At 17 square miles, the Quelccaya ice cap at 18,000 feet in Peru's Andes is a fair-sized glacier on the world stage, but in the tropics it's the tops. In the past 25 years, Quelccaya has lost ice that accumulated over a millennium and a half, glaciologists have concluded. What does all that disappearing ice way up in the mountains matter? "How much time do we have before 50 percent of Lima's or La Paz's water resources are gone?" researcher Douglas Hardy of the University of Massachusetts wondered in a recent interview.