As if rising sea levels weren't already enough of a threat, there are now concerns that melting glaciers could leak deadly mercury into the world's oceans, and from there into the food supply.
Researchers looking at rivers and fjords connected to the Greenland Ice Sheet - the second-biggest in the world, behind Antarctica - have found levels of mercury "comparable to rivers in industrial China".
"There are surprisingly high levels of mercury in the glacier meltwaters we sampled in southwest Greenland," said Florida State University scientist Jon Hawkings, who led the new study, published this week in journal Nature Geoscience.
"That's leading us to look now at a whole host of other questions such as how that mercury could potentially get into the food chain."
Mercury - chemical symbol Hg - is toxic to humans, and its presence in the environment has significantly worsened "due to anthropogenic pollution", particularly in the Arctic, where concentrations in marine animal and plant life "have increased by an order of magnitude over the past 150 years".
It's believed the region could be a "global Hg sink as prevailing atmospheric circulation carries Hg to northern latitudes", the study says.
Typically rivers would contain about "a salt grain-sized amount of mercury" for every Olympic-sized swimming pool of water - about 2.5 million litres. Mercury in the Greenland glacial meltwater rivers was found in concentrations up to 15 times that, and 200 times more was found in glacial flour - sediment created when a glacier scrapes across bedrock.
"We didn't expect there would be anywhere near that amount of mercury in the glacial water there," said Rob Spencer, associate professor of earth, ocean and atmospheric science at Florida State University.
"Naturally, we have hypotheses as to what is leading to these high mercury concentrations, but these findings have raised a whole host of questions that we don't have the answers to yet."
Such high concentrations are unlikely to be from human activity, they said, because there is much more mercury in the rivers than surrounding snow and ice, inconsistent with a pollution source, which would be less discriminate in its spread.
"We've learned from many years of fieldwork at these sites in western Greenland that glaciers export nutrients to the ocean, but the discovery that they may also carry potential toxins unveils a concerning dimension to how glaciers influence water quality and downstream communities," said University of Bristol glaciologist Jemma Wadham.
Glacial ice covers about 10 percent of the world's surface. If Greenland's ice melted, it would raise sea levels globally about seven metres - but the discovery suggests that would be far from the only problem if global temperatures aren't kept in check.
According to the World Health Organization, inhaling mercury "can produce harmful effects on the nervous, digestive and immune systems, lungs and kidneys, and may be fatal". Ingesting - or even absorption through the skin - can cause "tremors, insomnia, memory loss, neuromuscular effects, headaches and cognitive and motor dysfunction".
"All the efforts to manage mercury thus far have come from the idea that the increasing concentrations we have been seeing across the Earth system come primarily from direct anthropogenic activity, like industry," said Dr Hawkings. "But mercury coming from climatically sensitive environments like glaciers could be a source that is much more difficult to manage."
