Critical zones

For almost half a century, scientists have been measuring carbon dioxide in the air two miles above sea level in the middle of the Pacific Ocean. At first, Charles David Keeling counted 310 parts of carbon dioxide for every million parts of air. When he died in 2005, the number was 380. On May 9, 2013, the number topped 400, “a milepost,” wrote National Geographic’s Robert Kunzig, “on a far more rapid uphill climb toward an uncertain climate future.”

We might get wistful over the elegance of what is now known as the Keeling number: a solitary data point, like the Dow Jones industrial average, … » More …