May 27, 2013
Carbon-dioxide levels have increased every year, and earlier this month they reached the milestone of four hundred parts per million. No one knows exactly when levels were last this high; the best guess is the mid-Pliocene, about three million years ago, when sea levels were some seventy-five feet higher. President Obama will make a decision in the next few months about whether to approve the Keystone XL pipeline. The latest figures reveal what’s at stake.