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Mountain glaciers will lose their insulating air layer

A natural cooling mechanism has been shielding the world’s mountain glaciers from increasingly warm summer melt seasons, but new research by an international science team says the protective process will break down by mid-century.

Glaciers at that time will be fully susceptible to the warming overlying atmosphere, though the timing and extent of that response will vary by glacier.

The team’s work will improve modeling of how glaciers respond to a variety of atmospheric warming projections.

The team, which includes research assistant professor Pascal Buri of the University of ����ֱ�� Fairbanks Geophysical Institute, analyzed melt-season data from 350 weather stations covering 169 summer melt seasons on 62 glaciers. 

Results from the first-of-its-kind global-scale review of how mountain glaciers interact with the atmosphere were published Oct. 10 in . 

Thomas Shaw of the Institute of Science and Technology Austria is the lead author, with Buri among the eight co-authors from Austria, ����ֱ��, Switzerland, Italy and the United Kingdom.

The research includes five ����ֱ�� glaciers: Susitna Glacier, its east and west forks, and Maclaren Glacier, all in the ����ֱ�� Range; and McCall Glacier in the Brooks Range. McCall Glacier is the only Arctic ����ֱ�� glacier with a long-term mass balance record.

“What we understand much better now is how glaciers die, in a way,” Buri said.

“We now know there is a transition from a self-protecting cooling mechanism to a recoupling directly to the atmosphere,” he said. “And we know how this breakdown of the protecting boundary layer accelerates the demise of many mountain glaciers.”

The current near-surface temperature of mountain glaciers, when considered as a group, is 17% cooler than the ambient temperature.

That means glaciers can partially protect themselves from the atmosphere above and surrounding them — for now.

Projections under United Nations moderate and severe climate warming scenarios indicate that glaciers will reach their maximum self-cooling ability in the 2020s and 2030s. Mountain glaciers will then permanently lose the ability to cool themselves, leading to accelerated ice loss, the international team writes.

The science

The research focuses on the decoupling and recoupling of a mountain glacier’s near-surface air temperature with that of the overlying atmosphere. The strength and timing of a glacier’s annual melt cycle is influenced by the degree of decoupling or recoupling.

Decoupling is the self-cooling process currently protecting glaciers from the warming climate.

In a decoupled setting, a thermal boundary layer colder than the overlying atmosphere protects the glacier by slowing the summer melt season ice loss. This layer, which doesn’t extend to the surrounding terrain, can be a few meters to tens of meters thick and is formed and sustained by katabatic winds, masses of cold air that form at higher elevations over ice or snow and are pulled downhill by gravity.

“Glaciers are non-living things but can protect themselves,” Buri said. “And that’s quite fascinating.”

In a recoupled environment, increased turbulence in the overlying atmosphere, driven by warming or other weather changes, allows more heat to penetrate and break down the boundary layer. Temperature is generally uniform at a given elevation, whether on the glacier, at its edges or elsewhere in the area.

The glacier becomes susceptible to increased melting and shrinks until its mass is insufficient to generate the katabatic winds necessary to reform the protective boundary layer. Retreat accelerates and becomes irreversible under current and projected climate conditions.

A science oversight

The glaciological community hasn’t paid much attention to decoupling, Buri said, so the extent of self-cooling among the studied glaciers was unexpected. He previously believed the glaciers, collectively, had mostly lost their protective boundary layers.

The belief led Buri and other glaciologists to overestimate the atmosphere’s direct impact.

“This surprised me that we hadn’t passed this peak of maximum decoupling fully yet and that the boundary layers in many places were still intact, though weakening,” he said.

The findings have broad implications.

Current models don’t show boundary layers disappearing in mid-century when glaciers will recouple with the atmosphere. Such recoupling will further accelerate the demise of glaciers, the international research team projects.

Improving regional-scale glacier models by including processes such as decoupling and recoupling will increase accuracy of glacier mass loss predictions and related projections of meltwater runoff volume, sea level rise and impacts on ecosystems and infrastructure.

“We know about the committed mass loss of mountain glaciers under the current climate at the global scale,” Buri said. “Research, unveiling processes like this one, helps to understand that every fraction of a degree of temperature increase matters for the health of mountain glaciers.”

ADDITIONAL CONTACT: Pascal Buri, pburi@alaska.edu

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