.
Still, she added, science is advancing rapidly. Last year, researchers made predictions about how all the Earth's glaciers will evolve depending on what humans do or don't do about global warming.
The conclusions were not encouraging, but the scientific achievement was significant: According to the projections, even if countries meet the Paris Agreement goal of limiting warming to 1.5 degrees Celsius above pre-industrial conditions, about half of the world’s glaciers, or about 104,000, could disappear by 2100.