INAYA, PERU

On approach, Quelccaya grows from a white smudge in the distance to a glacier that dominates the horizon.

From 20 miles away, Lonnie Thompson stopped in his snowy tracks and pulled his binoculars out of a pack. Even from that distance, the Ohio State University scientist said he could tell that the sheet of gleaming ice at an elevation of 18,602 feet -- the largest tropical glacier in the world -- had shrunk.

"It looks like it's melted maybe 100 feet since last year," he said last month during a weeklong trip to the glacier high in the Andes Mountains.

That's not good. Especially in the dead of winter.

When Thompson, one of the world's foremost climate-change experts, reached base camp hours later, he learned that water was flowing from the glacier.

"It used to be we came here and it was frozen to the bottom," he said.

To Thompson, the rapid melting is transforming Quelccaya into the classic canary in a coal mine, warning of a potential catastrophe.

Most scientists agree that carbon dioxide, methane and other greenhouse gases are boosting the Earth's temperature. The warming has occurred so rapidly that every tropical glacier could disappear in a few decades.

Because wind and sea currents tie the tropics to the polar regions, what is happening to Quelccaya also is happening at the North and South poles.

Antarctic ice is melting. The fabled Northwest Passage -- a circuitous route, sought by explorers for centuries, between the Atlantic and Pacific oceans through the northern islands of Canada -- is now ice-free much of the year. Scientists think summer ice could disappear from the North Pole by the middle of this century.

Except for a few in Norway and Alaska, glaciers everywhere are in wholesale retreat.

"In 30 years, there will be no ice in Glacier National Park," Thompson said. A century ago, the park on the Montana-Canada border had 150 glaciers. Twenty-six remain.

The glacier atop Mount Kilimanjaro in Africa, with its 11,700-year ice record, will disappear within 20 years, experts predict.

At least 85 percent of Alaska's 1,500 glaciers are retreating, according to NASA scientists. They say the melting, which is reducing the weight of the ice on land, is triggering earthquakes.

Glaciers, Thompson said, are nature's water towers, storing it for drinking and power production, agriculture and tourism. More than 70 percent of the world's freshwater supply is stored in glaciers.

Layers of evidence

Ten years ago, conventional wisdom among scientists was that the tropics were the least affected by global warming.

But by the mid-1990s, Thompson, a scientist at the OSU Byrd Polar Research Center, had spent 20 years watching the world's largest tropical glacier melt before his eyes.

On this trip, four researchers from various institutes met Thompson and his fellow OSU scientists Henry Brecher, Victor Zagorodnov and Thompson's wife, polar scientist Ellen Mosley-Thompson.

The OSU crew tested a new lightweight ice drill, probed the depths of a glacial lake and collected plants entombed in the ice more than 5,000 years ago.

The glacier has been retreating about 100 feet each year. In some places, it is disappearing at a rate as fast as 500 feet annually.

Thompson's goal this year was to capture the weather history locked in the ice of Quelccaya and other critical glaciers before warming transforms them to puddles.

The ice contains secrets of ancient weather that shed light on current climate change. Dust, pollen, entombed microorganisms and gases in the ice cores that scientists pull from glaciers reveal glimpses of the weather through hundreds of thousands of years.

The ice record indicates the current period is the warmest in thousands of years. The frozen data fit a general picture of a global atmosphere being overwhelmed with carbon dioxide from burning coal, oil and gasoline.

Thompson thinks tropical glaciers can help hone climate models to make accurate predictions. Because reliable world weather records exist only for the past 150 years, the ice record is crucial to giving scientists an inkling of how current weather patterns fit in a much larger and older picture.

"The climate changes, the plants change," said Carl Reese, a geographer from the University of Southern Mississippi who studies pollen records from the Quelccaya glacier.

Earth's chaotic weather system has produced rapid, global changes several times in the past.

After the last ice age about 13,000 years ago, average temperatures suddenly dipped about 14 degrees Fahrenheit, and the lows lasted 1,200 years. During the shift, tundra replaced forests as far south as Virginia, and many species became extinct.

"Here on Quelccaya, we have a 2,000-year climate history, and it tells us that what is happening (now) is unusual in the last 2,000 years," said Mosley-Thompson, who studies polar ice records.

Ice cores from Mount Kilimanjaro, for example, indicate that Africa has had three catastrophic droughts in the past 11,700 years, including one 4,000 years ago that threatened the stability of ancient Egypt. Some glacial records, especially those in polar areas, go back hundreds of thousands of years.

Carbon dioxide from fossil fuels is only part of the equation leading to global warming and the retreat of glaciers.

"We have to consider other factors, like the massive deforestation in the Amazon," Mosley-Thompson said.

Trees tie up large quantities of carbon and also cool the atmosphere.

Looking for solutions

The United States, with less than 5 percent of the world's population, accounts for 25 percent of the world's industrial carbon-dioxide emissions.

"The big debate is, what do we do about it?" said Lara Hansen, a scientist with the World Wildlife Fund.

The group wants a reduction in carbon-dioxide emissions, especially from power plants.

The science, however, does not support Draconian measures, according to critics. And new restrictions on industrial emissions might not affect climate much, said H. Sterling Burnett, a senior fellow and environmental ethicist at the National Center for Policy Analysis in Dallas.

"I suspect sea levels will rise despite global warming -- they've risen 400 feet since the last ice age," he said.

Peter Kareiva, a scientist for the Nature Conservancy, has studied climate-change issues for more than a decade. He said the science is sound.

"You no longer have to look at models to appreciate the reality," Kareiva said. "Biologically, we're seeing huge responses. The ranges of plant and animal species are shifting, the times when buds burst, plants flower, birds lay eggs."

As the science has improved and the issue become more politicized, industry has begun to act. The courts are also involved. Several states recently sued Columbus-based American Electric Power and several other companies over greenhouse-gas emissions.

Even before the suit, AEP said it was moving to cut emissions, pledging to cap carbon-dioxide emission levels by 2006 and reduce them 10 percent.

"Climate change . . . clearly warrants taking some action," said John McManus, vice president of environmental services for AEP.

Cause and effect

Greenhouse-gas accumulation in the atmosphere, climate models show, could increase average global temperatures as much as 10 degrees Fahrenheit this century, enough to make Columbus feel like Tucson.

At the same time, precipitation could increase as much as 30 percent. But because of warmer temperatures, evaporation would increase and there would be more drought.

If this happens, lake and river levels will fall. Average Great Lakes water depths could drop 5 feet, and the flow through the St. Lawrence Seaway could be cut 40 percent.

Separating what occurs naturally and that caused by man is a challenge cited by both proponents and critics of climate-change theories.

"How much of the changing climate we're observing is due to human activity? We can't assign a specific number to it," said Neville Koop, a meteorologist with Weathernews America, a Japanese-based global company that advises shipping companies, construction firms and other businesses on weather trends.

Human activities, according to recent estimates, have added more than 900 billion tons of carbon dioxide to the environment since 1800. In a report published in July in the journal Science, National Oceanic and Atmospheric Administration scientists indicate that the additional carbon dioxide threatens to overwhelm the world's oceans.

That's a serious enough problem for Louisiana and other low-lying coastal areas. But it's absolutely deadly for nations such as Bangladesh, where most of the land lies only a few feet above sea level.

The Pacific island nation of Tuvalu, a nation of 10,000 people scattered across 26 islands, is 15 feet above sea level. The nation is making arrangements with Australia and New Zealand to take its citizens as refugees if seas eventually swamp the islands.

"It's a scary place to visit," Koop said. "The seas are coming up over the barrier reefs. They're on the front line, and they're seeing the consequences of what others are doing."

Even if fossil-fuel consumption is stopped immediately, however, it would take a century for the oceans, forests and soils to remove the carbon dioxide pumped into the atmosphere since 1900.

The view from Quelccaya

In the morning sunlight, it's easy to see what Thompson is talking about. Huge expanses of volcanic rock are visible, uncovered by the receding ice.

Quelccaya has diminished 20 percent in area since 1963. One of its main arms, Qori Kalis, has formed a deep lake at its base.

This arm has melted as much in the past 25 years as it did during the previous century.

Quelccaya still is massive -- a huge hulking cap of ice and snow about 6 miles long and an average of about 3 miles wide.

During thousands of years, it has expanded and contracted. In the "Little Ice Age" -- about 1500 to about 1880 -- the glacier was larger. Sections extended into the deep mountain valleys like arms of a gigantic snowy octopus.

At the top of the glacier, Thompson can look east to the Amazon basin. Sometimes he can see clouds of smoke rising from logging operations.

It's a scene being repeated in Asia, Africa, Central America and Siberia.

Without releasing one more ounce of carbon dioxide, there's already enough of the gas in the atmosphere to completely melt Quelccaya and the world's other tropical glaciers, he said.

There are too few trees and too many power plants and cars and, perhaps, too little time.

"We are witnessing a collision," he said, "between our civilization and the Earth."