24 Jan 2006
The authors of a study which revealed for the first time that growing plants emit the greenhouse gas methane now say their work has been widely misinterpreted by many in the media.
The results of the study, which were published in the January 12 edition of Nature, led some commentators to incorrectly conclude that planting trees to combat global warming was a waste of time. The Kyoto Protocol encourages forest planting because growing trees capture the main greenhouse gas, carbon dioxide, from the atmosphere and ‘lock it up ’ in plant material.
In a media statement posted last week on the website of the Max Planck Institute for Nuclear Physics, principal author Max Keppler says the study findings are only "preliminary." But he estimates that methane emissions by plants may reduce their net greenhouse gas absorption by up to 4 percent.
The climatic benefits gained from reforestation far exceed this relatively small negative effect, he says.
"The potential for reduction of global warming by planting trees is most definitely positive."
Here in New Zealand, scientists at Landcare Research think the effect may be closer to 1 percent and is only significant at really high temperatures (between 30 and 70 degrees C).
Some reportage on the joint study went so far as to blame plants for global warming. The authors emphasise that this is not the case.
"Emissions from plants are a natural source, they have existed long before man's influence started to impact upon the composition of the atmosphere. Emissions from plants thus contribute to the natural greenhouse effect and not to the recent temperature increase known as global warming."
Global warming, they say, is due to human activities.
The large-scale burning of fossil fuels worldwide, which is causing carbon dioxide levels in the atmosphere to increase, is the fundamental problem.
Methane has the second greatest effect on the climate, mainly due to rice cultivation and the digestive processes of ruminants like cattle and sheep. The concentration of methane in the atmosphere has almost tripled in the last 150 years.
NZ Forest Owners Association chief executive David Rhodes says a "fair degree of overseas media hype" followed the release of the story in Nature.
"By and large the subject was treated responsibly by New Zealand reporters who went to the trouble to get comment from local scientists.
"With respect to planted forests in New Zealand, any small amount of methane released by trees would generally be offset further by subtracting the methane that was previously produced by the grass they replaced. We are likely to be talking about a very insignificant factor.
"I don't think anyone will have stopped planting trees because of the report, though it was good for a few laughs during the summer silly season. Some cartoonists saw it as a godsend."
Mr Rhodes says forest owners are still negotiating with the government to get a market-based mechanism which will reward land owners for planting forests.
"New Zealand needs 60,000 hectares of new forest to be planted every year if it is to get close to meeting its Kyoto targets."
The Max Planck Society is named after the renowned German physicist Max Planck (1858 –1947) who is regarded as one of the ‘fathers ’ of quantum theory. Its research institutes perform fundamental public-good research in the natural sciences, life sciences, social sciences, and the humanities.
The study reported in Nature involved scientists at the Max Planck Institute, Heidelberg, Germany; the Institute for Marine and Atmospheric Research, Utrecht University, The Netherlands; and the Department of Agriculture and Rural Development, Belfast, Northern Ireland.