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Tropical glaciers are expected to retreat in a warming world. Now, a new analysis suggests these glaciers, located at high altitudes in the Andes Mountains, are smaller than they've been since the last major ice age -- an alarming milestone that could portend future glacial melting worldwide.

Publishing in the journal Science, an international team of researchers write that the bedrock now exposed at the margins of four such glaciers in the Andes has not seen the light of day since more than 11,700 years ago. That's when the last major ice age ended, beginning the current geological epoch known as the Holocene.

The researchers measured beryllium-10 and carbon-14 concentrations in the recently exposed rock to determine the last time the rocks were exposed. Both are formed when cosmic rays from outer space penetrate Earth's atmosphere and interact with the elements that make up Earth's surface. These long-lived isotopes accumulate in rocks and help scientists track the last time the rock was exposed.

"We found essentially no beryllium-10 or radiocarbon-14 in any of the 18 bedrock samples we measured in front of four tropical glaciers," Andrew Gorin, one of the researchers, said in a news release. "That tells us there was never any significant prior exposure to cosmic radiation since these glaciers formed during the last ice age."

The researchers write that other explanations, such as the glaciers being extremely susceptible to erosion, seem unlikely. Instead, they attribute the lack of beryllium-10 and radiocarbon-14 as evidence of "unprecedented" tropical warmth and global warming that has caused the glacier's ice cover to melt.

The glaciers may serve as a "canary in the coal mine," they say -- a warning that global warming has already raised Earth's temperature by 1.5 or 2 degrees Celsius and that more melting is at hand for other glaciers worldwide.