In the heart of Nepal's Sagarmatha National Park, I hike above the hamlet of Chukung, 11 miles south of Mount Everest, with Tenzing Sherpa, a former monk in the Tengboche monastery. We wander through lush yak meadows beneath the monolithic Lhotse, the world's fourth-highest mountain peak. Ravens clack and prattle as they wheel above us, and I scan the surrounding peaks, all more than 22,000 feet. Our faces are seared by the high-altitude sun, our backs chilled by the 10-degree air.
We scramble up the flank of a 200-foot-high hill of freshly churned rubble that stretches more than a mile. I reach the ridgeline, peer cau-tiously over an abrupt edge, then quickly draw back from a precipitous drop to a lake 150 feet below. I crawl again to the edge and watch a skein of late-morning mist rise from the water's surface, like steam from a cup of coffee.
Fifty years ago, Imja Tsho (tsho is Tibetan for lake) -- now about a mile in length and up to 295 feet deep -- did not exist. In-stead of looking down at a body of water, we would have looked out at the Imja Glacier, an upended maze of house-size blocks of ice and near-bottomless crevasses. Now, between breaths, Tenzing and I listen as rocks loosened from the gravelly glacial moraine by the morning thaw clatter and plunge to the water.
The rapid melting and recession of the Imja Glacier, and the simultaneous growth of Imja Tsho, have alarmed national park staff and the Sherpas who grow potatoes and run trekking lodges in scattered villages down valley. Glaciologists are concerned too, for they are seeing glacial lakes forming and filling faster than they can identify and catalog them. Ultimately, they fear, many will simply grow too large and burst through their moraines of unstable ice and rubble -- as happened in northern Bhutan in 1994.
That year, a mile-long glacial lake named Luggye Tsho, near Bhutan's border with Tibet, ruptured catastrophically. Over a period of several hours, the entire lake -- more than a billion cubic feet of water -- emptied out, sending a rampaging torrent down valley that swept away an artisans' colony near the town of Punakha, killing 23 people. Along Bhutan's border with India, 125 miles away, a hydrograph that measures the level of the nearby Sankosh River broke when the water rose to eight feet above normal. Now two nearby lakes, Thorthormi and Raphstreng, are also poised to overflow.
The Great Himalaya is home to thousands of glacial lakes. In 2001 the United Nations Environment Program identified 44 in Nepal and Bhutan that are at imminent risk of bursting. The task of tracking the growth of all these lakes is immense: The Himalaya contains more than 18,000 glaciers covering an area of 13,000 square miles. Directly to the north on the Qinghai-Tibetan Plateau, according to the Chinese Academy of Sciences, 46,298 glaciers blanket nearly 60,000 square miles. All told, this constitutes the largest area of ice outside the polar regions.
Global warming has forced into retreat virtually all of these glaciers, which are shrinking at a rate of 100 to 230 feet per year. Since the mid-1970s, average air temperatures across the Himalaya have risen by 1 degree centigrade, with higher sites warming twice as much as middle elevations. Glaciers reflect sunlight back into the atmosphere, but as they recede, more heat-absorbing rock and impounded glacial lake water are exposed to the sun, raising ambient temperatures and intensifying the melting process. This phenomenon is similar to what is occurring at the polar ice caps, as dark-colored ocean replaces the reflective ice sheet.
During the Little Ice Age, which ended as recently as 150 years ago, glaciers slowly bulldozed horseshoe-shape moraines of rubble and ice down valley. Now the melting tongues of some glaciers are found several miles up valley from these remnant moraines' farthest reaches. Some of these natural structures act as dams, trapping glacial meltwater. But they are not inviolate; beneath their surface rubble lies a core consisting largely of ice. As this ice melts, the moraine subsides and the lake fills with meltwater. "Piping," or infiltration of lake water through interstices in the ice and rubble, further weakens the moraine, turning it into the geologic equivalent of Swiss cheese.
The collapse of a moraine dam usually doesn't occur spontaneously. Most outbursts are triggered by a specific event, such as a landslide or an avalanche that sends rock and earth cascading into the water, generating a massive surge wave that can crest at well over 100 feet. These waves easily overtop the retaining moraine, causing a breach.
