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Chemistry World

 

Trees implicated in greenhouse gas conundrum


12 January 2006

An unexpected and startling discovery that plants emit millions of tonnes of greenhouse gas methane every year has plunged climate change discussions into disarray.

Trees and plants emit up to 30 per cent of the world's methane, Frank Keppler at the Max Plank Institute for Nuclear Physics, Heidelberg, Germany, and his colleagues claim. After discovering that fallen leaves, or plant litter, produced methane, Keppler investigated whether living plants also produce this highly reduced gas in air - an oxygen rich environment. He calculated that plants give off between 60 and 240 million tonnes of methane per year.

The news has shocked the atmospheric science community. 'I'm still amazed that people haven't seen it before,' said David Lowe, from the National Institute of Water and Atmospheric Research, Wellington, New Zealand. 'You wouldn't expect methane to come from plants and the air. You won't find any chemical reaction that people know about that would do that.'

Ed Dlugokencky from the US National Oceanic and Atmospheric Administration expects most scientists to react with disbelief. 'Since plants produce hydrocarbons such as isoprene and terpenes in large amounts and single carbon compounds such as methyl halides and methanol, I should not be too surprised by this result,' he told Chemistry World. Both Lowe and Dlugokencky commented that Keppler's experiments were so thorough there can be little room for doubt about the validity of Keppler's claims.

'It is a new pathway of [methane] formation,' said Keppler, 'it will change our thinking about plants and their role in climate change.'

Keppler has not yet worked out the details of the reaction. 'We know a possible precursor molecule in plants, for example pectin,' he said, but would not say any more about a possible mechanism.

Under Kyoto protocol rules countries are allowed to use forest sinks to offset emissions. If a country emits lots of carbon dioxide, but plants new forests, it can subtract the carbon dioxide that the trees remove. In light of the news that plants produce large quantities of another greenhouse gas, politicians and scientists will now be scrambling to calculate net carbon emissions from trees. 

'If you plant trees to try and remove carbon dioxide from the atmosphere, but the trees are actually producing methane at the same time, then the government departments are really going to have to know what those two levels are - what the net emission or sink would be,' said Lowe. New Zealand's ministry for agriculture and ministry for the environment has already contacted him to discuss an immediate investigation.

Keppler cautions against 'over interpreting' his results. He warns that it is not only net emissions from trees that need to be re-evaluated. Rising temperatures caused by greenhouse gases, and changing atmospheric conditions caused by changing carbon dioxide concentrations will lead to increased methane emissions from plants and emphasise global warming, he told Chemistry World. 'This effect could be much more pronounced than the benefit you get from other [reforestation] programmes,' he said. 

Katharine Sanderson             

References

F Kepler et alNature, 2005, 439, 187