Sitting on a flat volcanic plain 18,000 feet above sea level, the great Quelccaya ice cap of Peru is the largest piece of ice in the tropics. In recent decades, as scientists have watched it melt at an accelerating pace, it also has become a powerful symbol of global warming.
Yet the idea that the ice cap has retreated over time because of a change in temperature, rather than other possible factors such as reduced snowfall, always has been more of a surmise than a proven case. In fact, how to interpret the disappearance of glaciers throughout the tropics has been a scientific controversy.
Now, a group of scientists is presenting new findings suggesting that over the centuries, temperature is the main factor controlling the growth and retreat of the largest glacier emerging from the ice cap. If they are right, then Quelccaya’s recent melting could indeed be viewed as a symbol of the planetary warming linked to human emissions of greenhouse gases.
In a paper released Tuesday by the journal Geology, a group led by Justin S. Stroup and Meredith A. Kelly of Dartmouth College in Hanover, N.H., used elaborate techniques to date the waxing and waning over the past 500 years of the glacier, called Qori Kalis.
The group then compared the glacier’s movements with a record of ice accumulation on top of Quelccaya, obtained from long cylinders of ice drilled by the glaciologist Lonnie G. Thompson of Ohio State University.
The new paper suggests that the glacier sometimes grew during periods when the accumulation of ice in the region was relatively low, and conversely, that it retreated during some periods of high ice accumulation.
Kelly and Stroup conclude that the glacier is sensitive to temperature and that other factors, like the amount of snowfall, are secondary, thus supporting a view long held by Thompson that the glacier can essentially be viewed as a huge thermometer.
“The big driver is temperature,” said Thompson, who was not involved in the new paper.
Assuming it holds up, that is a sobering finding, considering how fast the Qori Kalis glacier is now retreating. Thompson documented last year that a part of the glacier that had apparently taken 1,600 years to grow had melted in a mere 25 years. He interpreted that as a sign that human emissions and the resultant warming have thrown the natural world far out of kilter.
