A study of Greenland's icesheet has revealed that a vast store of planet-warming methane appears to be more stable than thought, easing fears of a rapid rise in temperatures, a scientist said on Friday.
Methane is about 25 times more powerful at trapping heat in the atmosphere than carbon dioxide (CO2) and vast amounts of the compound are trapped in permafrost in the far northern hemisphere or in seabed deposits called clathrates.
Scientists have feared climate change could trigger a huge release of methane from the clathrate reservoir, sending global warming spiralling out of control.
An estimated 5,000 billion tonnes of carbon are locked up in these deposits, said Vasilii Petrenko of the Institute of Arctic and Alpine Research at the University of Colorado. "That's about equal to all of the oil, coal and gas reserves that we think we have," he told Reuters from Boulder, Colorado.
Petrenko and an international team of scientists spent six years studying air samples from vast blocks of Greenland ice to see if a rapid rise in temperatures about 12,000 years ago was triggered by methane from clathrates or another source.
The results showed the methane was most likely to have come from wetlands rather than the clathrates, deposits which resemble ice and are held in place on the ocean bed by high pressures and relatively low temperatures.
Petrenko said temperatures in Greenland 12,000 years ago had increased about 10 degrees Celsius in 20 years. But it took 150 years for methane levels in the atmosphere to increase by 50 percent. The rapid warming was driving the release of methane, he said, with the most likely sources being tropical wetlands and the vast northern wetlands created after the large-scale retreat of icesheets about 18,000 years ago.
GOING BACK IN TIME
Previous studies of ice core samples from Greenland and Antarctica had shown an increase in methane levels about 11,500 years ago came from the tropics and the northern wetlands.
But Petrenko and his team wanted to be sure of the source, particularly since a massive release of methane from clathrate deposits is believed to have sent temperatures soaring about 56 million years ago, when the planet was much warmer than today.
His team measured the amount of an isotope called carbon-14 (C14) in tiny amounts of methane extracted from air bubbles trapped in Greenland ice going back 12,000 years. C14 deteriorates at a known rate, so the scientists can use it to determine the age of the ice and also the likely source of the methane.
Methane from wetlands has different C14 "signature" than methane from clathrate deposits. "The project involved pushing the analytical techniques to a level no one has taken them before," Petrenko said.
Only about one trillionth of the methane from the air bubbles contained the carbon-14 isotope. The analysis was undertaken at Australian Nuclear Science and Technology Organisation. "The results definitely help us to say that it doesn't seem methane clathrates respond to warming by releasing lots of methane into the atmosphere, which is really good news for global warming."
But the sting in the tale were the ice core records showing methane levels rising as the planet warms. "We're warming now and we know that there's evidence of northern wetlands becoming more productive. If it's not clathrates, the wetlands might still drag the methane up."