Queshque Glacier, located in the Peruvian region of the Andes mountains, is one of four tropical glaciers where researchers collected samples of bedrock to better understand how glacial extent has changed during the past 12 000 years. Credit: Emilio Mateo, Aspen Global Change Institute
Queshque Glacier, located in the Peruvian region of the Andes mountains, is one of four tropical glaciers where researchers collected samples of bedrock to better understand how glacial extent has changed during the past 12 000 years. Credit: Emilio Mateo, Aspen Global Change Institute
Most glaciers worldwide are retreating because of climate change. To put the current retreat into perspective, researchers have conducted surveys that estimate historical glacial extent. They have found that many glaciers, including many high-altitude mountain glaciers, are not as small as they were in the past. By the early Holocene, roughly 12 000 years ago, Earth had come out of its last ice age, and variations in the planet’s orbit caused warmer summers and, consequently, smaller glaciers.
Tropical glaciers today, located in mountainous regions, appear to be decreasing in size more than glaciers in other regions. New results by Andrew Gorin of the University of California, Berkeley, and colleagues show that several glaciers in South America—where nearly all tropical glaciers are found—have retreated to a minimum extent not seen during any other time since the early Holocene, when humans were just starting to domesticate plants and animals for agriculture.
During that time in Earth’s history, the average temperature in most tropical regions was low, compared with today, before it started to sharply rise over the next several thousand years. One of the few previous measurements of Holocene glacial extent in South America goes back only 7000 years. The evidence indicates that the world’s largest tropical glacier, the Quelccaya ice cap in Peru, is smaller today than it was in the mid-Holocene. To better reconstruct how glacial extent may have changed further back in the early Holocene, Gorin’s colleagues took expeditions to several Andean glaciers starting in 2012 to collect bedrock samples that were once buried but have since been exposed at Earth’s surface.
Gorin and colleagues analyzed the samples using an established method among isotope geochemists: measuring the carbon-14 and the beryllium-10 composition of rocks. The radioactive isotopes are products of cosmic-ray reactions with silicon and oxygen in quartz, so they become concentrated in quartz bedrock only if it is exposed at the surface.
The radioactive isotope concentrations in nearly all the samples were low—so low, in fact, that the only way the samples could have that level is if they were never exposed to cosmic rays during the Holocene. That would imply that either ice covered the bedrock or the bedrock was exposed earlier in Earth’s history before it eroded away. The researchers’ modeling failed to support the erosion possibility. The most likely explanation, Gorin and his colleagues conclude, is that the melted glaciers exposed the bedrock and are smaller now than at any other time during the Holocene. Tropical glaciers provide a warning: As global temperatures continue to climb, more glaciers may soon reach similar minimums. (A. L. Gorin et al., Science 385, 517, 2024.)
This article was originally published online on 8 August 2024.
