GREAT FALLS — Healthy snowpack and cooler summers over the past four years have slowed melting of remaining glaciers in Glacier National Park in Montana, according to the U.S. Geological Survey.
"So the glaciers have paused in active retreat," said Dan Fagre, a research ecologist with the USGS' Northern Rocky Mountain Science Center in Bozeman who is stationed at Glacier.
But Fagre anticipates the glaciers, which are receding or disappearing, will likely resume retreating this year, if a forecast for El Nino-induced warmer temperatures comes to pass later this summer.
"This year we expect that any of the advantages the glaciers gained from the weather from the past few years will probably be erased," he said.
Fagre and other researchers studying glaciers in the park plan to return Wednesday to Sperry Glacier, which is in the center of the park south of Logan Pass.
Sperry also is at the center of USGS' efforts to monitor shrinking glaciers, and considered a benchmark for all of the park's remaining glaciers.
"So what Sperry does, most of the other glaciers are doing as well," Fagre said.
A glacier is a body of snow and ice that moves.
It forms when winter snowfall exceeds summer melting and retreats when melting outpaces accumulation of new snow.
"We've had a warming trend here that is about 1.8 times greater than the rest of the global average and that's enough to tip the balance for our glaciers," Fagre said.
In 1850, there were an estimated 150 glaciers of that size, compared to 25 today that are least 25 acres in size.
To be considered a true glacier, it must be at least 25 acres. Below that size, the ice is generally stagnant and does not move, unless it is on a steep slope.
Springs are beginning earlier, so snow has a longer period to melt in the summer, which is causing glaciers to retreat, Fagre said. There has been a 50-year decline snowpack in the area, he said.
At Glacier, researchers take photographs of glaciers and compare them to historic photos to map declines in the size.
Sperry gets a closer look with researchers traveling there several times a year to conduct "mass balance monitoring."
"Mass is how much ice there is there and balance indicates whether there's increase or decrease with neutral being zero," Fagre said.
The trip next week will involve measuring the snow at the end of the winter season, when the maximum amount of snow has fallen and before melting begins.
Researchers will return in the fall to take measurements on how much volume of snow and ice has been lost over the summer.
"The net balance tells us whether the glacier losing mass or gaining mass," Fagre said.
In the 10 years of the monitoring of Sperry, there has been a 5 percent decline in volume.
"We had this sort of pause," Fagre said of shrinking at Sperry Glacier and, by extrapolation, other glaciers. "They pretty much got as much snow as they needed."
Sperry covered 0.86 square kilometers in 2005, 0.83 in 2009 and 0.82 in 2013, illustrating the "pause" in its retreat as there was a 0.03 square kilometer loss from 2005 to 2009, but only 0.01 in the last four years, from 2009 to 2013, Fagre said.
This year, shrinking is expected to increase, based on the expectation of below-average precipitation and above average temperatures during the course of the summer as the El Nino strengthens.
The Climate Prediction Center just increased the probability of El Nino to 70 percent for this summer and fall, Fagre said.
"We think that there will not be enough snow left over to add to the glacier's size," Fagre said.
Much of the warming has occurred since the park was founded in 1910, he said.
A warming and dry period occurred from 1917 to 1941, based on tree-ring study, which Fagre called a "double whammy" for the glaciers.
That was followed by a cool and wet period that ended in 1976.
"Then basically temperatures have gone up enough since then that even when we have a cooler wet period, it's still too warm for the glaciers," he said.
A recent paper showed trout hybridization has increased due to warming stream temperatures, which reflects a loss of snow pack in glaciers, Fagre said. Its lead author was USGS researcher Clint Muhlfeld.
The paper says rapid increases in stream temperature and decreases in spring flow over the past several decades contributed to the spread of hybridization between native westslope cutthroat trout and introduced rainbow trout across the Flathead River system in Montana and British Columbia.
"It affects the whole ecosystem," Fagre said of receding glaciers. "And that's a clear example of those effects."
The park's relatively small alpine glaciers are good indicators of climate, the long-term average of daily weather conditions, according to the USGS. While occasional big winters or frigid weeks may occur, the glaciers are melting as long-term mean temperatures increase, the agency says.