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, outlined his fears at the annual meeting of the American Association for the Advancement of Science in San Francisco this week.

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 6 meters (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."

He also points towards Greenland with another warning. The Jakobshavn glacier is the island's largest outflow glacier, draining more than 6.5 percent of the ice cap. In the last decade, Thompson said, the glacier has doubled the speed it is sending ice out to the ocean.

"We're talking about huge amounts of ice and water going into the ocean in this one single case," Thompson said.

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 said. "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," he said.

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 phenomenon 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."

He says that Peru's Quelccaya offered another glimpse of a dangerous future for people living in that region. Over the years, as Qori Kalis retreated, a massively deep lake formed at its margin, high up a valley it has been contained by a natural dam. Last March, a massive chunk of the glacier broke off, tumbled downhill and splashed into that lake, sending a wall of water over the dam and cascading down into the valley.

"I've crossed this meadow that lies about 18 miles (30 kilometers) below Quelccaya perhaps 27 times and this is the first time I've ever seen this much sediment there, evidence of that recent flood."

He said that such events wouldn't have happened before 1991 since there was no lake there before that. "You see this unfolding also in the Himalayas where you have the retreat of glaciers and the formation of high-altitude lakes. Now the people in the valleys below face a new geological hazard."

Through aerial mapping and satellite images, historic photographs and current surveys, researchers now can paint a picture of just how much ice has vanished in recent decades. New measurements at Quelccaya, Kilimanjaro and other sites all show that these ice masses are shrinking at an alarming rate. Thompson plans new expeditions to both mountain sites later this year to add to the evidence.

Thompson recently returned from drilling ice cores with his Chinese collaborators at a new site, Naimona'nyi, a 20,000-foot (6,100-meter) 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.

Finding the plants that had been preserved under the Quelccaya ice was a real wake-up call, he believes. He and his colleagues at the Byrd Polar Research Center have found more than 50 additional sites with remarkable plant remains, most dating back to that 5,000-year-old mark.

"About 5,000 years ago, we had perhaps 300 million people living on the planet," he says. "Now there are more than 6.5 billion covering the globe. If you change the climate for many of these people, where will they go? There are fewer options today than there were back then."

This article has been adapted from a news release by Ohio State University