Global change: A green source of surprise

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Nature 439, 148-149 (12 January 2006) | doi:10.1038/439148a

Living terrestrial vegetation emits large amounts of methane into the atmosphere. This unexpected finding, if confirmed, will have an impact on both greenhouse-gas accounting and research into sources of methane.
On page 187 of this issue, Keppler et al. report the remarkable discovery that terrestrial plants emit methane into the atmosphere. Their results are startling, 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. Second, because the estimated emissions are large, constituting 10-30% of the annual total of methane entering Earth’s atmosphere.

In a series of carefully controlled experiments, Keppler and colleagues used gas chromatography and continuous-flow isotope-ratio mass spectrometry to find that methane is emitted from a wide variety of plant species under oxic conditions. Using 13C-labelled acetate substrates, they ruled out the possibility that the methane is produced by anoxic microbial activity. Going further, they showed that this vegetative source depends on sunlight and temperature, with emissions approximately doubling for each rise of 10 °C. The details of the methane-production mechanism are not known, but the authors do demonstrate that emissions are related to the quantity of pectin, a cell-bonding agent, that a plant contains.
To estimate the global methane emissions from vegetation, Keppler et al. make two main assumptions: first, that the emission rates they measured are representative values for short-lived biomass; second, that the emission estimates can be scaled relative to annual net primary productivity, and can distinguish between different types of environment and average daily hours of sunshine, and between differences in the period of vegetation growth. This type of approach, known as a bottom-up calculation, is commonly used to estimate global emissions from various methane sources and is notorious for producing a wide range of estimates. Additional constraints are applied to bottom-up estimates using methane isotopic data and inverse modelling techniques, but the errors remain large.

Most methane is lost from the atmosphere by oxidation, and estimates of this process are used in top-down calculations to deduce the amounts thus removed. For the methane budget to be balanced, the two techniques should agree when the atmosphere is near steady state. But this is rarely the case, as shown by Figure 1. The identification of a new source should prompt a re-examination of the global methane budget, and may ultimately help to reconcile the differences between the bottom-up and top-down techniques.

Meanwhile, Keppler and colleagues’ finding helps to account for observations from space of inexplicably large plumes of methane above tropical forests. They may also explain the current puzzling decrease in the global growth rate of atmospheric methane. Deforestation has led to a dramatic reduction in the Earth’s tropical forested area (more than 12% between 1990 and 2000). Keppler et al. calculate a corresponding decrease in methane emissions from tropical plants of between 6 million and 20 million tonnes over the same period. During that decade, the rate of methane accumulation in the


atmosphere slowed by about 20 million tonnes per year, suggesting that tropical deforestation may have contributed to the decrease.

Methane absorbs solar radiation strongly at infrared wavelengths, and is second only to carbon dioxide in its role in producing an enhanced greenhouse effect and warming the Earth. It also affects the way the atmosphere cleans itself of pollutants, and influences ozone depletion through the production of water vapour in the stratosphere. So methane has been the subject of intense scientific and political scrutiny, and is targeted for emissions controls under the Kyoto Protocol on climate change.

The predominant sources of atmospheric methane are biological. The main ones previously recognized were microbial activity in wetlands (including natural swamps and rice paddy fields) and the eructations of ruminant animals. The dramatic upswing in agriculture required to feed the Earth’s growing population has led to huge increases in rice culture and livestock farming in the past 250 years. The result has been large rises in methane emissions from both of these sources.
It was thought that methane production in flooded paddy fields was due to microbial activity in the anoxic environment of the paddy soils. In a ‘Kyoto world’, in which sources and sinks of greenhouse gases are added and subtracted like the columns in an accountant’s report, there are claims that new, ‘drier’ forms of paddy-field irrigation will lead to reduced methane emissions. But a study of rice plants has shown a strong link between the number and size of leaves on the plant and methane emissions: could the rice plants themselves be as significant a source of methane as the flooded paddy fields?

The implications of Keppler and colleagues’ work for the Kyoto Protocol include how reforestation and ruminant animals are treated in methane budgets. Under the Kyoto rules, reforestation since 1990 may be used as a CO2 sink to offset greenhouse-gas emissions from other sources; we now have the spectre that new forests might increase greenhouse warming through methane emissions rather than decrease it by sequestering CO2. And in certain countries with large numbers of sheep, cattle and other ruminant livestock, methane constitutes a significant fraction of total greenhouse-gas emissions. In such countries – Ireland and New Zealand, for example – ruminant animals graze on pastures that were originally forested. Given the findings of Keppler et al., it is possible that the forests that once occupied pasture may have produced as much methane as ruminants and grasses on the same land.

The new work will also influence studies of the history of Earth’s climate. Indications of past climate are often deduced from analyses of the concentration and isotopic composition of greenhouse gases in tiny air bubbles trapped in polar ice cores. Keppler and colleagues’ study shows that, in pre-industrial times, the relative contribution of methane to the atmosphere by direct emissions from plants could have been much larger than it is today. Measurements of isotopic values in methane derived from Antarctic ice cores show a signal


between AD 0 and 1200 that is inconsistent with theories of methane budgets being dominated by wetland sources7. A pre-industrial atmosphere containing large contributions of methane derived from vegetation can account for the observed isotopic signal. One of the further avenues of research will centre on the role of methane and vegetation in glacial- interglacial transitions.

This paper will undoubtedly unleash controversy, not the least of which will be political. But there are many scientific questions to be addressed. How could such a potentially large methane source have been overlooked? And what kind of mechanism could produce a highly reduced gas such as methane in an oxic environment? There will be a lively scramble among researchers for the answers to these and other questions.

1 David C. Lowe is in the Tropospheric Physics and Chemistry Group, National Institute of Water and Atmospheric Research, Private Bag 14901, Wellington, New Zealand – Email: d.lowe@niwa.co.nz.