Since 1974, glaciologist Lonnie Thompson has made the trek to the Quelccaya ice cap at least 27 times, drilling cores through to bedrock, taking samples and periodically monitoring its slow but accelerating retreat.

When glaciologist Lonnie Thompson returns to Peru's Qori Kalis glacier early this summer, he expects to find that half of the ice he saw during his visit there last year has vanished. What troubles him the most is his recent observations that suggest that the entire glacier may likely be gone within the next five years, providing possibly the clearest evidence so far of global climate change.

The fact that the Qori Kalis glacier, high in the Andes Mountains, is only one of many ice tongues retreating on the Quelccaya Ice Cap, the largest body of ice in the tropics, provides strong evidence of the warming that appears to be underway worldwide. Thompson, Distinguished University Professor in the School of Earth Sciences at Ohio State University and a world-acclaimed paleoclimatologist, recently outlined his fears at the annual meeting of the American Association for the Advancement of Science in San Francisco.

Since 1974, Thompson has made the trek to the Quelccaya ice cap at least 27 times, drilling cores through to bedrock, taking samples and periodically monitoring its slow but accelerating retreat. Ancient plant beds have been newly uncovered as the ice retreats. The first were discovered in 2002; more are uncovered each year, and carbon dating indicates that most have been buried for at least 5,000 years. They indicate that the current retreat of the ice exceeds any other retreat in at least the last 50 centuries.

Evidence from the analysis of those ice cores – as well as records from more than a dozen other remote ice fields across the globe over the past three decades – point to an increase in temperatures throughout the tropics.

Thompson notes that today's globally averaged temperature is thought to be only a few degrees cooler than the temperature at the height of the Eemian interglacial period, roughly 125,000 years ago when melting ice raised sea level nearly 20 feet. Recent model projections by the Intergovernmental Panel on Climate Change (IPCC) suggest that the globally averaged temperature at the end of the current century could be 3 degrees warmer than it is today, he says.

"It raises the question of whether there are delays in the climate system that haven't shown up as a change in sea level yet, but that will eventually come."

In 2001, he predicted that the famed "snows of Kilimanjaro" in Tanzania would disappear in 15 years as the glaciers atop that ancient volcano succumb to a warmer climate. If anything, he now wonders if his predictions were too conservative.

"Kilimanjaro is behaving just like Mount Kenya and the Rwenzori, both also in Africa, as well as the glaciers in the Andes and the Himalayas ," he says. "This widespread retreat of mountain glaciers may be our clearest evidence of global warming as they integrate many climate variables. Most importantly, they have no political agenda."

Aside from the sheer geophysical changes this represents, he worries most about what it means to the millions of people relying on these ice caps as major water supplies. He's quick to emphasize that 50 percent of the planet's surface area lies between 30 degrees north and south of the equator and that 70 percent of the people live there. This is also where climate phenomena that impact the entire planet originate, such as monsoons and El Ninos. "This is basically the weather engine for the world."

"These glaciers are going to be gone," he says. "If you a living at the base of one of these mountains, it doesn't matter why they're disappearing – only that they are. Millions of people are going to have to adapt to these changes, many of which will occur in some of the poorest regions of the globe."

Thompson recently returned from drilling ice cores with his Chinese collaborators at a new site, Naimona'nyi, a 20,000-foot ice field in Tibet near the western border of Nepal. They retrieved three cores to bedrock, each offering a record of the local climate, trapped in the ice. While the ice core has not been dated yet, preliminary analysis shows an increase in temperature over time that nearly mirrors the record from various other sites worldwide. "This may be very old ice," he says.