Eighteen years ago, Pastoruri, Peru’s best-known glacier and one of the biggest tourist attractions in the magnificent, snow-capped Cordillera Blanca, could be reached on a short horseback ride from the visitors’ parking lot.

Now from the end of the horse trail it takes another breathless half hour on foot at 5,000 meters (16,400 feet) above sea level to reach the leading edge of the ice. Ever more distant from the parking lot, Pastoruri has shrunk by 40% since 1995 and is technically no longer a glacier.

“It’s an ice cap,” says Marco Zapata, who heads the Glaciology and Water Resources Unit of Peru’s National Institute of Natural Resources (Inrena) in Huaraz. Within a decade, he says, it will probably have vanished altogether, a casualty of climate change.

The Andes are topped by the largest concentration of ice anywhere in the Earth’s tropics. While the melting of the tropical glaciers contributes little to global sea-level rise in comparison to the melting of ice in Greenland and Antarctica, the rapid retreat of Andean glaciers is threatening irrigation and drinking water supplies, hydroelectricity production and tourism.

It also is erasing a valuable record of the planet’s climatic history. In 1991, tropical Andean glaciers covered an estimated 1,065 square miles (2,758 sq kms), with 70% in Peru, 20% in Bolivia and the rest in Ecuador, Colombia and Venezuela.

Since then, glaciers have disappeared from Venezuela and are shrinking in the other countries. Figures from 2002 put the area at 962.6 square miles (2,493 sq kms), a loss of nearly 10% in just over a decade.

While the speed of the retreat varies from place to place, the trend is unrelenting. A 1987 aerial inventory of 18 of Peru’s 19 snowcapped ranges registered 3,044 glaciers covering 788 square miles (2,041 sq kms). Ten years later, satellite images showed the area had decreased by more than 20%, to 615.8 square miles (1,595 sq kms).

The Cordillera Blanca in the central department of Ancash, a popular hiking and climbing destination for Peruvian and international adventurers, is the country’s highest range and has about one-third of the country’s glaciers. In the 10 years between the surveys, however, the glaciated area in the Cordillera Blanca decreased by just over 15%.

To stay abreast of the changes, Zapata’s office currently is conducting a new nationwide glacier inventory. Among the points of particular interest will be the Santa River valley, known as the Callejón de Huaylas, which lies at the foot of the Cordillera Blanca. There, glaciers have long been a matter of life and death.

A glacial lake burst through a dam in 1941, sending a landslide thundering down on Huaraz. An avalanche that started on the north face of Huascarán, Peru’s highest mountain, buried the town of Ranrahirca in 1962, killing about 4,000 people. An earthquake in the same place in May 1970 shook loose another mass of ice, rocks and mud, burying the nearby town of Yungay, with a death toll that has been estimated at up to 18,000.

Although the glaciers can kill, farmers depend on them for irrigation. The shrinking of the glaciers concerns Tito Guillén, the 26-year-old mayor of the hamlet of Coyllur, just outside Huaraz.

“When we were kids, [the range] was a beautiful white blanket,” says Guillén, who worries Coyllur’s water source will dry up.

While rain waters crops from October through May, villagers rely on snow melt during the dry months of June, July and August. Concerned glacier-fed supplies will dwindle, the community is building canals and planting trees on surrounding hillsides to conserve water.

Guillén is encouraging his community to plan ahead, but such initiatives will not be quick to catch on. On the lower slopes of the Cordillera Blanca, many communities still irrigate by channeling glacial meltwater to flood their fields, an inefficient practice that can result in the loss of up to 70% of the water to evaporation, filtration in irrigation ditches and surface runoff.

Because glacial melt has been increasing, those farmers have not felt the pain of a decreased water supply and many do not see the need for more efficient irrigation techniques. As the glaciers shrink, however, civil engineer César Portocarrero sees the handwriting on the wall.

Portocarrero, who heads the Huaraz office of Practical Solutions, a nonprofit organization that works in the areas of appropriate technology and disaster mitigation, is encouraging farmers in the Cordillera Blanca to switch to drip irrigation.

Glaciers are an important source of drinking water in highland towns, as well as in the coastal capital of Lima, a desert city of nearly 9 million people. Quito, Ecuador, and the metropolitan area of La Paz-El Alto in Bolivia also depend on glacial melt to provide much of their drinking water.

In Ecuador, where Quito receives drinking water from the melting glaciers on the Antisana volcano, scientists have forecast that several mountain glaciers could disappear in the next 20 years. Between 1976 and 1997, the Cotopaxi volcano lost nearly one-third of its ice cover.

Bolivia’s Chacaltaya glacier—at 17,400 feet (5,300 meters) once touted as the world’s highest ski resort—has shrunk by 80% over the past two decades. Glaciers in the Condoriri range, which provide much of La Paz’s drinking water, have shrunk by 30% in the past 20 years.

Andean countries depend heavily on hydroelectricity, much of which during the dry season is derived from the flow of glacial melt. About 80% of Peru’s electricity comes from hydroelectric dams. A study published in June in the journal of the American Geophysical Union estimates that a 50% decrease in glacial runoff would reduce the capacity of one hydroelectric plant on the Santa River from 1,540 gigawatt hours to 1,250 gigawatt hours. Once the glaciers are gone, capacity would drop to 970 gigawatt hours.

Because the question seems to be not if, but when such runoff reductions occur, Inrena’s glacier inventory is aimed not just at mapping glacial retreat, but at determining how much fresh water remains in the country’s mountain reserves and how long it can be expected to last.

The calculation has been complicated by the lack of radar equipment for measuring glacier depth, Zapata says. His office has recently acquired the equipment, and his team is being trained to use it.

One expert who studies the depths of glaciers is Ohio State University professor Lonnie Thompson, winner of this year’s U.S. National Medal of Science and a member of the ice core paleoclimate research group at Ohio State’s Byrd Polar Research Center.

Thompson points out that accumulated ice, like the rings of a redwood tree, provides a window into history and serves as an early-warning system for global climate change.

He has been watching the world’s largest tropical ice cap—Quelccaya, in the department of Cusco, Peru—shrink since 1974, threatening one of the world’s best prehistoric climate records.

“Glaciers preserve a high-resolution temporal record of the past,” says Thompson. “Ice is the best recorder we have.” (See Q&A, this issue.)

When Thompson began studying Andean glaciers as a graduate student in 1979, the technology for drilling ice cores did not yet exist. He pioneered the use of portable, solar-powered equipment that could be hauled to the edge of the glacier on pack horses and then carried to the drilling site on foot.

He took the first core sample from Quelccaya in 1983 and a second in 2003. During the intervening decades, technology improved and his team expanded its range. The Byrd Polar Research Center now stores core samples of ice from glaciers around the world.

The samples contain detailed information about the history of climate and environment, but the technology for teasing the information out of the ice is still in its infancy.

“The first pioneers [of ice-core research] have just died,” Thompson says. “It’s a very young field and we’re just (now) learning new techniques.”

A crucial challenge for researchers is to take core samples from key glaciers and store them before the glaciers melt so they will be available when more superior analytical methods and technology are developed.

Glacial ice already is yielding valuable information, however. Scientists can determine annual precipitation from a glacier’s accumulated layers. The presence of dust indicates drought or volcanic eruptions, while pollen grains can signal changes in vegetation.

Oxygen isotopes reveal temperature, with the ratio of O18 to O16 isotopes increasing as the temperature rises. As a result, ice cores provide a record of El Niño events and changes in sea-surface temperature.

Meanwhile, air bubbles that have become trapped in the glacial ice provide the only record of the evolving composition of the world’s atmosphere over thousands of years. By analyzing this air, scientists can determine “the past concentration of greenhouse gases and the natural changes in greenhouse gases,” Thompson says. “You can measure oxygen, carbon dioxide and methane [from] long before human beings were a factor.”

Confirming human role

Comparing those measurements with figures from the years since the Industrial Revolution, he say, scientists have been able to confirm the human impact on climate change.

The ice cores read like a history book. The “little ice age”—four or five centuries that ended in the mid-19th century—shows up in samples from Quelccaya and other places in the Andes, as does the so-called medieval warm period that preceded it.

In the future, scientists should be able to extract more information from insects and bacteria trapped in the ice, Thompson forecasts.

Scientists forecast that it also will be possible to learn more about the Amazon’s climate history, including the natural frequency of forest fires before humans began clearing the rainforest, because snow falling on the Andean glaciers comes from air masses pushed upward from the Amazon.

Thompson says that of all the world’s glaciers, Quelccaya—which is 551 feet (168 meters) thick—is “the Holy Grail of tropical climate systems” because its ice-bound climatological record is so complete.

Since Quelccaya’s ice cap gets between 10 to 16 feet (three to five meters) of snow a year, the upper layers provide a week-to-week climate record, and the compressed, centimeter-thick layers at the bottom, which date from 315 AD, yield a year-to-year record.

While other ice sheets are older, Quelccaya’s clarity is unequaled. Says Thompson: “It’s a beautiful history for this part of the world.”

The samples, however, also underscore how accelerated glacial melting is threatening the historical record.

“What stands out in the [oxygen] isotopes is the last 100 years” of steady warming, Thompson says. The ice layers corresponding to modern history also contain elongated bubbles that signal melting and refreezing—a phenomenon not seen in older layers.

Last year, meteorological equipment on the ice cap recorded 47 days above freezing, including 18 when the temperature rose above 2ºC (35.6ºF).

Signs of acceleration

Thompson also monitors the retreat of the glaciers coming off the ice cap. One, the Qori Kalis glacier, was receding at the rate of six meters a year two decades ago, but is now shrinking at a 60-meter-per-year clip. Zapata also has found glacial retreat intensifying in the Cordillera Blanca.

As it melts, the Qori Kalis glacier is giving up more secrets. Thompson and his colleagues have found the remains of 5,200-year-old marsh plants in the area that had been covered with ice. The plants provide a glimpse of what the area looked like before it was glaciated, but they also indicate that the Andean climate is warmer now than it has been at any time in the past five millennia.

A variety of factors make glaciers melt faster as they shrink, exposing earth and rock that absorb more heat than they reflect. Global warming accelerates the process. And as global temperatures climb, the warming will be greater at high altitudes than at sea level, Thompson says.

If there is a 3ºC (5.4ºF) overall increase at sea level, the rise will be 3.3ºC (5.94ºF) in the upper Andes. And the consequences are unknown. “Climatologically, we’re in unfamiliar territory,” Thompson says.

Nevertheless, experts hope glacial melting will help catalyze a response to global climate change. “The glaciers are the most visible evidence of global warming,” Thompson says. “Their loss is apparent. And they have no political agenda.”