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Critical North American Mountain Region Melting at Fastest Rate in Centuries

Summer heat hitting glaciers harder than lower elevations

April 10, 2018 – One of the highest glaciers in the United States is melting at the fastest pace in 400 years, according to research from Dartmouth College, the University of New Hampshire and the University of Maine.

The study links increased melting observed on Mount Hunter in Alaska’s Denali National Park to rising summer temperatures in the region as well as atmospheric conditions created by a warming tropical Pacific Ocean.

In a surprise result, the research also found that the summit area studied is warming faster than low-lying regions in Alaska. The finding that warming is more severe at higher elevations could have a major impact on glaciers in the region and on global sea levels. 

“We have not seen snow melt like this in at least four centuries,” said Dominic Winski, a research assistant at Dartmouth and the lead author of the report. “Alaska was already known to be one of the fastest warming places on Earth. The fact that we found even faster warming high in the mountains is that much more noteworthy.” 

For the research, the Dartmouth-led team studied melt layers encoded in two ice cores drilled from the third highest peak in the Alaska Range. The cores from about 13,000 feet atop Mount Hunter reveal 60 times more annual melt today than there was during the pre-industrial period. 

While existing records provide hundreds of years of temperature data from lower elevations, the ice core melt layers allowed researchers to develop a high-elevation temperature record that extends 400 years into the past. This is a longer period than any other temperature record in the North Pacific region above 10,000 feet and creates the first high-elevation, ice core temperature record in the region that dates prior to the year 2000. 

The researchers installed a weather station on the high mountain glacier to collect temperature measurements in recent years, allowing them to understand the relationship between melt layers in the ice core and summer temperatures at the site.

“We know so little about how the high mountains are responding to climate change because it’s such a brutal environment to collect weather data – the temperature records are sparse and filled with gaps,” said Erich Osterberg, an assistant professor of earth sciences at Dartmouth who oversaw the study. “The best records of mountain warming are melt layers trapped in the glaciers themselves, preserving their climate history over the past several centuries.”

The study found that modern Mount Hunter temperatures are at least 2 to 3.5 degrees Fahrenheit warmer than during the period from the 18th century through the early 20th century. Nearly all of the warming occurred within the last 100 years, with the fastest warming being observed since the late 1970s. 

The research result showing that summer temperatures in Alaska are warming faster at high elevation than near sea level adds to the complex picture of current-day climate change. The distinction is especially important because the North Pacific region is home to thousands of square miles of alpine glaciers that are vulnerable to melting from warmer temperatures. 

Summer temperatures are known to be the primary determinant of ice gain and loss in the North Pacific. Other research cited in the study notes that Alaskan alpine glaciers have been receding rapidly over the last 50 years and are contributing steadily to global sea-level rise. 

“Our record provides a new twist on the rapid melting of Alaskan glaciers. It turns out that these high mountain glaciers have been exposed to more warming over the past century than we thought,” said Winski. 

As part of the study, the researchers found that the high-elevation melt layer record matches with warming sea surface temperatures in the tropical Pacific Ocean. The study team concludes that warm conditions in the tropical Pacific are likely responsible for part of the warmer weather observed on Mount Hunter.

The study suggests that warmer tropical Pacific Ocean temperatures have created a pattern of atmospheric waves extending toward Alaska through the upper atmosphere. These wave patterns enhance summertime high-pressure systems over Alaska and represent the likely link between the summer temperatures over Alaska and sea surface temperatures in the tropical Pacific. 

An earlier paper by the research team on the same ice cores found that higher tropical ocean temperatures are also linked to a doubling of snowfall on Mount Hunter during the winter, when it is always cold enough that precipitation falls as snow. 

“While it may seem counter-intuitive that the same location can get more snowmelt and more snowfall, what we found is that warming in the tropics gives a boost to precipitation in the winter and a boost to temperatures and melt in the summer. It’s another example of how complex relationships can lead to climate change surprises,” said Osterberg. 

According to the research, the ice core record observed in the study indicates that alpine regions surrounding the North Pacific may continue to experience accelerated warming with climate change, further threatening the already imperiled glaciers in the area. 

The study appears in the Journal of Geophysical Research: Atmospheres, a journal of the American Geophysical Union.