Plants: An ozone culprit?
Research suggests plants may emit methane, throwing off greenhouse gas accounting
New findings suggest that plants might produce methane in situ by a hitherto unknown process, and emit quantities representing 10-30% of the earth's atmospheric methane levels. If confirmed, the results -- published in this week's issue of Nature -- will require a re-evaluation of the planet's global methane budget, and have a significant impact on many fields, from the study of past climate changes to greenhouse gas accounting.
"At first, most people we showed our results to said -it can't be, something must be wrong," co-author Thomas Röckmann, from the Institute for Marine and Atmospheric Research in Utrecht, The Netherlands, told The Scientist. "It's more than not knowing it exists, people think it can't happen." But when it comes to greenhouse gases, knowledge is power, he added. "If you want to do something about the emissions and control them, you have to understand where they come from and where they go," said Röckmann.
Ultimately, the findings may lead to potential changes in the way governments manage greenhouse gases, consisting mostly of methane and carbon dioxide. For instance, some countries are trying to reduce their emissions by planting forests to absorb CO2 - but if that solution also results in more methane emissions, it will pose additional problems, according to David Lowe from the National Institute of Water and Atmospheric Research in Wellington, New Zealand, who was not a co-author.
Most of the methane emitted from natural sources is currently thought to originate from microbial activity in anaerobic environments, mainly in the earth's wetlands. This bacterial process is also involved in anthropogenic sources of methane, such as the soil of rice paddies and ruminants. Even though scientists did not expect plants to produce methane, Röckmann's team found that plants contain different methyl halides such as methanol or methyl chloride. They set out to examine whether plants do, in fact, emit the greenhouse gas.
When the researchers incubated detached leaf tissue or intact plants in methane-free chambers, they detected emissions. Using isotopic measurements, the researchers confirmed the releases originated from the plants, not the soil. If other types of living plants produce comparable amounts of methane globally, the researchers estimate plants could produce between 62 and 236 Teragrams (1 Tg=1012
grams) of methane every year, or 10-30% of the global methane budget.
"These are the findings of just one group. Now others around the world will have to make efforts to confirm these observations and to figure out the process that might be involved," said Lowe.
Röckmann's group did not investigate further how vegetation might produce methane, but hypothesized the mechanism may involve pectin, a cell-bonding agent in plants. "I personally would have liked to see more experiments to define the chemical process, but research is limited by time," Carl Brenninkmeijer, a researcher from the Max Planck Institute for Chemistry in Mainz, Germany, who did not participate in the study, told The Scientist. "It's difficult to understand how vegetal organic matter, even apple pectin, would produce methane in the presence of oxygen."
Experts agree that, if additional experiments confirm that plants contribute a significant amount of methane to the environment, researchers would have to re-examine global methane budget calculations. For instance, they would have to re-evaluate the estimated contributions of wetlands, rice fields, ruminants and other sources to the budget's global value, which remains stable at about 600 million tons per year.
These new findings may also provide insights into ongoing mysteries involving methane. For instance, between the ice ages and the warm ages, the only acknowledged source of methane was wetlands, but they don't explain the era's puzzling variations in the methane budget.
Despite the questions these findings may raise about greenhouse gases, the researchers insisted that efforts to reduce emissions should target man-made activities. "Emissions from plants are natural, they've been there long before man started to affect the atmosphere -they are not a factor in anthropogenic global change," said Röckmann. "They should be left untouched."
Links within this article
Frank Keppler et al. "Methane emissions from terrestrial plants under aerobic conditions," Nature, January 12, 2006.
S. Pincock, "Scientists demand action on climate," The Scientist, June 7, 2005.
A.J.S. Rayl, "Natural solutions to pollution," The Scientist, April 7, 2003.
D. Lowe, "A green source of surprise," Nature, Vol. 439, January 12, 2006.