The chunks of blue-tinted ice that splash into the sea from Alaska's calving tidewater glaciers make for dramatic images, but far more meltwater is flowing into the ocean from Alaska's mountain and inland glaciers, a new study has found.
In all, Alaska's melting glaciers are losing 75 billion tons of ice a year. That's enough, if melted, to cover the entire state with a foot of water every seven years, or to fill 30 million Olympic-sized swimming pools, according to the study, by scientists from the University of Alaska Fairbanks, the U.S. Geological Survey and University of Washington.
Most of that loss, about 94 percent, comes from glaciers located on land or by lakes, where melt is occurring at the surface where sunlight and atmospheric heat beats down, said the study, published online in the journal Geophysical Research Letters.
The findings are based on measurements of 116 Alaska glaciers. That includes data from new mapping of glacier boundaries, and precise measurements of glacial surface elevations, tracked by NASA through its Operation IceBridge using LiDAR technology.
The results upend a previously held belief that Alaska glaciers' contribution to global sea level rise would diminish somewhat once the major tidewater glaciers retreated back to shoreline, said Shad O'Neel, a USGS research geophysicist and a co-author of the study.
With the climate expected to keep warming and with nontidewater glaciers so important to the flow of water into the sea, "It's unlikely that the 75 gigatons a year is going to get smaller," O'Neel said.
Though there is an overall trend, each glacier has a unique story.
Columbia Glacier in Prince William Sound, one of the most-viewed and best-studied glaciers in the world, is a huge shedder of ice, making it an outlier in the tidewater category. The study's calculations put its annual loss of mass at more than 3 billion tons a year, and the huge glacier is thinning by 3.96 meters of water equivalent. Columbia has retreated more than 12 miles since 1980, according to the USGS.
But big glaciers high in the mountains, such as the Harding Icefield in Seward and Bering Glacier, which dumps its meltwater into a lake, are also losing significant mass each year, according to the measurements.
The much-smaller Yanert Glacier in Interior Alaska is thinning at nearly the same rate as Columbia Glacier, 3.01 meters of water equivalent a year, and losing 250 million tons of ice annually, according to the study,
Even some glaciers that are advancing are still losing mass because they are thinning, and some surging glaciers, which advance quickly as snow and ice slumps downhill, are also losing mass.
Only a few glaciers are gaining mass, according to the study's measurements. Among them are the Hubbard and Taku glaciers, tidewater glaciers in Southcentral and Southeast Alaska.
The variation among glaciers is to be expected, as each has unique contours and each has its own environmental circumstances, O'Neel said
"The glaciers are on the border of all the climate divides," he said. Climate zones are separated by mountains, so the fates of glaciers at the tops of those peak are dependent in part on slope location, he said.
But the overall trend is for melt and mass loss, and that might be accentuated in a year of sparse winter snowfall and record summer heat.
"This is probably going to be a pretty tough year for a lot of the glaciers," O'Neel said.