Andean glaciers have retreated to lowest levels in 11,700 years, news study finds
No longer covered in ice, Andean rocks signal the world’s glaciers are melting far faster than predicted, reports an international team of scientists led by Boston College researchers
Boston College
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A researcher collects a sample of bedrock from the Queshque Glacier in the Peruvian Andes Mountains. The samples show tropical glaciers have retreated to their smallest size in more than 11,700 years based on cosmogenic nuclide measurements of recently exposed bedrock, an international team of scientists, led by Boston college researchers, reports in the journal Science.
view moreCredit: Emilio Mateo, Aspen Global Change Institute
Chestnut Hill, Mass (8/1/2024) – Rocks recently exposed to the sky after being covered with prehistoric ice show that tropical glaciers have shrunk to their smallest size in more than 11,700 years, revealing the tropics have already warmed past limits last seen earlier in the Holocene age, researchers from Boston College report today in the journal Science.
Scientists have predicted glaciers would melt, or retreat, as temperatures warm in the tropics – those regions bordering the Earth’s equator. But the study’s analysis of rock samples adjacent to four glaciers in the Andes Mountains shows that glacial retreat has happened far faster and already passed an alarming cross-epoch benchmark, said 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.”
In other words, the glaciers may no longer be classified as being of the Holocene interglacial period, a significant epoch that saw the birth of civilization, where the flow of water and sea level dictated where towns and cities formed, and where agricultural and commercial activity emerged. Instead, they may be best classified by an epoch that may be well on its way to spelling their end: the Anthropocene.
The findings signal more of the world’s glaciers are likely retreating far faster than predicted, possibly decades ahead of a grim climatological schedule.
“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.
Glaciers have been retreating worldwide 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 several millennia, Shakun said. The team set out to determine how small tropical glaciers are today compared to their range over the last 11,000 years.
Researchers who formed the international team of scientists traveled to Colombia, Peru, and Bolivia to measure the chemistry of bedrock only recently uncovered in front of four melting glaciers spanning the tropical Andes. Two rare isotopes – beryllium-10 and carbon-14 – build up in bedrock surfaces when they are exposed to cosmic radiation from outer space, Shakun said.
“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, partnering with researchers from the University of Wisconsin and Tulane University on the American Cordillera project, then seeking 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 has been abnormally large, but was not yet progressing to an alarming level compared to ice melt across the entire Holocene, Shakun said. He and his team wanted to study a larger number of glaciers and use a technique that can unambiguously 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.
Journal
Science
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Recent tropical Andean glacier retreat is unprecedented in the Holocene
Article Publication Date
1-Aug-2024
Retreat of tropical glaciers foreshadows changing climate's effect on the global ice
University of Wisconsin-Madison
MADISON — As they are in many places around the globe, glaciers perched high in the Andes Mountains are shrinking. Now, researchers at the University of Wisconsin–Madison and their collaborators have uncovered evidence that the high-altitude tropical ice fields are likely smaller than they've been at any time since the last ice age ended 11,700 years ago.
That would make the tropical Andes the first region in the world known to pass that threshold as a result of the steadily warming global climate. It also makes them possible harbingers of what's to come for glaciers globally.
"We think these are the canary in the coal mine. The tropics would probably be the first place you'd expect ice to disappear, and that's what we're seeing," says Shaun Marcott, a professor of geoscience at UW–Madison. Marcott guided the research with colleagues at Boston College and Tulane University. Andrew Gorin, a former Boston College graduate student who is now at University of California, Berkeley, led the study, which appears in the Aug. 2, 2024, issue of the journal Science.
Glaciers grow slowly over time in regions where summer weather isn't warm enough to melt all of the previous winter's snowfall. Over time, unmelted snow collects and gets compacted and begins to move under its own weight, resulting in the year-round ice that defines a glacier.
Satellite imagery and on-the-ground observations have provided conclusive evidence for decades that high-altitude glaciers in the Andes are steadily shrinking as warmer temperatures cause them to melt more quickly than falling snow can replenish them.
What has remained unclear, though, is whether the glaciers' dwindling footprints are anomalously small compared to the rest of the period that began at the end of the last ice age, known as the Holocene. Meanwhile, glaciers in other parts of the world were smaller at some points in the early Holocene, when the global climate was warmer and drier than recent millennia.
“We knew that glaciers ebbed and flowed in the past, so we wanted to learn how the behavior of glaciers today — melting due to human-caused climate change — stacks up against their long-term fluctuations,” says Andy Jones, a UW–Madison doctoral student and study co-author.
To answer this question, the team of scientists analyzed the geochemistry of bedrock from areas near the edges of four glaciers in the high tropical Andes, choosing sites that satellite imagery showed were exposed by melting ice in only the last two or three decades.
The team specifically looked for evidence of two unique isotopes — basically chemical flavors — of a pair of elements with the bedrock's quartz crystals: beryllium-10 and carbon-14. These isotopes are only present in rock that has spent time at or near the Earth's surface as they result from interactions between the rock and cosmic rays, which are high-energy particles that constantly rain down on the planet from outer space.
Bedrock accumulates beryllium-10 and carbon-14 once it's exposed to the surface, so measuring the isotopes' concentrations in rock crystals near glaciers can be useful for understanding the previous extent of ice coverage.
The team found "remarkably low" concentrations of both isotopes in nearly all samples, suggesting that melting ice has exposed bedrock near the glaciers for the first time only recently in most of the sampled locations.
Additional analyses — and the fact that the extremely low concentrations were consistent across sample sites — made the researchers confident that melting ice, rather than erosion, exposed the bedrock.
"It's highly unlikely this is from erosion," says Marcott. "Because the multiple locations we went to all show the same thing."
This consistency points to a single likely conclusion, according to Marcott: The world's tropical glaciers, more than 99% of which are located in the Andes, are the first to shrink beyond what's been seen in the recent geologic past.
"Glaciers are very sensitive to the climate system that they live in," says Marcott. "They really are the place you would look to see some of the first big changes resulting from a warming climate. You can look to these glaciers and imagine what we might be looking at going into the future in other places like the Western United States, which is a no-ice scenario."
This research was supported by the National Science Foundation (EAR-1805620; EAR-1805133; EAR-1805892).
# # #
--Will Cushman, wcushman@wisc.edu, 608-263-1986
Journal
Science
Method of Research
Observational study
Subject of Research
Not applicable
Article Publication Date
1-Aug-2024
Current Andean glacier loss is unprecedented in the Holocene
Andean tropical glaciers are experiencing unprecedented retreat, according to a new study that reveals their current sizes are the smallest in over 11,700 years. “Our finding … identifies this region as a hot spot in our understanding of the changing state of the cryosphere,” say the authors. Glaciers act as important indicators of climate change, with their global retreat accelerating over recent decades. Examining this retreat in the context of the previous 11,700 years of the Holocene interglacial highlights the impact of modern global warming. Although many glaciers worldwide are smaller today compared to their sizes over the Holocene, it’s recognized that glaciers generally retreated to their smallest extents during the early to mid-Holocene. But today tropical glaciers are experiencing unprecedented retreat, possibly due to factors including increasing temperatures at high altitudes. The Andes – home to over 99% of tropical glaciers – are particularly affected, and may be the first to show significant impacts of human-induced climate change on a regional scale. To determine whether the magnitude of tropical glacier retreat in the Andes today exceeds the bounds of Holocene fluctuations, Andrew Gorin and colleagues analyzed paired cosmogenic carbon-14 (14C) and beryllium-10 (10Be) measurements in 20 bedrock samples collected near the margins of four tropical Andean glaciers. Cosmogenic nucleotide measurements can be used to estimate how long bedrock has been exposed, such as due to loss of overlying ice. Gorin et al. discovered that most bedrock samples had near-zero 14C and 10Be concentrations, suggesting that these locations were covered by ice throughout the Holocene and have only recently been exposed. Thus, the adjacent glaciers are likely smaller now than they have been at any point in the past 11,700 years. “Paired with a similar result in the Arctic, the early emergence of these small, fast-responding glaciers from their Holocene baseline may serve as a ‘canary in the coal mine,’ signaling an approaching contraction of higher-latitude glaciers beyond their interglacial range,” write Gorin and colleagues.
Journal
Science
Article Title
Recent tropical Andean glacier retreat is unprecedented in the Holocene
Article Publication Date
2-Aug-2024
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