http://www.nytimes.com/2016/10/04/science/atmospheric-carbon-dioxide-400-ppm.html 2016-10-03 23:45:05 A Milestone for Carbon Dioxide in the Atmosphere Scripps said that levels had recently risen above 400 parts per million, and were likely to stay that way “for the indefinite future.” === Climate science reached an unhappy milestone last week. And then things went a little crazy. One of the world’s most important sentinel sites for measuring levels of carbon dioxide in the atmosphere, at Mauna Loa, Hawaii, reported that levels had recently risen above the symbolically important figure of 400 parts per million, and were likely to stay that way “for the indefinite future.” Because rising amounts of carbon dioxide, a heat-trapping gas, in the atmosphere have been linked to In recent years, monitoring stations have reported periods in which CO Dr. James Hansen That is sobering news. But some of the reaction has been, to put it mildly, His father, Charles David Keeling, began carbon dioxide measurements on Mauna Loa and at other locations in the late 1950s. In an interview, Ralph Keeling stressed that the 400 parts per million is “a good yardstick,” but “to call it a tipping point is incorrect.” Gavin Schmidt, a scientist who heads a NASA climate research unit in New York, agreed. “400 ppm is definitely a milestone, but there’s no evidence it’s a tipping point,” he wrote in an email response to questions. One danger of alarmism, Dr. Keeling said, is that it can make people feel hopeless and throw up their hands. “The first step is just to stop the increase,” he said, no matter how high the numbers rise in the meantime. “There’s no punting on global warming.”