InsideClimate: For the first time in human history, concentrations of carbon dioxide in the atmosphere are expected to pass 400 parts per million across much of the Northern Hemisphere in May, according to scientists who study data from the Mauna Loa Observatory, the world's longest-running CO2 monitoring station.
While crossing the 400 ppm mark isn't a "tipping point" that signals climate catastrophe, scientists told InsideClimate News, it is an important symbolic milestone that underscores government inaction...