The Columbia glacier is smaller than it’s been for 900 years Ethan Welty/Getty

A major glacier in Alaska has retreated to its lowest point in 900 years as a consequence of global warming.

Glaciers around the world are in retreat. But the Columbia glacier is one of the most dramatic and well-documented cases, as well as the largest contributor to sea level rise out of the 50 or so glaciers that descend to the sea in Alaska.

Anders Carlson at Oregon State University and his colleagues have put the current ebb in the historical context of the past millennium, during which human contributions to climate change weren’t always so high.


In 2004, the team bored down into the mud at the bottom of Prince William Sound, the bay on the southern coast of Alaska that the glacier flows into, and examined the layers of sediment deposited over a period of about 1600 years.

The glacier recently receded past a geological fault line, either side of which are rocks of differing magnetic and chemical compositions. So Carlson and his colleagues looked for a corresponding shift in the sediment to find the last time the glacier crossed this fault.

Time capsules

The number and thickness of rings in the trunks of trees uncovered in the retreat of the glacier provided a timeline for the initial advance of the ice, and gave information about past climate conditions. By cross-referencing the sediments and tree data, the team found a matching point about 900 years ago.

Running climate simulators revealed that summer air temperatures about 1°C higher than normal between 1910 and 1980 led to the glacier thinning until it became unstable in the 1980s and triggered the rapid retreat over the past three decades. The team attributes that warming to human-caused climate change.

“What was surprising was the tight coupling between surface temperature of the glacier and its response,” says Carlson.

Trouble ahead

Linking the behaviour of glaciers that terminate in water with climate is difficult, especially in areas with tectonic activity. But the team’s approach has allowed them to do just that, says Julie Brigham-Grette at the University of Massachusetts, Amherst. “Because of the coupling of the tree rings and the sediments, they can make the case that this retreat is a response to temperature, and not the internal dynamics of the glacier,” she says.

The result is of wider importance, says Chris Rapley at University College London. “It shows that a small temperature increase of less than 2°C is sufficient to destabilise a glacier,” he says. International efforts to fight climate change are focused on limiting warming to 2°C, but are widely thought not to be sufficient to achieve that limit.

“Previous analyses have speculated that the warming acted as a trigger for the mechanical processes of the retreat, and this analysis provides evidence that this is the case,” Rapley says.

It’s unlikely that this is an isolated case. Alberto Reyes at the University of Alberta in Canada, one of the study’s authors, says that at some sites around the world, retreating glaciers are exposing trees that are some 7000 years old, indicating that those glaciers are now smaller than they have been in many thousands of years.

Journal reference: Geology, DOI: 10.1130/G38479.1

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