When it comes to transforming their environment, beavers have a lot in common with humans. They clear-cut trees and build dams to block streams, in the process radically altering the world around them. Now, it appears that beavers play a complex role in climate change, too. A new study suggests that beaver dams and the sediments corralled behind them sequester carbon, temporarily keeping greenhouse gases containing the element out of the atmosphere. But when the animals abandon these sites, the carbon leaks back out, contributing to global warming.

Before Europeans settled North America, as many as 400 million beavers inhabited an area covering just over nine-million square miles (about 60% of the continent). The wetlands that form behind their dams, as well as the floodplains that they groom nearby, provide habitat for many creatures. And although many ecologists are familiar with the biodiversity-boosting aspects of beaver activity, fewer are aware of beavers' role in carbon sequestration, says Ellen Wohl, a geoscientist at Colorado State University, Fort Collins.

In recent fieldwork in Colorado's Rocky Mountain National Park, Wohl studied the wetlands and floodplains upstream of beaver dams—areas collectively called "beaver meadows"—along 27 streams draining watersheds covering more than 400 square miles. She analyzed the carbon content of 29 samples of sediment collected along two of the larger waterways (one of which included remnants of 148 beaver dams, and the other had 100). Then, she combined these data with results from previous research to estimate the carbon content of beaver meadows throughout the region. Altogether, beaver meadows occupied about one-quarter of the total length of major streams in these watersheds, she notes.

Because the water table is elevated behind an intact beaver dam, oxygen can't get to much of the wood and other organic matter buried in sediments there, so it decomposes more slowly. In fact, Wohl says, wood buried in soggy beaver meadows can last about 600 years—longer than a typical log that falls in the forest. But when the water table drops and the soils dry out, decomposition begins to release carbon dioxide to the atmosphere.

In work published last year, Wohl and her colleagues found that sediment upstream of active beaver dams in the park contained about 12% carbon by weight, most of it locked in wood. But in research to be published in a forthcoming issue of Geophysical Research Letters, Wohl reports that, on average, only 3.3% of the sediment in abandoned beaver meadows is carbon.

That's a big difference. Today, abandoned beaver meadows contain about 736,000 metric tons of stored carbon—about 8% of the total stored in the soils of these watersheds. But if all the beaver dams were occupied with their wetlands intact, her previous data suggest, beaver meadows would be storing about 23% of the landscape's soil carbon, an estimated 2.7 million metric tons of organic carbon.

That figure may seem like small potatoes when global carbon dioxide emissions last year approached 36 billion tons (with a carbon content of about 9.8 billion tons). But extrapolated to all areas of North America where beavers have traditionally lived and then placed in the context of a preindustrial world, it suggests that beavers—as well as their relatively sudden removal from the landscape by trappers in pre-Colonial times—may have had a substantial effect on global climate.

"This is a really neat paper," says Justin Wright, an ecologist at Duke University in Durham, North Carolina. "I like the approach she took to get a landscape perspective."

The next step, Wright notes, is to look at a more complete assessment of beaver meadows' carbon budget that takes into account the greenhouse gases emitted by microbes that tend to thrive in moist sediments such as natural wetlands and rice paddies. Considering the overall balance between carbon storage and release in beaver meadows "is a really interesting question," Wohl says. "At this point, I don't know" what the answer is, she adds.

Beaver ponds can indeed be large sources of potent planet-warming greenhouse gases such as methane and nitrous oxide, says Jennifer Edmonds, an aquatic ecologist at the University of Alabama, Tuscaloosa. (Over the course of a century, methane traps heat in Earth's atmosphere about 25 times as effectively as carbon dioxide; nitrous oxide does so almost 300 times as effectively over the same interval.) But considering the whole landscape, she adds, "if I had to bet, I'd bet that [the beaver meadows] are storing more carbon than they're producing."

Nevertheless, Edmonds notes, the methane-producing microbes continue to thrive in a beaver meadow's sediment for many years after the dams have burst and sediments have begun to dry: "Beavers change streams in fundamental ways, and they leave a mark that lasts for decades."