Using satellite technology, researchers have for the first time mapped how much of the greenhouse gas methane is being emitted, and from where. Their results, described in this week's Science, show that the tropics emit the highest levels of methane.
The result tallies with the fact that rice and cattle farming — both activities widespread in the tropics — are the prime sources of methane in the atmosphere.
Measuring greenhouse gas emissions from satellites could become a useful tool in studies aiming to understand the distribution of these gases around the globe, says lead author Christian Frankenberg.
Indeed, NASA (the US National Aeronautics and Space Administration) is already planning a mission for 2007 that will measure the natural and human emissions of carbon dioxide.
Although methane is responsible for one-fifth of the effect of all greenhouse gases emitted by human activities, researchers agree that human emissions of carbon dioxide pose a greater threat to the global climate.
The researchers used a satellite to measure the intensity of sunlight that reached the satellite after passing through the Earth's atmosphere. Gases in the atmosphere, such as methane, absorb light energy. Different gases absorb different amounts of energy.
By measuring the 'leftover' energy after the light has passed through the atmosphere, the satellite can calculate which gases the light has had to pass through.
Frankenberg and his colleagues at the University of Heidelberg in Germany and the Royal Netherlands Meteorological Institute show that the highest methane emissions come from the Gangetic plains in India, Southeast Asia and parts of China.
After gathering their measurements, the team compared them with a computer model that works on current emissions records.
On the whole, their measurements matched estimates from the model, but discrepancies emerged in two regions: India and the tropics, including South America, Africa and Indonesia.
The measurements for India were lower than those in the model — which the team attributes to the model's overestimation of emissions from rice cultivation.
Over the tropics, however, the measurements were higher than expected. The authors suggest these extra emissions could be coming from plants, since the areas of discrepancy contain dense evergreen forest. Other sources could be wetlands, biomass burning, termites or cattle breeding.
The researchers suggest that the reason that this extra emission in the tropics has not been identified until now is because of tropical winds, which rapidly disperse the methane. As a result, ocean outposts that monitor these emissions pick up only part of the total.
Accurate measurements of methane emissions is important, as it is one of six greenhouse gases addressed by the Kyoto Protocol — the international treaty that aims to reduce greenhouse gas emissions caused by human activity by 2012.
The protocol only requires developed countries to reduce their emissions. However, developing countries, where methane-creating activities such as rice cultivation and cattle farming are common, are also significant contributors.
Benito Müller of the Oxford Institute of Energy Studies in the United Kingdom says that even though developing countries produce a lot of methane, this does not mean they should be obliged to carry the total burden of reducing them. The level of responsibility a nation bears for reducing emissions, says Müller, should take into account its economic capabilities.
Ultimately, says Frankenberg, the value of this new technology will lie with its capability of measuring other greenhouse gases, such as carbon dioxide. Frankenberg told SciDev.Net that we are reaching a balance between the methane produced and that absorbed by the planet's methane 'sinks'.
Carbon dioxide emissions have already overtaken methane in effects on climate change. Frankenberg adds that in view of the rise in carbon dioxide emission, "the importance of methane with respect to climate change in the future will decrease further".
Frankenberg says the fact that as a planet we are emitting more carbon dioxide that we can see the effects of, points to the presence of a carbon sink that is absorbing the excess. But at present, scientists are unable to tell whether this sink is in the ocean or in the biosphere.
This article was first published in Sciencexpress on 17 March 2005.
Reference: Science 308, 1010 (2005)
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