ScienceDaily reports that an expansion of wetlands and not a large-scale melting of frozen methane deposits is the likely cause of a spike in atmospheric methane gas that took place some 11,600 years ago. An international research team led by Scripps Institution of Oceanography at UC San Diego determined that the amount of carbon-14 isotopes in methane from air bubbles trapped in glacial ice was chemically not consistent with the melting of methane clathrates and therefore the release of huge amounts of methane during that time into the atmosphere (clathrates are solid forms of methane existing in seafloor deposits and permafrost, sequestered either by the intense pressure of the deep ocean or cold temperatures on land).
A view shared so far by many scientists was that a thawing of Arctic permafrost and warming of ocean waters could create a tipping point that would send planetary warming into overdrive. The fear has been that “a melting of only 10 percent of the world’s clathrate deposits would create a greenhouse effect equal to a tenfold increase in the amount of carbon dioxide in the atmosphere. For comparison, the warming trend observed in the last century has taken place with only a 30 percent increase of atmospheric carbon dioxide.”
The new study allays these fears, suggesting that clathrates do not respond to warming by releasing large amounts of methane into the atmosphere. The researchers arrived at this judgement by comparing the low levels of the carbon-14 isotope contained in methane currently present in Greenland’s air and carbon-14 trapped 11,600 years ago in the Greenland ice sheet. A “burst of methane took place immediately after an abrupt transition between climatic periods known as the Younger Dryas and Preboreal. During this event, temperatures in Greenland rose 10° C (18° F) in 20 years. Methane levels over 150 years rose about 50 percent, from 500 parts per billion in air to 750 parts per billion.” Crucially though, levels of carbon-14, which has a half-life of 5,730 years, were too high in the methane from that time to have come from clathrates, the researchers concluded; instead they went hand in hand with an expansion of wetlands, which produce large amounts of methane from bacterial breakdown of organic matter and are known to have spread during warming trends throughout history.
“This study is important because it confirms that wetlands and moisture availability change dramatically along with abrupt climate change,” said one of the researchers. “This highlights in a general way the fact that the largest impacts of future climate change may be on water resources and drought, rather than temperature per se.”