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 has also 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 like reduced snowfall, has always 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.
Qori Kalis is hardly an outlier, though: land ice is melting virtually everywhere on the planet. That has been occurring since a 500-year period called the Little Ice Age that ended in about 1850, but the pace seems to have accelerated substantially in recent decades as human emissions have begun to overwhelm the natural cycles.
In the middle and high latitudes, from Switzerland to Alaska, a half-century of careful glaciology has established that temperature is the main factor controlling the growth and recession of glaciers.
But the picture has been murkier in the tropics. There, too, glaciers are retreating, but scientists have had more trouble sorting out exactly why.
That glaciers should exist at all in the warmest part of the Earth is perhaps strange; they do so only in high, cold mountain regions. The tropical glaciers receive intense sunlight virtually year-round. Ice atop these glaciers can sometimes vaporize without even passing through a stage as liquid water. Over short periods, at least, the tropical glaciers appear to be sensitive to changes in clouds and many other factors.
One group of scientists is coming to the conclusion that even in these conditions, temperature is nonetheless the main factor controlling the ebb and flow of tropical glaciers over centuries.
But a second group believes that in some circumstances, at least, a tropical glacier's long-term fate may reflect other factors. In particular, these scientists believe big changes in precipitation can sometimes have more of a role than temperature.
In interviews and emails, scientists from both groups praised the new paper for its reconstruction of the Qori Kalis glacier's movements, a feat that required a decade of intensive labor.
A core finding is that the Peruvian glacier was expanding during the Little Ice Age. That adds to a growing body of research suggesting that the cooling during that mysterious event was global in scope, which may in turn help scientists determine the causes.
"I think it's a great study," said Aaron E. Putnam of the Lamont-Doherty Earth Observatory at Columbia University, who has done extensive work on glaciers in New Zealand. "They do something that I haven't seen done in such an elegant way."
But some scientists were critical of the paper's broader claim about temperature as the controlling factor for the glacier. "I actually believe that finding is probably accurate, but I don't see that they make a compelling case for that with this study," said Douglas R. Hardy of the University of Massachusetts, Amherst, who has done extensive work on Quelccaya, including documenting a recent, sharp increase of air temperatures.
Hardy and several of the other critics noted that the Kelly paper's temperature conclusion depended strongly on a record of ice accumulation over centuries that Thompson had compiled by drilling into Quelccaya. The ice has been compressed over time, so the evidence requires considerable interpretation.
All of the scientists involved in the debate over tropical glaciers believe that global warming is a problem and that human emissions pose a long-term threat to the planet. But the unresolved controversy has served as fodder for skeptics of global warming, who say the scientists do not really know what is going on.
The biggest scientific battle has been fought not over Quelccaya but over Mount Kilimanjaro in Tanzania. There, too, Thompson has asserted that the glaciers atop the mountain - the "snows of Kilimanjaro," in Ernest Hemingway's phrase - are disappearing because of planetary warming.
But Hardy and other scientists like Georg Kaser of the University of Innsbruck in Austria have argued that it is actually a series of other factors, primarily a reduction in precipitation, that is starving the Kilimanjaro glaciers. That group says the precipitation decline could be, at least in part, a secondary effect of global warming, caused by rising temperatures in the Indian Ocean.
Kelly is now looking for evidence that may shed light on the Kilimanjaro debate.
Her method involves dating ridges of rock and debris, known as moraines, that glaciers leave at their far edges. Mount Kilimanjaro does not have the right kind of rock, but she has begun a study of glacial moraines in the Rwenzori Mountains, 500 miles away in Uganda, that could eventually show whether glaciers in Africa tend to behave in the same way as the one in Peru.