Andean rocks, no longer covered in ice, show that the world’s glaciers are melting much faster than expected
Rocks recently exposed to the sky after being covered in prehistoric ice show that tropical glaciers have shrunk to their smallest size in more than 11,700 years, revealing that the tropics have already warmed beyond limits last seen in the early Holocene era, Boston College researchers report in the journal Science.
Scientists had predicted that glaciers would melt, or retreat, as temperatures warmed in the tropics, the regions bordering the Earth’s equator. But analysis of rock samples adjacent to four glaciers in the Andes mountain range show that glacier retreat has been much faster and has already surpassed an alarming inter-epoch benchmark, said Jeremy Shakun, an associate professor of Earth and environmental sciences at Boston College.
“We have pretty strong evidence that these glaciers are smaller now than they have been at any time in the past 11,000 years,” said Shakun, a paleoclimatologist and co-author of the report. “Since modern glacier retreat is driven mostly by rising temperatures — as opposed to decreased snowfall or changes in cloud cover — our findings suggest that the tropics have already warmed outside their Holocene range and into the Anthropocene.”
In other words, the glaciers can no longer be classified as being from the Holocene interglacial period, a major epoch that saw the birth of civilization, when water flow and sea level dictated where towns and cities formed, and where agricultural and commercial activity emerged. Instead, they may be better classified as belonging to an epoch that may be about to spell its end: the Anthropocene.
The findings suggest that more of the world’s glaciers are likely retreating much faster than previously predicted, possibly decades earlier than predicted by climate science.
“This is the first major region on the planet where we have strong evidence that glaciers have crossed this important benchmark – it’s a ‘canary in the coal mine’ for glaciers around the world,” Shakun said.
Glaciers have retreated worldwide over the past century, but until now the magnitude of this retreat compared to natural fluctuations over the past millennia was unclear, Shakun said. The team set out to determine the current size of tropical glaciers compared to their extent over the past 11,000 years.
Perito Moreno Glacier
The researchers who formed the international team of scientists traveled to Colombia, Peru and Bolivia to measure the chemistry of bedrock recently discovered in front of four melting glaciers spanning the tropical Andes. Two rare isotopes — beryllium-10 and carbon-14 — accumulate on bedrock surfaces when they are exposed to cosmic radiation from outer space, Shakun said.
“By measuring the concentrations of these isotopes in newly exposed bedrock, we can determine how long ago the bedrock was exposed, which tells us how often glaciers were smaller than they are today — much like a sunburn tells us how long someone has been in the sun,” Shakun explained.
Shakun led the project with Andrew Gorin, a former graduate student from British Columbia, and partnered with researchers at the University of Wisconsin and Tulane University on the American Cordillera project, then sought samples and data from colleagues at Aix-Marseille University, the National University of Ireland, the Aspen Global Change Institute, Ohio State University, Union College, Grenoble Alpes University and Purdue University.
“We found no beryllium-10 or radiocarbon-14 in any of the 18 bedrock samples we measured in front of four tropical glaciers,” said Gorin, now a PhD student at UC-Berkeley. “That tells us there was never significant prior exposure to cosmic radiation since these glaciers formed during the last ice age.”
Twenty years ago, researchers at Peru’s Quelccaya Ice Cap, the world’s largest tropical ice sheet, found rooted plant remains melting into the ice margin as it retreated. Radiocarbon dating showed the plants were 5,000 years old, indicating that Quelccaya had been larger than its size at the time of the study throughout that interval; otherwise, the plants would have decayed if there had been a prior period of exposure, Shakun said.
The Quelccaya findings suggest that modern ice retreat has been abnormally large, but not yet at an alarming level compared with the melting of the entire Holocene, Shakun said. He and his team wanted to study a larger number of glaciers and use a technique that could unambiguously show whether a glacier was ever smaller than it is today.
Shakun and his colleagues have applied the same technique to glaciers across the American Cordillera, from Alaska to Tierra del Fuego. The team published the results of their North American sampling last year and plans to publish results from southern South America soon.
“Once we’ve done that, all these studies can be brought together into a global perspective on the current state of glacier retreat,” Shakun says.
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Image: A researcher collects a bedrock sample from the Queshque Glacier in the Peruvian Andes. The samples show that tropical glaciers have retreated to their smallest size in more than 11,700 years, based on measurements of cosmogenic nuclides in the newly exposed bedrock. Credit: Emilio Mateo, Aspen Global Change Institute
