Rocks exposed to the sky after being covered in prehistoric ice show that tropical Andean glaciers have shrunk to their smallest size in more than 11,700 years.
The finding reveals that the tropics have already warmed beyond limits last seen in the Holocene geological era, Boston College researchers report in the journal Science.
Scientists have 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 from four glaciers in the Andes Mountains shows that glacial retreat has occurred 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 last 11,000 years,” said Shakun, a paleoclimatologist and co-author of the report.
“Since modern glacier retreat is primarily driven by rising temperatures, rather than reduced snow cover or changes in cloud cover, our findings suggest that the tropics have already warmed beyond their Holocene range and into the Anthropocene.”
GLACIERS ENTER THE ANTHROPOCENE
In other words, glaciers can no longer be classified as belonging to the Holocene interglacial period, a major epoch that saw the birth of civilization, where water flow and sea level dictated where towns and cities formed, and where agricultural and commercial activity emerged. Instead, they are better classified by an epoch that may be on its way to heralding its end: the Anthropocene.
The findings indicate that more of the world’s glaciers are likely retreating much faster than previously projected, possibly decades ahead of a grim climate timeline.
“This is the first major region on the planet where we have solid evidence that glaciers have crossed this important benchmark; it’s a ‘canary in the coal mine’ for glaciers everywhere,” Shakun said.
Glaciers have been retreating around the world over the past century, but it has been unclear how the magnitude of this retreat compares to the range of natural fluctuations over the past few millennia, Shakun said. The team set out to determine how small tropical glaciers are today compared to their range over the past 11,000 years.
Researchers who formed the international team of scientists traveled to Colombia, Peru and Bolivia to measure the chemistry of newly discovered bedrock in front of four melting glaciers spanning the tropical Andes. Two rare isotopes, beryllium-10 and carbon-14, accumulate on bedrock surfaces when 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 in the past the bedrock was exposed, which tells us how often glaciers were smaller than they are today — sort of like how a sunburn can tell you how long someone was in the sun,” Shakun said.
Shakun led the project with Andrew Gorin, a former graduate student from British Columbia, in collaboration with researchers at the University of Wisconsin and Tulane University on the American Cordillera project, and 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 virtually 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 doctoral student at UC-Berkeley. “That tells us there was never any significant prior exposure to cosmic radiation since these glaciers formed during the last ice age.”
THE CASE OF QUELCCAYA
Twenty years ago, researchers on the Quelccaya Ice Cap in Peru, the world’s largest tropical ice sheet, found remains of rooted plants melting from the ice margin as it retreated. Radiocarbon dating showed those plants were 5,000 years old, indicating that Quelccaya had been larger than its size at the time of that study throughout that interval; otherwise, the plants would have disintegrated if there had been a prior period of exposure, Shakun said.
Those Quelccaya findings suggested that modern ice retreat has been abnormally large, but it was not yet progressing at an alarming rate compared with ice melting throughout the Holocene, Shakun said. He and his team wanted to study a larger number of glaciers and use a technique that can unambiguously show whether a glacier was once smaller than it is today.
Shakun and his colleagues have been applying the same technique to glaciers along the entire length of the American Cordillera, from Alaska to Tierra del Fuego. The team previously published results from their North American sampling last year and aims to publish results from southern South America soon.
“Once we do that, all these studies can be put together into a global perspective on the current state of glacier retreat,” Shakun said.
