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Methane growth slowing, but why?

Ice coring

Image courtesy CSIRO

The rate of increase in global levels of the greenhouse gas methane has been slowing over the past twenty years, say Australian scientists, but the source of the savings is unknown.

Overall, methane levels have increased dramatically over the past 200 years, said Dr David Etheridge of CSIRO's Division of Atmospheric Research. Its concentration in the atmosphere now averages 1700 parts per billion, compared to 700 parts per billion 200 years ago.

But while methane levels are still increasing, they are doing so more slowly than they were between 1960 and 1980, suggesting methane emissions are stabilising, said Dr Etheridge.

Dr Etheridge compared direct measurements of methane levels in the atmosphere from the Cape Grim monitoring station in Tasmania, which go back to the late seventies, with data from ice cores in Antarctica going back 1000 years. His findings were published in the Journal of Geophysical Research.

Methane levels were fairly stable up until the industrial revolution 200 years ago, said Dr Etheridge. This suggests that human activities such as burning of biomass, fossil fuel exploration and distribution, rice farming, livestock and landfills were the main source of the increase.

However, how much methane comes from each of these different sources is largely unknown, said Dr Etheridge.

"The question is not how much is being produced and consumed, but which sources are contributing the most," said Dr Etheridge. "The slowdown in methane growth is good news, but we don't know exactly why it's happening yet. The next thing is to try and locate the 'smoking gun'."

Analysis of the isotopic ratios, or chemical signatures, of methane samples to identify their source is now proceeding. Methane from fossil fuels, for example, has no carbon 14, so measuring the amount of this isotope in a sample tells scientists what proportion of that sample came from fossil fuels.

One possibility is that that oil and gas technology has become more efficient over the past 20 years and reduced the amount of methane leakage, said Dr Etheridge.

However, he said the slower rate of increase in methane levels was no cause for complacency about human contributions to global warming. "Until we know what the sources are, we don't know whether it's just an aberration."

In another study published in the current issue of Science, US researchers have added to the evidence that human-caused emissions of greenhouse gases, including methane and carbon dioxide, are partly responsible for warming of the earth's atmosphere.

Tom Wigley at the National Center for Atmospheric Research, Richard Smith at the University of North Carolina, and Benjamin Santer at the Lawrence Livermore National Laboratory analysed the past 115 years of global temperature measurements using a new statistical technique which helped separate the effects of human greenhouse gas emissions from natural climate variability.

They found that while some of the increase could be attributed to variations in the amount of radiation from the sun, "solar forcing alone is insufficient to explain the behavior of the observed temperature data."

Tags: science-and-technology