About 15,000 researchers have gathered in San Francisco this week to discuss earthquakes, water resources and planetary science, but climate change was the topic of the day.

Barnett studies snowpack at high altitudes in the Western United States and estimates the region's snow accumulation decreased an average of 20 percent between 1950 and 1999.

Only about one quarter of this decrease can be reliably explained by natural temperature variations. Computer modeling shows the remainder is "a slam dunk" attributable to human activity, said Barnett.

About 50 percent of the fresh water consumed by people worldwide comes from mountains, so the rate at which snowpack is disappearing is worrying, said Daniel Fagre, an ecologist who works for the US Geological Survey in Glacier National Park in Montana.

Fagre said only about 25 of 150 glaciers that once dotted Glacier National Park remain. Initial data projected that, for the first time in more than 1,000 years, the park would be without ice floes by 2030, but more recent estimates project the icebergs may be lost even before then, Fagre said.

"The glaciers of Glacier National Park will be gone in our lifetimes," Fagre said, noting that big horn sheep now graze in the park locations where glaciers once stood.

Scientists say climate change is caused by carbon dioxide which traps heat in the Earth's atmosphere and raises temperatures. Other speakers at the meeting said climate change is also a cyclical process that accelerates once it has begun.