The importance of carbon dioxide as a greenhouse gas in climate change is spurring studies to assess each source where the gas is emitted and each ‘sink’ (like forests and soils) where it is absorbed – and how much carbon comes and goes from each. At the Meeting of the Americas, David Butman of Yale University presented the first estimate of  how much carbon enters the atmosphere annually from the rivers and streams of the United States—a quantity researchers keep track of in petagrams or trillions of kilograms. Butman, a doctoral student, and his advisor Peter Raymond, also of Yale, say that U.S. waterways are a natural source that emits about a fifth of a petagram of carbon.  That’s a significant amount, the researchers say–comparable to the net amount of carbon that U.S. forests absorb. This new estimate suggests that waterways in temperate areas around the globe together give off about a petagram of carbon.

– Peter Weiss, AGU science writer