Greenhouse methane levels stop rising
Atmospheric levels of the second most important greenhouse gas, methane, finally seem to have stopped rising, Australian researchers have announced.
After two centuries of strong increases, methane emissions have not risen since 1999, making the scientists confident that this was a genuine and significant trend, not just a short-term variation. If the trend continues, they say concentrations of methane will start to fall.
"Over the past four years there has been no growth in atmospheric methane concentrations compared to a 15% rise over the preceding 20 years and a 150% rise since pre-industrial times," said Dr Paul Fraser, a chief research scientist at CSIRO Atmospheric Research. "This is a very exciting result."
"Methane is the second most important gas after carbon dioxide. It is responsible for a fifth of the enhanced greenhouse effect over the past 200 years."
The good news emerged from air samples taken at Cape Grim, in Tasmania, which boasts some of the cleanest air in the world. It is a key station in a global atmospheric monitoring network.
Methane is more than 20 times more potent that carbon dioxide as a greenhouse gas and contributes to human-induced climate change. It is released by agriculture (mainly rice, cattle and sheep), from landfill waste, mining, the use of fossil fuels (coal, oil and gas) as well as from natural wetlands.
Although scientists are unsure why methane emissions have stopped growing, they first noticed a slowdown in the mid-90s and they think they know where reductions have happened.
"We can say that the main falls have been in the northern hemisphere, which accounts for 90% of the world's fossil fuel emissions, and in its high latitudes," Fraser said. "That suggests it is due to human activities and that it's not from agriculture.
"We suspect it's from the Russian gas fields, either because of a drop in consumption due to the economic downturn there or because of better control of leaks from the reticulation system."
The growing trend towards recovering methane emitted by landfill waste dumps may also be playing a part.
"If this global decline in methane emissions continues, global atmospheric methane concentrations will start to fall," he said.
"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."
If methane emissions had kept rising over the past decade at the old rate, they would be more than 10% higher than they are today, he said. Even China's emergence as a large and fast-growing economy that uses large volumes of natural gas for industry was unlikely to cause emissions to rise again overall.
Natural gas produces less greenhouse gas emissions relative to coal and oil and awareness of its value as a fuel had led to far greater global efforts to capture and use methane efficiently.
The methane story appears to be echoing that of ozone-depleting chemicals used by industry, whose levels rose strongly for decades last century. Their rate of increase began to slow in the late-80s, after concerns about the thinning of the Earth's ozone layer. They levelled off by the mid-90s after tougher controls were imposed on chemical use and have since fallen about 7%.
Other major greenhouse gases, carbon dioxide and nitrous oxide, continue to rise steadily.