By dating boulders exposed by the retreat of glaciers at the end of the last Ice Age, researchers have resolved an apparent anomaly during the time of the last great climate change. The process has provided valuable evidence for the likely impact of the current increase in atmospheric carbon dioxide on the world’s remaining rivers of ice. 

"Glaciers are particularly sensitive to climate change, and their ongoing retreat globally is considered to be a robust signal of global warming over the last two centuries," a team led by Professor Jeremy Shakun of Boston College report in Nature Communications. Consequently, tracking their expansion and decline helps us understand climatic history. 

Being covered by ice protects rocks against radiation. Thus, it is possible to estimate the time a boulder has been at the surface by measuring the presence of the isotopes Berylium-10 and Helium-3, both of which are produced through the impact of cosmic rays. Shakun's team performed these measurements on 1,116 glacial boulders left behind by 195 moraines on five continents. 

Others have made similar measurements before, but produced puzzling results. Instead of finding that glaciers retreated as carbon dioxide concentrations in the atmosphere rose, earlier studies suggested the retreat began before carbon dioxide concentrations started to rise. Taken at face value, such results would bring into question our understanding of how the Earth's climate operates.  

"There had been a long-standing mystery about why these boulders were uncovered at the time they were, because it didn't properly match the increase in greenhouse gases," said Shakun. From 19,000 to 12,000 years ago, carbon dioxide rose from 180 to 280 parts per million (ppm) in the atmosphere. "The data now show that as soon as the greenhouse gas levels began to rise, the glaciers began to melt and retreat." 

Regional warming does not always align with global trends, thanks to factors such as shifts in ocean currents. While the sites studied by Shakun are skewed towards western North and South America, they include locations in Australia, which is now too warm (and flat) to support glaciers even at its highest altitudes, as well as the Alps and eastern Africa.

“We find that there was regional variability in the timing of glacier fluctuations superimposed on a global pattern of broadly synchronous retreat that was largely coincident with the rise in CO₂,” the authors report.

“Our results suggest that greenhouse gases were the major driver of global-scale glacier retreat, while other factors modulated glacier responses regionally,” the paper adds. Indeed, carbon dioxide levels in the atmosphere explains 81% of the glacier variability the authors tracked.

The authors also note that it took a rise of just 80 ppm to set off a rapid decrease in glaciers. "This study validates predictions that future glacial loss will occur due to the ongoing increase in greenhouse gas levels from human activities," said Professor Peter Clark of Oregon State University, who is a co-author on the study. "We could lose 80-90 percent of the world's glaciers in the next several centuries if greenhouse gases continue to rise at the current rate."