Mystery Glaciers Growing as Most Others Retreat

John Roach
for National Geographic News
June 22, 2009
Two South American glaciers are displaying strange behavior for the times: They're growing.

Most of the 50 massive glaciers draped over the spine of the Patagonian Andes are shrinking in response to a global warming, said Andrés Rivera, a glaciologist at the Center for Scientific Studies in Valdivia, Chile.
(Related: "Mountain Glaciers Melting Faster Than Ever, Expert Says.")

But the Perito Moreno glacier in Argentina and Pio XI glacier in Chile are taking on ice, instead of shedding it.

"What is happening … is not well understood," Rivera said.

Theories center on the geography and topography of the glaciers; the depth and temperature of the waters where the glaciers end; and how quickly, or slowly, they react to changes in the climate.

Yet overall, "if you account for the gains and losses of all of Patagonia's glaciers, they are [still] losing huge amounts of ice," Rivera pointed out.

Climate Insensitivity?

One hypothesis for the 3-mile-wide (5 kilometer-wide) Perito Moreno's advance is the glacier's apparent insensitivity to changes in what glaciologists call the equilibrium line on glaciers, Rivera said.

Roughly equivalent to the snow line, the equilibrium line is the elevation above which the glacier is growing, due to snow accumulation, and below which the glacier is melting.

When this line moves higher up a hill or a mountain due to rising temperatures, for example, more of the glacier is situated in the melting zone, and the glacier retreats.

But because Argentina's Perito Moreno glacier is so steep in the area where the equilibrium line falls, climate shifts don't impact the line's movement much, at least as it relates to the height of the mountain, Rivera noted.

As a result, the amount of of ice lost or gained is minimal.

"No Convincing Result"

It could also be that Perito Moreno simply hasn't got all that much to lose.

The lake where Perito Moreno ends—Lago Argentino—is shallower than the bodies of water at the ends of most glaciers.

Most glaciers calve, or release ice, in deep water, but not Perito Moreno, where the calving rates are higher than on other Patagonian glaciers.

That means less of the glacier is in the melting zone below the equilibrium line.

As heavy snowfall above the equilibrium line pushes the glacier downhill, the glacier breaks up when it hits the lake, Rivera explained.

Such impacts kept the glacier from growing longer when the climate was cooler, and thus more likely to expand, he said.

If Perito Moreno had extended into a deep lake area, it would have become a longer glacier, and Earth's recent warming trend would be causing the glacier to melt and its ice to retreat more easily, Richard Alley, a glaciologist at Pennsylvania State University in University Park, said in an email.

"Instead, we have a shorter glacier, with less [of a] zone where the warming can cause melting, but a large high-elevation [snow and ice] accumulation zone," Alley added.

As for the Pio XI glacier in Chile, some scientists have attempted to explain its advance as a glacial surge, a periodic and sudden expansion of a glacier that is little understood but is thought to be unrelated to external forces.

But the Chilean glaciologist, Rivera, said the evidence is inconclusive.

"At the end of the day, there is not a clear, convincing result for this research," he said.

"So, I am not sure why these glaciers are advancing."

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