CORVALLIS, Ore. – A dramatic increase about 12,000 years ago in levels of atmospheric methane, a potent greenhouse gas, was most likely caused by higher emissions from tropical wetlands or from plant production, rather than a release from seafloor methane deposits, a new study concludes.
This research, to be published Friday in the journal Science, contradicts some suggestions that the sudden release of massive amounts of methane frozen in seafloor deposits may have been responsible for – or at least added to – past periods of rapid global warming, including one at the end of the last ice age.
The findings were made with analysis of carbon isotopes from methane frozen in Greenland ice core samples, by researchers from Oregon State University, the University of Victoria, University of Colorado, and the Scripps Institution of Oceanography at the University of California-San Diego.
For climate researchers, an understanding of methane behavior is significant because it is the second most important “greenhouse gas” after carbon dioxide. Its atmospheric concentration has increased about 250 percent in the last 250 years, and it continues to rise about 1 percent a year.
“Methane is a gas that makes a significant contribution to global warming but has gone largely unnoticed by the public and some policy makers,” said Hinrich Schaefer, a postdoctoral research associate in the OSU Department of Geosciences. “Its concentration has more than doubled since the Industrial Revolution, from things like natural gas exploration, landfills, and agriculture. We need to know whether rapid increases of methane in the past have triggered global warming or just been a reaction to it.”
To better answer this question, researchers studied two stable isotopes of carbon found in methane, that can provide a better idea of where the methane came from during a period thousands of years ago when Earth was emerging from its most recent ice age, and entering the interglacial period that it is still in. At that time, methane concentration went up 50 percent in less than 200 years.
Several things naturally produce methane, including biomass burning, geologic sources, wetlands, animals, and aerobic production by plants, a mechanism that was unknown until just recently. And huge amounts of methane – with more carbon stored in them than all the known oil and gas fields on Earth – are found in methane hydrates on the seafloor. In this setting, the cold temperatures and pressure keep the methane stable and prevent it from entering the atmosphere.
But some researchers have theorized that something might release the trapped seafloor methane – submarine landslides, a drop in pressure caused by dropping sea levels, or warming of ocean waters.
If that happened, it might cause a huge increase in atmospheric levels of methane and global warming. Some have hypothesized that this may be one of the factors that have helped cause cyclical ice ages – as ice levels rise and sea levels drop, methane might be released from the seafloor hydrates, causing global warming and an end to the ice age. Then the process would start over again.
“There have been estimates that releasing even 1 percent of the methane hydrates in the seafloor could double the atmospheric concentration of methane,” said Ed Brook, an associate professor of geosciences at OSU and co-author on the study. “So we looked to the past to see if that may have happened during previous periods of rapid global warming.”
Based on their isotopic analysis of the methane from the Greenland ice cores, the researchers concluded that it did not come from seafloor hydrate deposits or “gas bursts” of methane associated with them. The most likely candidates, they said, were higher emissions from tropical wetlands or larger amounts of plants, or some other combination of sources.
If the rise in methane had come from seafloor hydrate deposits, the study found, the atmospheric levels of methane would have had a different isotopic “signature” than they actually did.
There are still important questions to answer about methane in Earth’s atmosphere and the role it may play in global warming, the scientists said. For one thing, the current understanding of methane sources and sinks does not completely explain the isotopic signature of methane now found in the atmosphere. This indicates that estimates of methane emissions, including the human-made contribution, may have to be revised.
There are also concerns about methane trapped in permafrost across much of the Arctic regions. There are significant amounts of methane in this permafrost that could be released if it melted and organic material associated with melting permafrost could cause further increases in methane. This might cause “a fairly significant rise in the total level of atmospheric methane of around 20 percent,” Schaefer said.
By largely ruling out major bursts of methane from seafloor deposits during a period of global warming, however, this study suggests there may not be any “reinforcing” greenhouse mechanism from that cause. The increase in tropical wetlands or other factors that caused a large, rapid methane increase at the end of the last ice age may be relevant to future changes in methane, but changes in land use and vegetation cover due to human activities complicate the analysis of this issue, the scientists said.
OSU researchers are international leaders in the study of past climate changes, some of which have been surprisingly rapid - on the order of years or decades - and mechanisms that cause them. The studies were funded by the National Science Foundation, American Chemical Society and other grants.