Along with increasing sea levels, melting glaciers are putting something else into the world's oceans -- a huge load of organic carbon that has the potential to change marine chemistry and ecosystems, says a newly published study by a team of mostly Alaska scientists.
By 2050, the cumulative global load of organic carbon swept into the sea by glacial melt -- the of bits of old flora and fauna and their byproducts that have been absorbed, covered or ground up by the moving ice -- is expected to be 48 million metric tons according to the study, published Monday in the journal Nature Geoscience.
That is 1.5 times the amount of annual carbon runoff from the Amazon River, which has the biggest flow of all the world's rivers.
Of that organic carbon expected to be dumped into marine waters, nearly a third -- an estimated 15 million metric tons -- will be attributable to a warming climate and the acceleration of glacier melt, the study said.
The study analyzes already published data from glaciers in Alaska, Greenland, Svalbard, Antarctica, the Himalayas and other sites to calculate expected deposits of organic carbon dropped into the oceans. The researchers, from the University of Alaska Southeast, the U.S. Geological Survey and other institutions, used information gathered at well-studied glaciers in those parts of the world and pieced that into a model intended to calculate runoff and carbon deposits from all the world's glaciers.
"This is the first time it's been done in a comprehensive, data-driven way," said lead author Eran Hood, an environmental science professor at UAS.
Polar ice sheets and mountain glaciers cover about a tenth of the earth's surface and hold about 70 percent of the planet's fresh water, according to the USGS.
The accelerated melt of glaciers gets attention because of its contribution to sea-level rise, but the "real take-home message" of the new study is that there will be "not just changes to the level of the ocean but changes to the chemistry and the food web," Hood said.
Organic carbon is eaten by microbes and is at the base of the food web, he said. But it can also break down into inorganic carbon, which changes marine chemistry in other ways, he said.
Amounts of runoff and carbon loads will vary year to year, Hood said, because glacier dynamics vary. But even temporarily stable glaciers will be contributing to the loads, he said. "Even if a glacier doesn't lose mass, there's a lot of melt that comes off of it each year," he said.
Whether it will be a good or bad thing for more carbon to be flushed into the oceans from accelerated melt of glaciers is yet unknown, Hood said.
"When you're putting more of this stuff in from the glacier melt, you can change the downstream food web. But we don't really know what the changes are," he said. "It could be a good thing in some areas, but we're not at the point of understanding this."
One expected chemical effect of glacial melt is the accelerated acidification in the Gulf of Alaska and similar nearshore waters close to such tidewater ice.
Scientists from the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory reported this week on preliminary results from a project that measured water chemistry throughout the summer in Prince William Sound.
The project used three remotely operated "gliders" -- small watercraft similar in appearance to surfboards – and a local cruise ship to ferry measuring equipment around the glacier-fed sound.
The project is the most comprehensive effort ever to track the corrosive characteristics of water at different sites in the sound and the nearby northern Gulf, Evans said. Up to now, the scientists studying ocean acidification in the Gulf region have had to rely on relatively short-term water-sampling cruises conducted at the beginning and end of the summer, he said. But this project was collecting data daily for four months, he said.
While glacial runoff is not acidification in itself, it can exacerbate the change in pH by mixing fresh water with low calcium content into calcium-containing salt water, according to the researchers. In addition, meltwater from glaciers is known to quickly absorb carbon dioxide from the atmosphere, adding to the forces behind acidification, the researchers said.
Evans said data from the summer's project is still being analyzed, but that already some interesting results have emerged. One striking result, he said, was the discovery of hotspots near glaciers where calcium and associated pH levels were particularly low.
"I was pretty shocked at the lower values that were observed near the glacier and the extent of the chronic effect," he said.
Another big lesson was that the methods and equipment deployed last summer in the sound worked, Evans said. The gliders survived being on the open water amid numerous other vessels and changing wave conditions, and the researchers were particularly pleased with the reliable measurements taken by instruments placed on a passenger vessel operated by Major Marine Tours.
The ship "went up to the glacier every day and that saw the hotspot region up close," Evans said.
The partnership with the tour company and cooperation the project got from Prince William Sound fishermen and other mariners could wind up as a model for future projects, he said.
"It's hard to do ship-based oceanography because it's so expensive, so this is one way of getting around it," he said.