Don’t blame plants for global warming

Don’t blame plants for global warming

EDITOR’S SUMMARY: A week after announcing their surprising discovery that plants release 10 to 30 percent of the world’s methane—a potent greenhouse gas—researchers from the Max Planck Institute for Nuclear Physics warn that plants should not be blamed for recent global warming.

The scientists say that because emissions from plants are a natural source, they have existed long before man’s influence started to impact atmospheric concentrations of greenhouse gases. Anthropogenic emissions—especially agricultural cultivation—are responsible for the well-documented increase in atmospheric methane since pre-industrial times. Emissions from plants contribute to the natural greenhouse effect and not to the recent temperature increase usually referred to as “global warming”.

“The potential for reduction of global warming by planting trees is most definitely positive,” said Frank Keppler, a scientist involed in the research. “The fundamental problem still remaining is the global large-scale anthropogenic burning of fossil fuels.”

The new comments are included in the following release from the Max Planck Institute for Nuclear Physics. For reference, the original release announcing the research is also included.

In a recent study (Nature, 12 January 2006), scientists from the Max Planck Institute for Nuclear Physics, Utrecht University, Netherlands, and the Department of Agriculture and Rural Development for Northern Ireland, UK, revealed that plants produce the greenhouse gas methane. First estimates indicated that this could account for a significant proportion of methane in the atmosphere. There has been extended media coverage of this work with unfortunately, in many instances, a misinterpretation of the findings. Furthermore, the discovery led to intense speculations on the potential relevance of the findings for reforestation programs in the framework of the Kyoto protocol. These issues need to be put in the right perspective.

The most frequent misinterpretation we find in the media is that emissions of methane from plants are responsible for global warming. As those emissions from plants are a natural source, they have existed long before man’s influence started to impact upon the composition of the atmosphere. It is the anthropogenic emissions which are responsible for the well-documented increasing atmospheric concentrations of methane since pre-industrial times. Emissions from plants thus contribute to the natural greenhouse effect and not to the recent temperature increase known as “global warming”. Even if land use practices have altered plant methane emissions, which we did not demonstrate, this would also count as an anthropogenic source, and the plants themselves cannot be deemed responsible.

Furthermore, our discovery led to intense speculation that methane emissions by plants could diminish or even outweigh the carbon storage effect of reforestation programs with important implications for the Kyoto protocol, where such programs are to be used in national carbon dioxide (CO2) reduction strategies. We first stress that our findings are preliminary with regard to the methane emission strength. Emissions most certainly depend on plant type and environmental conditions and more experiments are certainly necessary to quantify the process under natural conditions. As a first rough estimate of the order of magnitude we have taken the global average methane emissions as representative to provide a rough estimate of its potential effect on climate. These estimates (for details, see below) show that methane emissions by plants may slightly diminish the effect of reforestation programs. However, the climatic benefits gained through carbon sequestration by reforestation far exceed the relatively small negative effect, which may reduce the carbon uptake effect by up to 4 per cent. Thus, the potential for reduction of global warming by planting trees is most definitely positive. The fundamental problem still remaining is the global large-scale anthropogenic burning of fossil fuels.

Details of calculations used:

In our study, we have linked global methane emission estimates to plant growth, which is generally quantified as net primary productivity (NPP). On a global basis NPP amounts to ~62 x 1015 g of carbon/yr, which corresponds to an uptake of 227 x 1015 g of CO2/yr. On the emission side, our study suggests annual global methane emissions by plants of 62-236 x 1012 g/yr CH4. Thus, for each kg of CO2 assimilated by a plant roughly 0.25 to 1 to 4 g of CH4 is released. During growth of a new forest, up to 50% of plant tissue is lost again in the short term through decomposition of plant litter of leaves and roots. This then doubles the estimate to 0.5 to 2 g methane emitted per kg of CO2 assimilated and stored in plants for longer periods. Over a 100-year horizon, the global warming potential of methane is ~20 times higher than that of carbon dioxide. Thus, for climate, the benefits gained by reforestation programs would be lessened by between 1 and 4 per cent due to methane emissions from the plants themselves.

In the last few years, more and more research has focused on the biosphere; particularly, on how gases which influence the climate are exchanged between the biosphere and atmosphere. Researchers from the Max Planck Institute for Nuclear Physics have now carefully analysed which organic gases are emitted from plants. They made the surprising discovery that plants release methane, a greenhouse gas – and this goes against all previous assumptions.

Equally surprising was that methane formation is not hindered by the presence of oxygen. This discovery is important not just for plant researchers but also for understanding the connection between global warming and increased greenhouse gas production (Nature, January 12, 2006).

Methane is the greenhouse gas which has the second greatest effect on climate, after carbon dioxide. The concentration of methane in the atmosphere has almost tripled in the last 150 years. Methane is best known as natural gas, currently an important energy source. Nonetheless, only part of the methane uptake in the atmosphere is due to industrial activities connected to energy production and use. More important for the increase of methane in the atmosphere is the increase in so-called “biogenic” sources, e.g., rice cultivation or domestic ruminants related to the rise in the world’s population. Nowadays, methane in the atmosphere in fact is largely of biogenic origin.

Until now, it has been assumed that biogenic methane is formed anaerobically, that is, via micro-organisms and in the absence of oxygen. In this way, acetate or hydrogen and carbon dioxide are transformed into methane; they themselves are created in the anaerobic decomposition of organic materials. The largest anoxic sources of methane are wetlands and rice fields, as well as the digestion of ruminants and termites, waste disposal sites, and the gas produced by sewage treatment plants. According to previous estimates, these sources make up two-thirds of the 600 million tonnes worldwide annual methane production.

Scientists from the Max Planck Institute for Nuclear Physics have now discovered that plants themselves produce methane and emit it into the atmosphere, even in completely normal, oxygen-rich surroundings. The researchers made the surprising discovery during an investigation of which gases are emitted by dead and fresh leaves. Then, in the laboratory and in the wild, the scientists looked at the release of gases from living plants like maize and ryegrass. In this investigation, it turned out that living plants let out some 10 to 1000 times more methane than dead plant material. The researchers then were able to show that the rate of methane production grew drastically when the plants were exposed to the sun.

Although the scientists have some first indications, it is still unclear what processes are responsible for the formation of methane in plants. The researchers from Heidelberg assume that there is an unknown, hidden reaction mechanism, which current knowledge about plants cannot explain – in other words, a new area of research for biochemistry and plant physiology.

In terms of total amount of production worldwide, the scientists’ first guesses are between 60 and 240 million tonnes of methane per year. That means that about 10 to 30 percent of present annual methane production comes from plants. The largest portion of that – about two-thirds – originates from tropical areas, because that is where the most biomass is located. The evidence of direct methane emissions from plants also explains the unexpectedly high methane concentrations over tropical forests, measured only recently via satellite by a research group from the University of Heidelberg.

But why would such a seemingly obvious discovery only come about now, 20 years after hundreds of scientists around the globe started investigating the global methane cycle? “Methane could not really be created that way,” says Dr. Frank Keppler. “Until now all the textbooks have said that biogenic methane can only be produced in the absence of oxygen. For that simple reason, nobody looked closely at this.”

The fact is that, in order to determine the quantity of emissions, scientists indeed have to make very careful measurements. The researchers from Heidelberg conducted most of their experiments in methane-free air, in order to factor out the high natural background of methane. Furthermore they used isotope analysis to show beyond doubt that this was an undiscovered process of methane production. By “looking closely” – despite established opinion – they made a discovery that will require textbooks to have their passages about methane production rewritten.

Following up on this discovery, the scientists now will continue laboratory work, as well as field and remote sensing studies, to better quantify the strength of these methane emissions. A related exciting question is which role the biosphere has played in methane production in the history of the earth, and what kind of influence rising global temperatures and carbon dioxide concentration have on the production of methane from plants. Answers to these questions are important for understanding the feedback mechanisms between climate change and greenhouse gas production.

This is a modified new release from Max-Planck-Gesellschaft

Article published by Rhett Butler

Climate Change, Climate Science, Earth Science, Green

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