Beaver Ponds May Exacerbate Warming In Arctic, Scientists Say

An anonymous reader quotes a report from The Guardian: The stream through western Alaska never looked like this before. In aerial photography from the 1980s, it wove cleanly through the tundra, thin as thread. Today, in satellite images, it appears as a string of black patches: one large pond after another, dozens of meters apart. It's a transformation that is happening across the Arctic, the result of landscape engineering on an impressive scale. But this is no human endeavor to reshape the world. It is the work of the North American beaver, and there is no sign of it stopping. Were the waddling rodents making minor inroads, researchers may never have noticed. But the animals are pouring in, pushing north into new territories. The total number of animals is far from clear, but the ponds they create are hard to miss: in the Arctic tundra of Alaska alone, the number of beaver ponds on streams have doubled to at least 12,000 in the past 20 years. More lodges are dotted along lakes and river banks.

The preponderance of beavers, which can weigh as much as 45kg, follows a collapse in trapping and the warming of a landscape that once proved too bleak for occupation. Global heating has driven the shrubification of the Arctic tundra; the harsh winter is shorter, and there is more free-running water in the coldest months. Instead of felling trees for their dams, the beavers construct them from surrounding shrubs, creating deep ponds in which to build their lodges. The new arrivals cause plenty of disruption. For some communities, the rivers and streams are the roads of the landscape, and the dams make effective roadblocks. As the structures multiply, more land is flooded and there can be less fresh water for drinking downstream. But there are other, less visible effects too. The animals are participants in a feedback loop: climate change opens the landscape to beavers, whose ponds drive further warming, which attracts even more paddle-tailed comrades. Physics suggested this would happen. Beaver ponds are new bodies of water that cover bare permafrost. Because the water is warm -- relatively speaking -- it thaws the hard ground, which duly releases methane, one of the most potent greenhouse gases.

Scientists now have evidence this is happening. Armed with high-resolution satellite imagery, Tape and his colleagues located beaver ponds in the lower Noatak River basin area of north-western Alaska. They then analyzed infrared images captured by Nasa planes flying over the region. Overlaying the two revealed a clear link between beaver ponds and methane hotspots that extended for tens of meters around the ponds. 'The transformation of these streams is a positive feedback that is accelerating the effects of climate change, and that is what's concerning,' says Tape. 'They are accelerating it at every one of these points.' Because the Nasa images give only a snapshot in time, the researchers will head out next year to measure methane on the ground. With more measurements, they hope to understand how the emissions vary with the age of beaver ponds: do ponds release a steady flow of methane, or does the release wane after a decade or two? 'What's happening here is happening on a huge scale,' says Ken Tape, an ecologist at the University of Alaska, Fairbanks, who is tracking the influx of beavers into the sparse northern landscape. 'Our modeling work, which is in progress right now, shows that this entire area, the north slope of Alaska, will be colonized by beavers by 2100.'

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