One of the largest areas of interconnected glaciers in North America is melting twice as fast as before 2010, a team of scientists reported Tuesday, calling it an “exceptionally troubling” sign that land ice could be disappearing even faster in many places than previously thought.
The Juneau Ice Field, which stretches across the Coast Mountains of Alaska and British Columbia, lost 1.4 cubic miles of ice per year between 2010 and 2020, the researchers estimate. That’s a sharp acceleration from decades before, and even sharper compared with the mid-20th century or earlier, the scientists said. All told, the ice field has lost a quarter of its volume since the late 1700s, part of a period of glacier expansion known as the Little Ice Age.
As societies add more carbon dioxide to the atmosphere that drives global warming, glaciers in many areas could reach tipping points where they begin to melt more quickly, said Bethan Davies, a glaciologist at the University of Newcastle in England who led the new research.
“If we reduce carbon, we have more hope of preserving these beautiful ice masses,” Dr Davies said. “The more carbon we add, the greater the risk that we remove them irreversibly and completely.”
The fate of Alaska’s ice is of enormous importance to the world. In no other region of the planet is melting glaciers predicted to contribute more to global sea level rise this century.
The Juneau Ice Field covers 1,500 square miles of rugged terrain north of Juneau, the state capital. The region has become warmer and rainier over the past half century, meaning the glaciers have a longer melt season and less snow to replenish them.
The ice field contains 1,050 glaciers. Or at least in 2019.
To reconstruct how the ice has evolved over the past two and a half centuries, Dr Davies and her colleagues combined decades of glacier measurements with information from satellite images, aerial photographs, maps and surveys. They looked at tree-ring and peat studies to understand the past environment. They also went out onto the ice themselves to verify what they saw from the satellites.
The changes they discovered are dramatic.
Scientists found that all of the ice field’s glaciers retreated between 1770 and 2019. More than 100 glaciers disappeared completely. Nearly 50 new lakes formed as glaciers melted and water pooled.
The scientists also found that the rate at which the ice field lost volume slowed somewhat in the mid-20th century. It increased after 1979, and then accelerated further after 2005.
This acceleration, the scientists said, could have to do with the way the whiteness of the ice — its albedo, as glaciologists call it — affects melting and vice versa. As snowfall decreases, more rocks and boulders are exposed in the ice. These darker surfaces absorb more solar radiation, causing the ice around them to thin even faster. Tourism and wildfires also deposit soot and dust on the glacier surface, further accelerating melting.
Another factor, Dr Davies and her colleagues say, is that as the ice field thins, more of it will be at a lower altitude. This exposes more of its broad, flat surface to warmer air, causing it to thin even faster.
Scientists are aware that glacier melt is influenced by this kind of self-amplifying feedback, said Martin Truffer, a physicist at the University of Alaska Fairbanks who was not involved in the new research. In general, models of glacier change still don’t capture enough of these physical complexities, Dr. Davies said. “If you want to know how this ice field is going to behave, you want to know that the physics is realistic,” she said.
Still, she added, science is advancing rapidly. Last year, researchers published predictions of how every glacier on Earth will evolve, depending on what humanity does, or doesn’t do, about global warming.
The scientific achievement was significant, even if the conclusion was not encouraging. According to projections, even if countries meet the Paris Agreement goal of limiting warming to 1.5 degrees Celsius above pre-industrial conditions, about half the world’s glaciers, some 104,000 in total, could be gone by 2100.