PUNAKHA, BHUTAN — High in the Himalayas, above this peaceful valley where farmers till a patchwork of emerald-green fields, an icy lake fed by melting glaciers waits to become a "tsunami from the sky."
The lake is swollen dangerously past normal levels, thanks to the global warming that is causing the glaciers to retreat at record speed. But no one knows when the tipping point will come and the lake can take no more, bursting its banks and sending torrents of water crashing into the valley below.
Such floods from above have hit Punakha before, most recently in 1994, a calamity that killed about two dozen people and wiped out livelihoods and homes without warning. But scientists say a new flood could unleash more than twice as much water and be far more catastrophic.
Unfortunately, Punakha's residents are not alone in this picturesque Buddhist kingdom in having the threat of death and destruction hanging over their heads like an environmental sword of Damocles. Because of Earth's rising temperatures, at least 25 glacial lakes in Bhutan are at risk of overflowing and dumping their contents into the narrow valleys where much of the country's population lives.
Like many poor countries, isolated Bhutan is paying for the environmental damage wreaked by the developed world and the expanding economies of nations such as China and India, whose fossil-fuel consumption and greenhouse-gas emissions are pushing global temperatures relentlessly upward.
But the added, perhaps more bitter, irony here is that Bhutan probably has done more to safeguard its environment than almost any other country.
A land of breathtaking vistas, little pollution and great biodiversity, Bhutan regards conservation as one of its most important public-policy goals -- an anchor of "gross national happiness," the quirky measure of development concocted by the former king and upheld by his son, the current occupant of the throne.
Sustainable development is the official mantra. By law, the country's forest cover, including blue pine, cypress, spruce and hemlock, must never drop below 60%. Snow leopards, Himalayan black bears, barking deer and red pandas roam unmolested in the national parks and wildlife reserves that account for a quarter of Bhutan's territory. A sanctuary in the east is famous as the only one in the world set aside for the yeti -- or migoi, the mythical Abominable Snowman.
"This country is committed to being conducive to environmental sustainability and not to be harmful to the world, but the impact of climate change is coming anyway," said Doley Tshering of the United Nations Development Program office in Thimphu, the capital. "You know you haven't created the problem, [yet] you know you're probably having the worst of it."
Some shifting weather patterns are already being felt.
"The winters are not so cold. The hot season is arriving much earlier," Tshering said. "Even fruit trees that would not fruit in Thimphu, that people just planted as ornamental flowers, are now starting to fruit."
Less benign are diseases such as malaria and dengue fever, common in the lower-lying, warmer south, which are now appearing at higher altitudes.
Officials are also worried that any changes to Bhutan's monsoon season could deal a major blow to agriculture, the main source of income for about 70% of the country's population of fewer than 700,000 citizens. Estimates of the population vary -- other agencies have put the figure as high as 2.3 million.
But possibly the most dramatic effect of global warming on Bhutan can be seen in its glaciers -- or, perhaps more accurately, not seen.
On satellite images taken in 2000 and 2001, some of the smaller ice sheets along Bhutan's 200-mile stretch of the Himalayas could no longer be found, according to a report last year by the International Center for Integrated Mountain Development and the U.N. Environment Program.
Experts estimate that Bhutan's glaciers are retreating by as much as 100 feet annually. The loss has grave consequences for the country's long-term development: Bhutan relies heavily on selling hydroelectric power, which accounts for about a third of national revenue.
"In the short run, we'll have increased summer flows, but after 40 years, it'll dry up," said Thinley Namgyel, a senior officer at Bhutan's National Environment Commission.
Of more immediate concern is the risk of floods from fast-filling glacial lakes.
In 1994, the Luggye lake burst and sent water hurtling down into Punakha. Now, a neighboring lake, the Thorthormi, poses an even greater peril.
Fed by a separate glacier, the Thorthormi has bulked up to alarming size and is in danger of swamping a third body of water, the Raphstreng. In a nightmare scenario, the two lakes could merge, punch through the natural but unstable moraine dams holding them back, and go cascading into the valley, picking up debris as they thunder downhill.