"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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