Peru’s Kecheke Glacier is retreating due to global warming
Emilio Mateo/Aspen Global Change Institute
Andean glaciers are almost certainly smaller than they have been for at least the past 130,000 years, a study of rocks exposed by melting ice has found.
“Frankly, this surprised us,” says Andrew Golin of the University of California, Berkeley, “and we think it’s clear evidence that at least one part of the world has moved away from the comfortable climatic conditions that have fostered the development of human civilization.”
The Andes are so high that many permanent glaciers exist in the tropics. In fact, almost all of the world’s tropical glaciers are found in the Andes.
For decades it has been clear that global warming is causing these glaciers to thin and retreat, but it has been unclear how this compares to what happened in the more distant past.
Gorin and his colleagues analyzed 20 samples of rocks recently exposed by the retreat of four tropical glaciers in the Andes. They looked at carbon and beryllium isotopes, which form when exposed rocks are hit by cosmic rays, and can reveal when a glacier last retreated beyond a particular point.
Similar studies in the world’s north have found that glaciers were at their smallest thousands of years ago, in the middle of the current interglacial period, because changes in Earth’s orbit caused more sunlight in the north during winter, causing glaciers to retreat, Gorin said.
Although the northern glacial retreat during the Interglacial Period was a regional rather than global phenomenon, the researchers expected to find a similar phenomenon in the Andes at the time, but the levels of the isotype they found were so low they were barely detectable.
“This is a wake-up call,” Gorin said. “It’s like a canary in the coal mine for any mountain glacier.”
“We’re quickly passing climate milestones that we thought were decades away,” he says, “and we chose the specific locations on these glaciers that we sampled with the implicit assumption that these glaciers are smaller than they’ve ever been in human history.”
The results of the study directly show that these glaciers have never retreated as much as they are today in the past 11,700 years – prior to this point, the entire planet was in a global ice age, and work by other research teams has shown that the tropics were colder at that time.
Although the study does not say so, Gorin agreed when asked that this means Andean glaciers have shrunk to their smallest size since at least the last interglacial period, about 130,000 years ago.
“I would be willing to bet everything you say that these glaciers are currently at their smallest since the last interglacial period is true,” he says, “but the limitations of the techniques we used to address this problem mean we can’t definitively prove that’s the case, so we don’t say so in the paper.”
“This is a stunning study,” said Liam Taylor of the University of Leeds in the UK. “Without a doubt, Andean glaciers are in a state not seen since the Holocene epoch began 11,700 years ago, and the science shows conclusively that this is a direct result of climate-altering human activities.”
Taylor said the retreat of glaciers is already affecting agriculture, drinking water supplies, sanitation and hydroelectric power in the region because the glaciers act as reservoirs, storing snowfall in the winter and releasing meltwater in the summer.
“Many of the glaciers in the region are now past ‘peak water level,’ meaning that the meltwater that provides freshwater downstream is drying up,” he said.
Floods caused by glacial lakes bursting in Peru over the past century have killed tens of thousands of people, says Stephen Harrison of the University of Exeter in the UK, and more such disasters are likely around the world as mountain glaciers retreat.
Climate models predict that mountain glaciers will lose more than 90 percent of their ice by the end of the century, leaving only a few small glaciers in the highest regions, he says.
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