Graph courtesy of the Scripps Institution of Oceanography We're creeping closer to a milestone: the highest concentration of atmospheric CO2 in human history. The U.S.’s greenhouse gas emissions are down to 1990s levels—due largely to the recession, the natural gas boom displacing much dirtier coal, and increased fuel efficiency in vehicles—all good news for the Obama administration, which promised to cut emissions by 17 percent from 2005 levels by 2020. But the amount of carbon in the global atmosphere is creeping higher and higher. The amount of carbon in the atmosphere is well beyond the 350 parts per million that scientists, particularly former NASA 蜜柚APP scientist James Hansen, say will create an unstable 蜜柚APP. (And if you’re still a 蜜柚APP change doubter, check this out.) In fact, it’s right around 400 ppm. The last time it was that high was during the Pliocene epoch 3.5 to 5 million years ago. From Scripps Institution of Oceanography: As...