In July, at least 90 million cubic yards of glacial ice and rock plummeted from the Aru Range of mountains in western Tibet. Those who witnessed the collapse said that the avalanche lasted no longer than five minutes.

The devastation within such a short time was immense. In places, the ice deposits ran 30 feet deep. The avalanche buried 3.7 square miles — an area equivalent to two-thirds of the Los Angeles airport campus. Debris struck Dungru village, in Tibet’s Rutog County, killing nine herders. More than 100 yaks perished, as did 350 sheep. NASA, which documented the debris field via satellite, described the collapse as one of the largest recorded avalanches in history.

The collapse, at first, left climatologists perplexed. But now an international team of scientists has found the most likely culprit: an unusual slick of meltwater beneath the glacier, created by unusually warm temperatures.

“The site of collapse is baffling. … The Rutog avalanche initiated at quite a flat spot. It doesn’t make sense,” Tian Lide, a glaciologist at the Chinese Academy of Sciences, which operates a scientific station in the area, told Nature in August. The Aru Glacier collapsed again in September. No one was reported injured.

Certain glaciers surge, advancing a matter of yards in short bursts. “When the glaciers collapsed in western Tibet,” wrote Tian in an email to The Washington Post on Monday, “it was first assumed to be [the result of] surging-type glaciers.”

In 2002, a 1.5-mile-long section of glacier in Russia — the Kolka Glacier, known to surge — broke free, traveling 11 miles in six minutes and killing more than 100 people. Perhaps the Aru Glacier was acting like its Russian precursor.

But glaciers that surge tend to be lengthy and wide. The Aru Glacier is small and frigid and clings to the mountain like a tongue to a frozen flagpole. And glacial surges are cyclical. There was no indication in remote-sensing data nor in reports from local herders that the Aru Glacier had surged, Tian said.

Surges are measured in yards, too, not miles. “This is new territory scientifically,” University of Oslo glaciologist Andreas Kaab said to NASA in September. “It is unknown why an entire glacier tongue would shear off like this. We would not have thought this was even possible before Kolka happened.”

Tian and his colleagues in China and at Ohio State University say they may have an answer. Climatologists had assumed the Tibetan glacier was frozen to mountain bedrock at subzero temperatures, making the Aru Glacier a type of glacier known as “cold-based.”

Cold-based glaciers have not changed significantly since the Little Ice Age, Tian said, referring to a period of cooling between the 14th and 19th centuries. The Aru Glacier was expected to change little in the future. The glaciers there were “quite stable,” he said, “with low local precipitation, low glacier movement and cold temperatures.”

As the scientists wrote recently in the Journal of Glaciology, “This is the first known occurrence of an unexpected, instantaneous collapse of a cold-based glacier in a nonvolcanic region.”

Based on GPS imagery and mathematical modeling, coupled with the climate data in the region, the researchers hypothesize that the Aru Glacier had begun the process of becoming a polythermal glacier — that is, a mixture of subzero ice as well as ice warmed to the melting point.

“The cold-based glaciers are not expected to slide at the bottom of the glacier,” Tian said, “but polythermal glaciers probably will.”

It was possible that meltwater pooled at the bottom of the glacier, forming a lubricant that enabled the swift collapse. Ohio State University earth scientist and study co-author Lonnie Thompson pointed to the fact that the avalanche happened so quickly, in just four to five minutes, as key evidence. “Given the rate at which the event occurred and the area covered, I think it could only happen in the presence of meltwater,” he said in a news release.

The source of the meltwater, if it existed, remains unknown. But Thompson noted that the area has steadily warmed. The weather station closest to the glacier reported the temperature increased 2.7 degrees Fahrenheit over 50 years, he said. Such warming may seem insubstantial — but it was enough for melted snow to seep below the glacier, Thompson said. The weather station also recorded high levels of precipitation in the 40 days before the avalanche.

“What we are worrying is that it is probably an extreme way for the cold-based glacier to respond to the climate warming,” Tian told The Post.

The July collapse “raises concerns that future events are possible and may pose risks for inhabitants of this region,” the glacier scientists concluded in the paper. “If the climate warming in the region is the primary cause of the Aru Glacier collapse, then it will not be the last one.”

Tibetan scientists warn of rapidly melting glaciers at the 'roof of the world,' and think global warming could be the culprit.

More from Morning Mix