Paris - The Arctic Ocean could be a significant contributor of methane, a powerful greenhouse gas, scientists reported on Sunday.
Researchers carried out five flights in 2009 and 2010 to measure atmospheric methane in latitudes as high as 82° north.
They found concentrations of the gas close to the ocean surface, especially in areas where sea ice had cracked or broken up.
The study, published in the journal Nature Geoscience, wonders if this is a disturbing new mechanism that could accelerate global warming.
"We suggest that the surface waters of the Arctic Ocean represent a potentially important source of methane, which could prove sensitive to changes in sea-ice cover," it says.
If so, the Arctic Ocean would add to several identified "positive feedbacks" in Earth's climate system which ramp up the greenhouse effect.
One such vicious circle is the release of methane from Siberian and North American permafrost.
The thawing soil releases methane that has been locked up for millions of years, which adds to global warming - which in turns frees more methane, and so on.
But this is the first evidence that points to a methane contribution from the ocean, not the land, in Arctic latitudes.
Levels of methane in the atmosphere are relatively low, but the gas is 20 times more effective that carbon dioxide (CO2) at trapping solar heat.
Scientists have been struggling to understand the movements of the methane curve.
There was a rapid increase in levels due to post-World War II industrialisation, followed by a period of relative stability in the 1990s and more recently, by another rise.
The new paper, led by Eric Kort at the California Institute of Technology Caltech), says measurements of methane over some parts of the ocean were comparable to coastal eastern Siberia where there has been permafrost thaw.
Noting that around 10 million square kilometres of the Arctic Ocean are subject to summer melting of sea ice, "the emissions rate we encountered could present a source of global consequence", it says.
The source of the sea methane is unclear, it stresses.
The gas is unlikely to have been belched from sediment in the continental shelf as it was found at locations over the deep ocean. One idea is that it comes from microbes at the ocean surface.