Growth of Atmospheric Methane Stablizes, May Slow Global Warming

IRVINE, California, November 21, 2006 (ENS) - Levels of atmospheric methane, a potent greenhouse gas, have stayed nearly flat for the past seven years after rising for more than two decades, according to new research led by a Nobel Prize winning scientist at the University of California-Irvine.

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.

"If one really tightens emissions, the amount of methane in the atmosphere 10 years from now could be less than it is today," said Professor F. Sherwood Rowland, co-recipient of the 1995 Nobel Prize for chemistry.

"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," Rowland said.

UC Irvine Professor of chemistry and earth system science F. Rowland Sherwood. He and fellow chemist Mario Molina won the 1995 Nobel Prize for chemistry for their discovery that chlorofluorocarbons in aerosol sprays and coolants were damaging the Earth’s protective ozone layer. (Photo courtesy UC Irvine)

Methane, the major ingredient in natural gas, warms the atmosphere through the greenhouse effect and helps form ground-level ozone, an ingredient in smog.

Methane is one of the six greenhouse gases governed by the Kyoto Protocol, an international treaty under which 35 industrialized countries are legally bound to cut the emission of greenhouse gases an average of 5.2 percent between 2008 and 2012.

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, landfills, cattle, and rice paddies. Rice grows mostly in flooded fields, where bacteria in waterlogged soil releases methane. Methane also is produced by termites and wetlands.

One reason for the slowdown in the growth of methane concentration may be repairs made to oil and gas lines and storage facilities to prevent leaks, which can release methane into the atmosphere.

Other reasons include a slower growth or decrease in methane emissions from coal mining, rice paddies and natural gas production.

Rowland and his team, Donald Blake, professor of chemistry and earth system science, and researchers Isobel Simpson and Simone Meinardi, used canisters to collect sea level air in locations from northern Alaska to southern New Zealand.

Donald Blake, UC Irvine professor of chemistry, studies atmospheric gases that can affect urban air quality, climate change and stratospheric ozone depletion. (Photo courtesy UC Irvine)

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, with a few sharp upward fluctuations, which scientists have linked to events such as the eruption of Mt. Pinatubo in 1991 and the Indonesian and boreal wildfires in 1997 and 1998.

From December 1998 to December 2005, the samples showed a near-zero growth of methane, ranging from a 0.2 percent decrease per year to a 0.3 percent gain.

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 100 years and has been accumulating 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. But to stabilize this greenhouse gas, we would have to cut way back on emissions," Rowland said.

"Methane is not as significant a greenhouse gas as carbon dioxide, but its effects are important," he said. "The world needs to work hard to reduce emissions of all greenhouse gases."

The methane research will be published in Thursday's online edition of "Geophysical Research Letters."

The National Aeronautics and Space Administration, NASA, and the Gary Comer Abrupt Climate Change Fellowship supported this research.