This finding indicates that methane may no longer be as large a global warming threat as previously thought, and it provides evidence that methane levels can be controlled.
Scientists also found that pulses of increased methane were paralleled by increases of ethane, a gas emitted during fires. This is further evidence, they say, that methane is formed during biomass burning and that large-scale fires can be a big source of atmospheric methane.
Professors Sherwood Rowland and Donald Blake of the University of California, Irvine, and researchers Isobel Simpson and Simone Meinardi, say that one reason for the slowdown in the growth of methane concentration may be leak-preventing repairs made to oil and gas pipelines and storage facilities, which can release methane into the atmosphere. Other reasons may include slower growth or actual decrease in methane emissions from coal mining, rice paddies, and natural gas production, they say.
"If one really tightens emissions, the amount of methane in the atmosphere 10 years from now could be less than it is today. We will gain some ground on global warming if methane is not as large a contributor in the future as it has been in the past century," said Rowland, a co-recipient of the 1995 Nobel Prize for discovering that chlorofluorocarbons in such products as aerosol sprays and coolants were damaging the Earth's protective ozone layer. The research will be published 23 November in Geophysical Research Letters.
Methane, the main ingredient of natural gas, warms the atmosphere through the greenhouse effect and helps form ozone, a component of smog. Since the Industrial Revolution in the late 1700s, atmospheric methane has more than doubled. About two-thirds of methane emissions can be traced to human activities, such as fossil-fuel extraction, rice paddies, landfills, and cattle farming.
Scientists in the Rowland-Blake laboratory use canisters to collect sea-level air in locations from northern Alaska to southern New Zealand. They then measure the amount of methane in each canister and calculate a global average.
From 1978 to 1987, the amount of methane in the global troposphere increased by 11 percent, a more than one percent increase each year. In the late 1980s, the growth rate slowed to between 0.3 percent and 0.6 percent per year. It continued to decline into the 1990s, but with a few sharp upward fluctuations, which scientists have linked to non-cyclical events such as the eruption of Mount Pinatubo in 1991 and Indonesian and boreal wildfires in 1997 and 1998. Then, from December 1998 to December 2005, the samples showed a near-zero growth of methane, ranging from an annual 0.2 percent decrease to a 0.3 percent gain.
Along with methane, the scientists also measured levels of other gases, including ethane, a by-product of petroleum refining that also is formed during biomass burning, and perchloroethylene, a chlorinated solvent often used in the dry cleaning process. Ethane levels followed the peaks and valleys of methane over time, but perchloroethylene showed a different pattern. This finding provides evidence that biomass burning can on occasion, as in Indonesia in 1997 and Russia in 1998, be a large source of atmospheric methane, the researchers say.
They say there is no reason to assume that methane levels will remain stable in the future, but the fact that leveling off is occurring now indicates that society can do something about global warming. Methane has an atmospheric lifetime of about eight years. Carbon dioxide, the main greenhouse gas that is produced by burning fossil fuels for power generation and transportation, can last a century and has been accumulating steadily in the atmosphere.
"If carbon dioxide levels were the same today as they were in 2000, the global warming discussion would leave the front page," Rowland said. "But to stabilize this greenhouse gas, we would have to cut way back on emissions. Methane is not as significant a greenhouse gas as carbon dioxide, but its effects are important. The world needs to work hard to reduce emissions of all greenhouse gases."
NASA and the Gary Comer Abrupt Climate Change Fellowship supported this research.
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