An analysis of ancient Greenland ice suggests a spike in the greenhouse gas methane about 11,600 years ago originated from wetlands rather than the ocean floor or from permafrost, a finding that is good news according to the University of Colorado at Boulder scientist who led the study.
Methane bound up in ocean sediments and permafrost, called methane clathrate, has been a concern to scientists because of its huge volume, greenhouse gas potency and potential for release during periods of warming, said Vasilii Petrenko, a CU-Boulder postdoctoral fellow and lead study author. If just 10 percent of methane from clathrates -- an ice-like substance composed of methane and water -- were suddenly released into Earth's atmosphere, the resulting increase in the greenhouse effect would be equivalent to a 10-fold increase in atmospheric carbon dioxide, he said.
Using carbon 14 as a "tracer" to date and distinguish wetland methane from methane clathrates, an international team determined the methane jump 11,600 years ago likely emanated primarily from Earth's wetlands. "From a global warming standpoint, this appears to be good news," said Petrenko of CU-Boulder's Institute of Arctic and Alpine Research, lead author on a paper that was published in Science on April 24.
Methane is the third most powerful greenhouse gas behind water vapor and C02 and accounts for roughly 20 percent of the human-caused increase in the greenhouse effect.
As Earth emerged from the last ice age, temperatures in some places in the Northern Hemisphere shot up about 18 degrees Fahrenheit in just 20 years, said Petrenko. Scientists have been concerned that such abrupt warming events could trigger huge oceanic methane "burps" caused by the dissociation of seafloor clathrates, providing a positive climate feedback mechanism that could drive up Earth's temperatures still further.
"If we found that clathrates release a lot of methane to the atmosphere during abrupt episodes of warming, that could signal big trouble for the planet, " said Petrenko. "But even though wetlands appear be the primary source, it's still something to be concerned about."
Methane emitted from human activities like rice cultivation, livestock, the burning of grasslands, forests and wood fuels, gas leaks from fossil fuel production and waste management activities have nearly tripled methane concentrations in Earth's atmosphere in the past 250 years, Petrenko said. The amount of carbon held in methane clathrate deposits on Earth may equal the amount of carbon in all oil, coal and gas reserves on the planet, he said.
Study co-authors were from the Scripps Institution of Oceanography, Oregon State University, the Australian Nuclear Science and Technology Organisation, the National Institute of Water and Atmospheric Research in New Zealand, Danish Technical University and the Commonwealth Scientific and Industrial Research Organisation in Australia. Petrenko conducted most of the research as part of his doctoral thesis at the Scripps Institution of Oceanography under Professor Jeffrey Severinghaus.
The research team extracted several tons of ancient ice from the western margin of the Greenland ice sheet at a site called Pakitsoq, the largest ice samples ever recovered for a climate change study. The researchers cut the ice into blocks with electric chain saws, dumped 17 cubic feet at a time into a vacuum melting tank heated by powerful propane torches, and transferred ancient air released from bubbles in the ice into cylinders for subsequent laboratory analysis, Petrenko said.
The effort, which lasted five field-seasons, was "an undertaking of epic proportions," said Petrenko. "This was the first measurement of its kind, and we really pushed the envelope," he said. "It represents a major advance in analytical methods for studying ancient ice."
Methane clathrates are only stable in conditions that combine cold temperatures and high pressures. Some scientists suspect that a swift and massive warming in the early Cenozoic era about 56 million years ago may have been triggered by huge methane releases from clathrates into the atmosphere, Petrenko said.
Methane levels in Earth's atmosphere increased about 2 percent from about A.D. 1 to 1000 and decreased by 2 percent from 1000 to 1700, which may have been due in part to decreased landscape burning by indigenous people in the Americas devastated by introduced diseases, according to a 2005 CU-Boulder study. 暗网禁区 60 percent of atmospheric methane is now generated from human-related activities, according to the International Panel on Climate Change.
The 2009 Greenland ice study was funded by the National Science Foundation, the American Chemical Society and several other agencies. Petrenko's postdoctoral fellowship at CU-Boulder is funded by The University Corporation for Atmospheric Research.