Researchers at the government's Commonwealth Scientific and Industrial Research Organisation (CSIRO) described the findings of their study as exciting although they admit to being unsure why it is happening.

The research, conducted by the CSIRO's greenhouse gas monitoring facility at Cape Grim on the southern island state of Tasmania, shows there has been no growth in the gas over the last four years after a 15 percent rise in the preceding 20 years and a 150 percent rise since pre-industrial times.

"This is a very exciting result," said CSIRO Atmospheric Research chief research scientist Paul Fraser.

Methane, acknowledged to be some 23 times more potent than carbon dioxide, is responsible for a fifth of the enhanced greenhouse effect over the past 200 years.

The bad news, however, is that global emissions of carbon dioxide, the most important greenhouse gas, are more difficult to control and look set to continue to increase.

Methane is released into the atmosphere from agriculture such as the production of rice, cattle and sheep, from landfills and from the mining and use of the fossil fuels coal, oil and gas, as well as from natural wetlands.

"Although we can't be certain why methane concentrations have levelled out, we think it is in response to emissions declining due to better management of the exploration and use of fossil fuels and the increasing recovery of landfill methane.

"If this global decline in methane emissions continues, global atmospheric methane concentrations will start to fall.

"Global emissions of the most important greenhouse gas, carbon dioxide, are difficult to control, and are set to continue to increase, despite the efforts of the Kyoto Protocol and similar initiatives.

"This makes the good news on methane all the more important."

Man-made global warming has been blamed for rising world temperatures and freak weather in recent years, with France, for instance, forecasting 2003 will be the country's hottest year since records began 150 years ago.

NASA scientists in the United States say the polar ice cap is melting at an alarming rate due to global warming, with satellite images showing the ice cap continuing to shrink.

The part of the Arctic Ocean that remains frozen all year round shrank at a rate of 10 percent per decade since 1980.

SPACE.WIRE