Scientists have anticipated that glaciers would retreat as tropical temperatures rise. However, the study's analysis of rock samples near four Andean glaciers demonstrates that the retreat has occurred much faster, surpassing a significant historical threshold, explained Boston College Associate Professor of Earth and Environmental Sciences Jeremy Shakun.
"We have pretty strong evidence that these glaciers are smaller now than they have been any time in the past 11,000 years," said Shakun, a paleoclimatologist and co-author of the report. "Given that modern glacier retreat is mostly due to rising temperatures - as opposed to less snowfall, or changes in cloud cover - our findings suggest the tropics have already warmed outside their Holocene range and into the Anthropocene."
The glaciers may no longer be classified as being from the Holocene, a period that fostered the rise of civilization. Instead, they now appear to belong to the Anthropocene, an era potentially marking their demise.
The findings suggest that more of the world's glaciers may be retreating much faster than previously predicted, potentially decades ahead of forecasts.
"This is the first large region of the planet where we have strong evidence that glaciers have crossed this important benchmark - it is a 'canary in the coalmine' for glaciers everywhere," said Shakun.
While glaciers globally have been retreating over the past century, it has been unclear how this retreat compares to natural fluctuations over millennia. The team aimed to determine the current size of tropical glaciers relative to their range over the last 11,000 years.
An international team of researchers traveled to Colombia, Peru, and Bolivia to analyze the chemistry of bedrock recently exposed by four melting tropical Andean glaciers. Beryllium-10 and carbon-14 isotopes accumulate in bedrock surfaces when exposed to cosmic radiation, said Shakun.
"By measuring the concentrations of these isotopes in the recently exposed bedrock we can determine how much time in the past the bedrock was exposed, which tells us how often the glaciers were smaller than today - kind of like how a sunburn can tell you how long someone was out in the sun," Shakun said.
Shakun led the project with former BC graduate student Andrew Gorin, collaborating with researchers from the University of Wisconsin and Tulane University on the American Cordillera project, and gathering samples and data from colleagues at Aix-Marseille University, the National University of Ireland, Aspen Global Change Institute, Ohio State University, Union College, University Grenoble Alpes, and Purdue University.
"We found essentially 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 any significant prior exposure to cosmic radiation since these glaciers formed during the last ice age."
Twenty years ago, researchers at the Quelccaya Ice Cap in Peru, the largest tropical ice mass in the world, found rooted plant remains melting out of the ice margin as it retreated. Radiocarbon dating showed that those plants were 5,000 years old, indicating Quelccaya had been larger than its size at the time of that study for that whole interval- otherwise the plants would have decayed away if there was a prior period of exposure, Shakun said.
Those Quelccaya findings suggested that modern ice retreat had been unusually large but not yet at a critical level compared to ice melt across the entire Holocene. Shakun and his team sought to study a larger number of glaciers using a technique that could definitively show if a glacier was ever smaller than 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 the results of its North American sampling last year and aims to publish the results from southern South America soon.
"Once we do that, then these studies can all be put together into a global perspective on the current state of glacier retreat," said Shakun.
Research Report:Recent tropical Andean glacier retreat is unprecedented in the Holocene
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