NOAA’s Mauna Loa Observatory in Hawaii. (NOAA via AP) Scientists who measure and forecast the concentrations of greenhouse gases in the atmosphere said Monday that we may have passed a key turning point. Humans walking the Earth today will probably never live to see carbon dioxide concentrations in the atmosphere once again fall below a level of 400 parts per million (ppm), at least when measured at the iconic Mauna Loa Observatory in Hawaii, where the longest global record of Co2 has been compiled. “Our forecast supports the suggestion that the Mauna Loa record will never again show CO2 concentrations below the symbolic 400 ppm within our lifetimes,” write the researchers, led by Richard Betts of the U.K. Met Office’s Hadley Center, in Nature Climate Change. The study was conducted with colleagues from the Hadley Centre and Ralph Keeling of the Scripps Institution of Oceanography in La Jolla, Calif. […]