Melting of one of North America’s largest ice fields has accelerated and may soon reach an irreversible tipping point. This is the conclusion young research colleagues and I have published on the Juneau Icefield, which straddles the Alaska-Canada border near Alaska’s capital, Juneau.
In the summer of 2022, I skied across the flat, smooth, white plateau of the ice field, accompanied by other researchers, sliding in the footsteps of the person in front of me under a hot sun. From that plateau, about 40 large interconnected glaciers descend toward the sea, with hundreds of smaller glaciers on the surrounding mountaintops.
Our work, now published in Nature Communications, has shown that Juneau is an example of a climate “feedback” in action: as temperatures rise, less and less snow remains during the summer (technically: “late summer snow” yes This in turn leads to the ice being exposed to the sun and higher temperatures, which means more melting, less snow, and so on.
Like many Alaskan glaciers, Juneau’s are very heavy, with lots of ice and snow at high elevations above the snow line in late summer. This formerly held the tongues of the glaciers down. But when the late summer snowline creeps up to the upper plateau, then suddenly a large amount of a very heavy glacier will be newly exposed to melting.
This is what is happening now, every summer, and the glaciers are melting much faster than before, causing the ice field to become thinner and thinner and the plateau to become lower and lower. Once a threshold is crossed, these feedbacks can accelerate melting and drive a steady loss of snow and ice that would continue even if the world stopped warming.
The ice is melting faster than ever
Using satellites, photos, and old rock piles, we were able to measure ice loss along the Juneau Icefield from the end of the last “Little Ice Age” (about 250 years ago) to the present day. today. We saw that the glaciers began to shrink after the end of that cold period around 1770. This ice loss remained constant until around 1979, when it accelerated. It accelerated again in 2010, doubling the previous rate. Glaciers there shrank five times faster between 2015 and 2019 than they did from 1979 to 1990.
Our data shows that as the snowpack decreases and the summer melt season lengthens, the ice field is darkening. Fresh, white snow is highly reflective, and much of the strong solar energy we experienced in the summer of 2022 is reflected back into space. But the bottom of the summer snowpack is growing, and now it’s happening frequently right on the plateau of the Juneau Icefield, which means old snow and glacier ice are being exposed to the sun. These slightly darker surfaces absorb more energy, increasing the melting of snow and ice.
As the ice field plateau thins, ice and snow reserves at higher elevations are lost and the plateau area decreases. This will make it increasingly difficult to stabilize or even recover the ice field. This is because the warmer air at low altitudes drives further melting, leading to a tipping point of no return.
Long-term data like these are critical to understanding how glaciers behave and the processes and tipping points that exist within individual glaciers. These complex processes make it difficult to predict how a glacier will behave in the future.
Joining parts of the most difficult figure in the world
We used satellite data to reconstruct how big the glacier was and how it behaved, but that really limits us to the last 50 years. To go back further, we need different methods. To go back 250 years, we mapped moraine ridges, which are large piles of debris deposited at the mouth of a glacier and places where glaciers have scoured and polished rock.
To check and build on our map, we spent two weeks in the ice field and two weeks in the rainforest below. We camped among moraine ridges, suspending our food high in the air to keep it safe from bears, yelling to warn off moose and bears as we bushwhacked through the rainforest and fought off mosquitoes thirsting for our blood. .
We used aerial photographs to reconstruct the ice field in the 1940s and 1970s, in the era before available satellite imagery. These are high-quality photos, but they were taken before global positioning systems made it easy to locate exactly where they were taken.
A number also had some minor damage over the years – some Sellotape, a tear, a fingerprint. As a result, the individual images had to be stitched together to make a 3D view of the entire ice field. Everything was like doing the hardest puzzle in the world.
Work like this is essential as the world’s glaciers are melting fast – all together they are currently losing more mass than the Greenland or Antarctic ice sheets, and the rate of thinning of these glaciers worldwide has doubled over the past two decades.
Our longer time series show how sharp this acceleration is. Understanding how and where “feedbacks” are causing glaciers to melt even faster is essential to making better predictions of future changes in this important region.
Bethan Davies, Senior Lecturer in Physical Geography, University of Newcastle. This article is republished from The Conversation under a Creative Commons license. Read the original article.