Science: 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...