Tropical glaciers hit geological low

MADRID (Euronews) – Rocks exposed to the sky after being covered by prehistoric ice show that tropical glaciers in the Andes have shrunk to their smallest size in more than 11,700 years.

The findings suggest that warming in the tropics has already exceeded limits set during the Holocene geological epoch, Boston College researchers report in the journal Science.

Scientists predict that as temperatures rise in tropical regions bordering Earth’s equator, glaciers will melt or retreat.

But Jeremy Shakun, an associate professor of earth and environmental sciences at Boston College, said an analysis of rock samples from four glaciers in the Andes showed that the retreat was much faster and had exceeded alarming benchmarks between different time periods.

“We have pretty strong evidence that these glaciers are smaller now than at any time in the last 11,000 years,” said Shakun, a paleoclimatologist and co-author of the report.

“Because modern glacier retreat is primarily due to rising temperatures, rather than reduced snow cover or changes in cloud cover, our results suggest that warming in the tropics has already occurred beyond the Holocene and Anthropocene.”

Glaciers enter the Anthropocene

In other words, the glaciers are no longer part of the Holocene interglacial period, a crucial era that saw the birth of civilization, when water flows and sea levels dictated the formation of towns and the emergence of agriculture and commerce. Instead, they are better categorized under an era that may soon be coming to an end: the Anthropocene.

The findings suggest that more of the world’s glaciers may be retreating much faster than predicted, potentially decades ahead of a dire climate timeline.

“This is the first major region on Earth with solid evidence of a glacier crossing this important landmark; it’s the ‘canary in the coal mine’ for glaciers everywhere,” Shakun said.

Glaciers around the world have been retreating over the past century, but it’s unclear how the magnitude of this retreat compares to the range of natural fluctuations over the past few thousand years, Shakun said. The research team set out to determine how small tropical glaciers are today compared to their extent over the past 11,000 years.

The researchers, who formed an international team of scientists, traveled to Colombia, Peru and Bolivia to measure the chemical composition of bedrock recently discovered in front of four melting glaciers across the tropical Andes. When exposed to cosmic radiation from outer space, two rare isotopes, beryllium-10 and carbon-14, accumulate on the bedrock surface, Shakun said.

“By measuring the concentrations of these isotopes in newly exposed bedrock, we can determine how long the bedrock has been exposed in the past, which can tell us how often glaciers were smaller than they are today, a bit like being able to tell you how long someone’s sunburn lasted when they were in the sun,” Shakun said.

Shakun led the project with former BC graduate student Andrew Gorin, working 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, Université Grenoble Alpes and Purdue University.

“We found almost no beryllium-10 or radiocarbon-14 in the 18 bedrock samples we measured in front of four tropical glaciers,” said Goering, now a doctoral student at the University of California, Berkeley. “This tells us that these glaciers have never had significant exposure to cosmic radiation since they formed during the last ice age.”

The Quelccaya Case

Two decades ago, researchers at Peru’s Quelccaya Ice Sheet, the world’s largest tropical ice sheet, found the remains of rooted plants melting from the ice edge as it retreated. Radiocarbon dating put the plants at 5,000 years old, suggesting that Quelccaya was larger throughout this time than it was when it was studied; otherwise, if there had been prior contact, the plants would have disintegrated, Shakun said.

The findings at Quelccaya suggest that the scale of modern ice retreat is unusually large, but not yet alarming compared with the melt during the entire Holocene, Chakun said. He and his team want to study more glaciers and use a technique that can definitively show whether a glacier was once smaller than it is today.

Chakun and his colleagues have been applying the same technique to glaciers in mountain ranges across the U.S., from Alaska to Tierra del Fuego. The team previously published results from sampling in North America last year and plans to publish results from southern South America soon.

“Once we do that, all of these studies can be integrated into a global perspective to understand the current state of glacier retreat,” Shakun said.