In the new research, a thousand feet of additional melting has exposed plants that laboratory analysis shows to be about 6,300 years old. The simplest interpretation, Dr. Thompson said, is that ice that accumulated over approximately 1,600 years melted back in no more than 25 years.

“If any time in the last 6,000 years these plants had been exposed for any five-year period, they would have decayed,” Dr. Thompson said. “That tells us the ice cap had to be there 6,000 years ago.”

Meredith A. Kelly, a glacial geomorphologist at Dartmouth College who trained under Dr. Thompson but was not involved in the new paper, said his interpretation of the plant remains was reasonable.

Her own research on Quelccaya suggests that the margins of the glacier have melted quite rapidly at times in the past. But the melting now under way appears to be at least as fast, if not faster, than anything in the geological record since the end of the last ice age, she said.

Global warming, which scientists say is being caused primarily by the human release of greenhouse gases, is having its largest effects at high latitudes and high altitudes. Sitting at high elevation in the tropics, the Quelccaya ice cap appears to be extremely sensitive to the temperature changes, several scientists said.

“It may not go very quickly because there’s so much ice, but we might have already locked into a situation where we are committed to losing that ice,” said Mathias Vuille, a climate scientist at the State University at Albany in New York.

Throughout the Andes, glaciers are now melting so rapidly that scientists have grown deeply concerned about water supplies for the people living there. Glacial meltwater is essential for helping Andean communities get through the dry season.