Where did those pine beetles come from? Us. Climate change:
SARAH GARDNER: But wait a minute. Explain for us how this little beetle has anything to do with climate change? Because, I mean, my understanding is that the pine beetle is a native species, right? It's always been there. And a lot of westerners believe the only reason it's gotten out of hand is because we haven't been thinning out the forests enough, right?
SAM EATON: That hasn't helped. And you throw in fire suppression and the beetles basically have an all-you-can-eat buffet of lodge pole and Ponderosa pine. But the scientists I talked to -- like Jesse Logan, who's been studying the beetles for decades -- say the main thing driving this outbreak is human-caused global warming.
JESSE LOGAN: It's by the actions of people. It's directly our actions that are taking these forests out.
SAM: Let me connect the dots here. Logan says pine beetles have always been held in check by deep winter freezes. But that 2-degree increase in average temperatures you mentioned earlier, Sarah, has meant fewer cold snaps -- especially in the high elevations of the Rockies. Basically, the pine beetle couldn't have asked for better breeding conditions [Sarah Gardner and Sam Eaton, "Climate Change in Our Own Backyards," Marketplace, 2009.10.27].
Marketplace is running a big series on climate change; part 4 runs this evening (SDPB Radio, 19:00 CDT). Each piece is lengthy and worth the listen. The series webpage includes lots of resources on climate change science and economic impacts.