Glaciers along the southeastern rim of the Tibetan Plateau temporarily defied a warming climate around 9,000 years ago. Now a new model helps explain how they accumulated ice even though other glaciers in the region waned, scientists say.
Cyclical variations in the Earth’s orbit cause long-term fluctuations in the amount of sunlight reaching the top of the atmosphere, also known as insolation, in the Northern Hemisphere during summer months. Just after the most recent ice age ended, about 10,000 years ago, the Northern Hemisphere’s insolation was on the rise and the climate was warmer than it is today, says Summer Rupper, a climatologist at Brigham Young University in Provo, Utah. Field data indicate that most glaciers in the region were retreating around 9,000 years ago, but in one area — the high mountains that stretch from the central Himalayas eastward into southern China — the ice masses inexplicably grew. Now, analyses by Rupper and her colleagues reported in an upcoming Quaternary Research suggest how those glaciers amassed ice in a warming climate.
The researchers looked at previous studies that had used 18 different models to compare global climate about 6,000 years ago with today’s climate. Few studies have focused on the period about 9,000 years ago, but the team’s analyses show that the climate at that time was comparable to conditions around 6,000 years ago, Rupper says.
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Most of these regional, low-resolution models agree that about 6,000 years ago, summertime temperatures at most sites in central Asia averaged between 2 degrees and 6 degrees Celsius higher than they are today, Rupper notes. Accordingly, many of the region’s glaciers — and particularly those along the western and northern rims of the Tibetan Plateau — melted back to higher altitudes. But those along the southeastern rim of the Tibetan Plateau actually grew, in some cases advancing to points as much as six kilometers beyond today’s limits.
Previously, researchers have suggested that an increase in monsoon precipitation in the Himalayas caused the glacial growth, Rupper says. But the new analyses show that the added snowfall — measuring anywhere between 3 and 7.5 meters of extra snowfall each year — could account for less than 30 percent of the glacial advance. The largest contributor to the anomalous glacial growth was an increase in cloudiness throughout the eastern Himalayas, which caused temperatures there to drop several degrees below the average temperature in other parts of central Asia at the time. A smaller yet substantial contributor was an increase in evaporative cooling, which likely stemmed from stronger winds, Rupper says. Neither cloudiness nor evaporative cooling have been considered in most previous models of glacial behavior, she notes.
This is great work, says Lewis Owen, a glacial geologist at the University of Cincinnati. The team’s new model of glacial behavior “should get people to thinking,” he adds.
The new model may also help explain glacial behavior in the Karakoram range in central Asia today, says Bruce Molnia of the U.S. Geological Survey in Reston, Va. He and other researchers have noted that glaciers in arid and largely cloud-free Afghanistan are shrinking substantially, but those just 300 kilometers or so to the south in Pakistan — an area that experiences increased monsoon precipitation in the summer months — are growing.