Global warming: Blame the forests

Baobab forest in Madagascar. A recent study has revealed that living plants may emit up to a third of the methane entering the Earth's atmosphere, contributing to global warming.

Source: Guardian Unlimited

They have long been thought of as the antidote to harmful greenhouse gases, sufferers of, rather than contributors to, the effects of global warming. But in a startling discovery, scientists have realised that plants are part of the problem.

According to a study published recently, living plants may emit up to a third of the methane entering the Earth's atmosphere.

The result has come as a shock to Earth Observation Scientists. "This is a genuinely remarkable result," said Richard Betts of the climate change monitoring organisation, the Hadley Centre. "It adds an important new piece of understanding of how plants interact with the climate."

Methane is second only to carbon dioxide in contributing to the greenhouse effect. "For a given mass of methane, it is a stronger greenhouse gas, but the reason it is of less concern is that there's less of it in the atmosphere," said Dr Betts.

But the concentration of methane in the atmosphere has almost tripled in the last 150 years, mainly through human-influenced so-called biogenic sources such as the rise in rice cultivation or numbers of flatulent ruminating animals. According to previous estimates, these sources make up two-thirds of the 600m tonnes worldwide annual methane production.

Frank Keppler, of the Max Planck Institute for Nuclear Physics, who led the team behind the new research, estimated that living plants release between 60m and 240m tonnes of methane per year, based on experiments he carried out, with the largest part coming from tropical areas.

David Lowe, of the National Institute of Water and Atmospheric Research in New Zealand, said the new work, published in Nature, is important for two reasons. "First, because the methane emissions they document occur under normal physiological conditions, in the presence of oxygen, rather than through bacterial action in anoxic environments," he wrote in an accompanying article. "Second, because the estimated emissions are large, constituting 10-30% of the annual total of methane entering Earth's atmosphere."

Yadvinder Malhi, a specialist in the relationship between vegetation and climate at Oxford University, said the plant source of methane had probably been missed in the past because scientists have a poor understanding of the way methane circulates in the atmosphere. "There are a variety of sources and sinks of methane and there are huge error bars on those terms," he said. "What's been uncertain is where the methane is coming from and where it's going. Unlike carbon dioxide, methane is much more dynamic; it lasts about 10 years in the atmosphere."

Biogenic methane has traditionally been assumed to come from organic materials as they decompose in oxygen-free environments. But Dr Keppler found plants emit the gas even in normal, oxygen-rich surroundings: between 10 and 1 000 times more methane than dead plant material. When the plants were exposed to the sun, the rate of methane production increased. "Until now all the textbooks have said that biogenic methane can only be produced in the absence of oxygen," Dr Keppler said. "For that simple reason, nobody looked closely at this."

The discovery sheds further light on the complex relationship between greenhouse gases and the environment. "If you're after predictions of global average temperature, it won't make a huge amount of difference," said Dr Betts. "But it shows how complicated it is to exactly quantify reforesting or deforesting in comparison with current fossil fuel emissions."

It will also intensify debates on whether targets in climate change treaties such as the Kyoto Protocol should be based entirely on carbon emissions, which are easily measured, or also take sinks into account, which remove carbon from the atmosphere but are more difficult to measure.

For climate scientists, the new work clears up a few unexplained features in the environment.
"The rate of methane increase in the atmosphere has slowed down in the last 10 years and there was no really convincing explanation of why that's been going on," said Dr Mahli. "This paper argues that tropical deforestation may be a factor there."

In addition, the new research could help to explain the source of plumes of methane observed by satellites over tropical forests. "The sheer biomass of the forest may be a factor there," said Dr Mahli.

The fact that plants produce methane does not mean that planting forests is a bad idea, however. "Putting a tree where there was no tree before locks up a lot of carbon and this [new research] perhaps reduces the overall benefit of that by a fraction," said Dr Mahli.

Some mysteries remain: how and why plants produce methane is unclear. Dr Keppler's team said the search for an answer is likely to open up a new area of research into plant biochemistry.