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Beavers damned for increasing threat from methane

December 21, 2014, by Alex Kirby

Dams built by beavers create shallow pools in which carbon builds up and generates methane.
Image: Makedocreative via Wikimedia Commons

The growth of the world’s beaver population to more than 10 million has led to a big increase in one of the main greenhouse gases that cause climate change. LONDON, 21 December, 2014 − For a picture of industrious innocence, beavers are hard to beat. Yet they now find themselves facing a grave charge: they are, it seems, responsible for increasing greenhouse gas emissions. The problem, Canadian scientists say, lies in the shallow ponds that form behind the dams the beavers build. The ponds are essential to the animals’ way of life. Unfortunately, they’re also good places for generating methane. Methane is a powerful greenhouse gas, and in the short term it does much more damage than the far more abundant carbon dioxide. There is now international agreement that methane is 34 times more potent than CO2 over a century, but 84 times more over a much shorter timespan – just 20 years. And two decades can be crucial in trying to slow the rate of climate change.

Trapping limited

Colin J. Whitfield, of the University of Saskatchewan, led a study − published in the journal AMBIO − from which he estimated that beaver numbers in Eurasia and the Americas have grown so much that the methane emissions the ponds produce are now 200 times higher than in 1900. Between the 16th and the 19th centuries, the fur trade nearly led to the beavers’ extinction worldwide. After trapping was limited and they were re-introduced to their natural ranges, the number of North American (Castor canadensis) and Eurasian (Castor fiber) beavers began to grow. The North American beaver has also been introduced to parts of Eurasia and South America.

“This suggests that the contribution of beaver activity to global methane emissions may continue to grow”

Beavers build dams in rivers to create standing open-water ponds and wetlands. The ponds are usually shallow, with dams seldom more than 1.5 metres high. The study found that carbon builds up in the oxygen-poor pond bottoms, and methane is then generated. Unable to dissolve adequately in the shallow water, it is released into the atmosphere. The team estimated the size of the current global beaver population and determined the area covered by beaver ponds. They found that global beaver numbers have grown to over 10 million, damming more than 42,000 sq kms of aquatic pond areas, bordered by over 200,000 kms of shoreline habitat. At the end of the 20th century, they say, beavers contributed up to 0.80 teragrams (or 800 million kilograms) of methane to the atmosphere annually. This is about 15% of the input from wild cud-chewing animals such as deer or antelopes. “Continued range expansion, coupled with changes in population and pond densities, may dramatically increase the amount of water impounded by the beaver,” Whitfield says. “This, in combination with anticipated increases in surface water temperatures, and likely effects on rates of methanogenesis, suggests that the contribution of beaver activity to global methane emissions may continue to grow.”

Copious amounts

Beavers are not alone in unwittingly worsening climate change. Ruminants − animals that chew the cud − emit copious amounts of methane, prompting concerns about the impacts on the atmosphere of an increasingly meat-based human diet. Now comes news that another species may have to step up and accept some of the blame for a warming world. Scientists from Woods Hole Research Center, in the US, told the American Geophysical Union fall meeting in San Francisco that Arctic ground squirrels may be playing a greater role in climate change than previously thought. They say the animals are hastening the release of greenhouse gases from the permafrost, accelerating an existing positive feedback that means the warming temperatures help the frozen soil to thaw and emit still more greenhouse gas. − Climate News Network

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