Alaska’s heaviest glaciers are approaching an irreversible tipping point

Melting of one of North America’s largest ice fields has accelerated and could soon reach an irreversible tipping point. That’s the conclusion new research colleagues and I have published about the Juneau Ice Field, which straddles the Alaska-Canada border near the Alaskan capital of Juneau.

In the summer of 2022, accompanied by other researchers, I skied across the flat, smooth and white plateau of the ice field, gliding in the footsteps of the person in front of me under the hot sun. About 40 huge, interconnected glaciers descend from this plateau to the sea, with hundreds of smaller glaciers on mountain tops all around.

Our work, now published in The nature of communication, showed that Juneau is an example of a climate “feedback” in action: as temperatures rise, less and less snow remains over the summer (technically: the “end-of-summer snow line” rises). This in turn leads to the ice being exposed to sunlight and higher temperatures, which means more melting, less snow, and so on.

Like many Alaskan glaciers, Juneau is summit heavy, with lots of ice and snow at high elevations above the late summer snow line. This used to keep the glacier tongues lower. But when the snowline creeps up to the summit plateau in late summer, suddenly large amounts of heavy glaciers are newly exposed to melting.

This 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 sink lower and lower. Once a threshold is exceeded, these feedbacks can accelerate melting and lead to a permanent loss of snow and ice that would continue even if the world stopped warming.

The ice is melting faster than ever

Using satellites, photographs, and old rock piles, we were able to measure ice loss on the Juneau Ice Field from the end of the last “Little Ice Age” (about 250 years ago) to the present. We saw that the glaciers began to shrink after this cold period ended around 1770. This ice loss remained constant until 1979, when it accelerated. In 2010, it accelerated again and doubled the previous pace. Glaciers there shrank five times faster between 2015 and 2019 than between 1979 and 1990.

Our data shows that as snow falls and the summer melt season lengthens, the ice field darkens. Fresh white snow is very reflective and much of the strong solar energy we experienced in the summer of 2022 is reflected back into space. But in late summer, the snowline rises and is now often found directly on the Juneau Icefield plateau, meaning older snow and glaciers are 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 surface area decreases. As a result, it will become increasingly difficult for the ice field to ever stabilize or even recover. This is because warmer air at low altitudes drives further melting, leading to an irreversible tipping point.

Longer-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 the glacier will behave in the future.

The hardest puzzle in the world

We used satellite records 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 on glacial till, and where glaciers have cleaned and polished the bedrock.

To check and follow up on our mapping, we spent two weeks on the ice field itself and two weeks in the rainforest below. We camped between moraine ridges, hanging food high in the air to protect it from bears, yelling to warn off moose and bears as we made our way through the rainforest, and fighting off mosquitoes thirsting for our blood.

We used aerial photographs to reconstruct the ice field in the 1940s and 1970s, before satellite imagery was readily available. These are high-quality photos, but they were taken before global positioning systems made it easy to locate where they were taken.

Some of them have also been slightly damaged over the years – some Sellotape, tears, thumbprints. As a result, the individual images had to be stitched together to create a 3D image of the entire ice field. It was all like doing the hardest puzzle in the world.

Work like this is crucial because the world’s glaciers are melting rapidly – ​​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 shows how significant this acceleration is. Understanding how and where “feedbacks” cause glaciers to melt even faster is essential to better predict future changes in this important region.

This article is republished from The Conversation under a Creative Commons license. Read the original article.