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. A land of breathtaking vistas, little pollution and great biodiversity, Bhutan regards conservation as one of its most important public-policy goals. 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 percent. Snow leopards, Himalayan black bears, barking deer and red pandas roam unmolested in the national parks and wildlife reserves that account for one-quarter of Bhutan's territory. A sanctuary in the east is famous as the only one in the world set aside for the yeti