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Plants Exhale Methane, Add to Greenhouse Effect, Study Says

Brian Handwerk
for National Geographic News
Updated March 7, 2006
 
Editor's Note: Published on January 11, the original version of this
story misinterpreted a scientific study as saying that plants
contribute to global warming. According to a later press release by the study's
authors
, that characterization was inaccurate, and we have
amended our story.


Grasses and other green growth may produce 10 to 30 percent of Earth's annual methane output, a new study reports.

Until the data were unveiled in this week's Nature, scientists had believed that plant-related methane formed only in oxygen-free environments, such as bogs.

But a team of European researchers identified a large range of plants that release methane under normal growing conditions. The gas also seeps from dead plant material.

David Lowe is an atmospheric chemist with the National Institute of Water and Atmospheric Research in Wellington, New Zealand. He wrote a review article accompanying the study.

According to Lowe, "We now have the specter that new forests might increase greenhouse warming through methane emissions rather than decrease it by being sinks for carbon dioxide."

"The identification of a new source should prompt a reexamination of the global methane budget."

Escaping Notice

The potentially enormous natural source of greenhouse gases had thus far escaped notice, which experts say is not surprising.

Thomas Roeckmann, a study co-author and atmospheric scientist with the Institute for Marine and Atmospheric Research in Utrecht, the Netherlands, said a very close look was needed to spot the emissions.

"The emissions per plant are rather small, and one has to look quite carefully to detect the increase above the high natural background of [methane]," he said. "So you would not find those emissions by chance."

Estimating the total global production of methane and other greenhouse gasses is far from an exact science, Lowe said.

"People who prepare the emission budgets use a bottom-up technology. Someone will make a measurement in a swamp somewhere and simply extrapolate that measurement upwards to represent all the world's swamps. They'll measure emissions from a cow or a sheep and extrapolate that upwards to include all of the world's animals.

"As you can imagine, there are huge errors. The science is so inexact that you could easily fit a new source like this into the estimates."

The new research may explain the large plumes of methane hovering over tropical forests that several satellites have spotted.

"That has been puzzling, and no one could explain it—but this provides an explanation," Lowe said. "It's [likely to be] the trees themselves that are producing that methane."

Unknown Contribution

Plants are just one of several natural sources of greenhouse gasses. Scientists believe that livestock like cattle and sheep may be responsible for some 20 percent of global methane emissions.

Just one full-grown dairy cow can emit some 100 to 130 gallons (400 to 500 liters) of methane each day as a product of digestive microbes breaking down their grassy diet.

Methane is considered a key greenhouse gas because it traps heat inside the Earth's atmosphere about 20 times more effectively than carbon dioxide.

Scientists have long known that anaerobic bacteria produce large levels of methane while breaking down plant material in bogs, such as Siberia's vast peatlands.

But the scope of production from oxygen-using live plants could be far higher—and have more dramatic results for the planet's climate.

Research suggests that rising temperatures could boost plants' methane production, which might help to retain heat and spark further temperature increases in a self-perpetuating cycle.

Just how big of a greenhouse cycle might be created?

"The size of the effect is difficult to quantify," Roeckmann, of the Dutch Institute for Marine and Atmospheric Research, said.

"It will depend on how sensitive the emissions are to various environmental parameters, [such as] temperature, increased carbon dioxide, and humidity."

"This is just a first research paper," Lowe added. "You really need others to get out there and confirm these measurements independently. You'll see lots of labs getting involved with this research."

Historical Context

The findings also may prompt scientists to revisit conclusions on historical climate change and its possible causes.

"We know that there have been periods of very rapid climate change, and ice cores have shown [corresponding] changes in levels of gases like carbon dioxide and methane," Lowe said.

"It has been thought that the land had reverted very rapidly to swamps, but it could be that in fact the forests played quite a major role in rapidly changing temperatures."

Roeckmann explained that global levels of plant and animal matter have also fluctuated during different historical eras.

"Our new results at least show the possibility that this could have a significant climate impact," he said.

"We postulate in our paper that the direct plant source made up roughly 50 percent of the global [methane production] in pre-industrial times."

"Thus [methane] emissions from plants represent a [newly discovered] process that can lead to warming and cooling of the Earth, certainly on glacial-interglacial timescales but, potentially even more important, on longer geological timescales.

"And of course this also affects the understanding of the current conditions."

Lowe notes that any reassessment of current climate change models could include some interesting political ramifications.

For example, the Kyoto protocol—an international treaty designed to try and curb climate change—requires complex accounting that holds countries to specified greenhouse gas emissions limits.

"Several countries are counting their forests as vegetative sinks for carbon dioxide," he said.

"But are you absorbing more carbon dioxide than you are [possibly] releasing methane? I suppose that the Kyoto protocol accountants are going to be working overtime trying to figure that one out."

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