Tropical glaciers offer a glimpse of our harsh climate reality

Tropical glaciers have shrunk to their smallest size in over 11,700 years, according to researchers at Boston College.

These results, derived from rocks newly exposed after being covered by prehistoric ice, indicate that the tropics have warmed beyond levels last seen in the early Holocene epoch.

Unexpected melting of tropical glaciers  

Scientists have long predicted that tropical glaciers would melt as temperatures rise in regions near the Earth’s equator. However, the analysis of rock samples near four glaciers in the Andes Mountains shows that this retreat has occurred much faster than expected, crossing a significant climatological threshold. 

“We have pretty strong evidence that these glaciers are smaller now than they have been any time in the past 11,000 years,” said co-author Jeremy Shakun, an associate professor of earth and environmental sciences at Boston College.

“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.”

Tropical glaciers: A sign of the times

This shift means that glaciers, traditionally classified within the Holocene interglacial period – an epoch characterized by the development of human civilization – may now be better categorized within the Anthropocene, an era defined by significant human impact on Earth’s geology and ecosystems.

The implications of this study are stark, suggesting that many of the world’s glaciers are likely retreating much faster than previously predicted, possibly by decades.

“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,” Shakun warned.

Climate change vs. natural fluctuations 

While it is known that glaciers have been retreating worldwide over the past century, it has been unclear how this retreat compares to natural fluctuations over the last several millennia. To address this, the research team set out to determine the current size of tropical glaciers compared to their size over the past 11,000 years.

An international team of scientists traveled to Colombia, Peru, and Bolivia to analyze the chemistry of bedrock recently exposed by retreating glaciers in the tropical Andes. They measured concentrations of two rare isotopes – beryllium-10 and carbon-14 – which accumulate in bedrock surfaces when exposed to cosmic radiation. 

“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 explained.

“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 lead author Andrew 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.”

Largest tropical ice mass 

The results of this investigation are supported by research conducted twenty years ago at the Quelccaya Ice Cap in Peru, the largest tropical ice mass in the world. There, researchers discovered rooted plant remains melting out of the ice margin, dated to be 5,000 years old. 

The findings indicated that Quelccaya had remained larger than its current size for millennia, as the plants would have decayed if previously exposed. 

Shakun noted that while these findings suggested modern ice retreat was abnormally large, it had not yet reached an alarming level compared to the entire Holocene. 

Studying a broad range of glaciers

The team aimed to study a broader range of glaciers to conclusively determine if any had ever been smaller than today.

Shakun and his colleagues have applied this technique to glaciers along the entire American Cordillera, from Alaska to Tierra del Fuego. They published their North American results last year and plan to release findings 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,” Shakun said.

This research underscores the urgency of addressing climate change, as the rapid retreat of tropical glaciers highlights the profound impact of rising temperatures. The evidence points to a need for global action to mitigate these effects and preserve vital ecosystems.

The research is published in the journal Science.

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