STEVE INSKEEP, Host:
One problem of accounting for carbon is trying to figure out how much carbon goes into forests and comes out again. That's important because forests could become carbon banks, as they're called, storing carbon and keep it out of the atmosphere. The legislation pending in Congress would pay people to grow trees. But as NPR's Christopher Joyce reports, carbon banking is a tricky business.
CHRISTOPHER JOYCE: Here's forest ecologist Matthew Hurteau from Northern Arizona University.
M: Without regular surface fire, the litter, branches, coarse woody debris build up over time, and then when fire does occur, end up releasing more energy and carbon.
JOYCE: But any fire puts CO2 into the air. So do frequent small fires put more or less carbon into the atmosphere than the occasional massive fire?
M: You know, that is really a question that we're looking for the answer to right now.
JOYCE: Gregory Latta is a forest economist at Oregon State University who studied how Northwestern forests might be affected by warming. In some places, warming could slow growth, and that means less carbon in the bank.
M: At the lower elevations, they have moisture. And as you crank up the temperature in the growing season, that moisture evaporates. And so it's kind of taking away from them a little bit.
JOYCE: Latta adds that a warmer climate would probably have the opposite effect on higher-altitude forests - they'd grow faster. In the end, Northwestern forests might see overall increases, but that may not apply elsewhere.
M: So you can't really put this broad brush across the whole area and say, here's the impact of climate change. It's going to really impact different areas depending on what their starting point was.
JOYCE: Then there's another trend to watch: drought. Climate models predict the West will get drier. David Breshears, at the University of Arizona, says scientists are seeing more dead and dying trees already.
M: There's a potential for kind of a double-whammy between having warmer temperature and drought, which could trigger fire or tree die-off, and that'll pose a challenge in terms of how much carbon can you store in a system.
JOYCE: Christopher Joyce, NPR News.
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