Giant craters on the Arctic sea floor were formed when methane gas previously trapped in ice was released with such force it blew through bedrock, Norwegian researchers say.

A study published in the latest edition of the journal Science says that during the last ice age, a sheet of ice up to two kilometres thick lay on the floor of the Barents Sea off Norway, holding vast amounts of methane in hydrate form — an ice-like mix of gas and water.

According to the researchers, when a warming climate caused the ice sheet to dissipate around 12,000 years ago, the methane concentrated in mounds and then was "abruptly released," causing the craters.

"To disturb the bedrock that much, we feel pretty certain that it's not something that can be done by gas bubbles just seeping up. It must have been a catastrophic event," said lead author Karin Andreassen, a professor at the Centre for Arctic Gas Hydrate, Environment and Climate at the Arctic University of Norway.

Methane continues to seep out into the water to this day, Andreassen said, through more than 600 "gas flares" that remain near the craters.

Methane gas in northern waters is also an issue in Canada, Scott Dallimore, a research scientist at the Geological Survey of Canada, told CBC News.

This August 2009 photo shows methane gas bubbles in the Mackenzie River Delta in the Northwest Territories. Scott Dallimore, a research scientist at the Geological Survey of Canada, says the Mackenzie Delta region and the Beaufort Sea are both areas in the country's north where methane gas appears. (Rick Bowmer/Associated Press)

Like the craters observed in the Norwegian study, there are "pockmarks" that are actively releasing gas in areas of the Beaufort Sea, which stretches across the coasts of the Northwest Territories and Yukon Territory, Dallimore said.

But unlike the "blowout" the Norwegian researchers believe formed the craters in the Barents Sea, the Beaufort Sea pockmarks developed with a slower release of gas that weakened the sediments on the sea floor, Dallimore said.

Scientists studying the release of gas into the ocean agree it has critical environmental implications.

"It adds on to the acidification of the ocean and changing of the chemistry of the ocean and the ecosystems of the ocean," Andreassen said.

Whether the phenomenon also pushes greenhouse gases out of the water and into the atmosphere is an area requiring further research, the Norwegian study says.

At this point, it doesn't appear the methane rising from the floor of the Barents Sea is reaching the air, Andreassen said.

That could be due to the depth of the Barents Sea, Dallimore said, because when methane is released as a gas, the bubbles rapidly dissolve into the ocean. The deeper the water, the further the bubbles have to travel, so the methane is more likely to dissolve by the time it reaches the surface.

But the Beaufort Sea shelf is more shallow, he said, so it's possible for methane bubbles to reach the atmosphere.

"It's a topical concern," Dallimore said, noting that there is a program that assesses these types of potential "geohazards" in Canada, but more study is needed.