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Max Planck Institute: Don’t Blame the Plants

The authors of a recent study published in Nature that revealed the surprising discovery that plants produce the greenhouse gas methane have issued a press release correcting widespread media misinterpretations of their results.

These misinterpretations ranged from blaming plants for global warming to calling into question the relevance of reforestration programs under the Kyoto protocol.

The scientists from the Max Planck Institute for Nuclear Physics, Utrecht University, Netherlands, and the Department of Agriculture and Rural Development for Northern Ireland, UK take pains to clarify those points.

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 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.

In the study, the authors 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 (62 Petagrams—quadrillion grams) of carbon/yr, which corresponds to an uptake of 227 x 1015 g of CO2/yr.

On the emission side, the study suggests annual global methane emissions by plants of 62–236 x 1012 g (Teragrams—trillion grams) of CH4 (methane)/yr. 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 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% due to methane emissions from the plants themselves, according to the authors’ calculations.




But what about cow farts? Sorry, cow methane. The number of cows and the amount of land needed to graze them or feed them (70% of agricultural land?) is surely anthropogenic, no?


The warming-reductions are only for the short term; IIRC methane is oxidized to CO2 on the time scale of 10 years, so any increase in warming potential is strictly time-limited.

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