Atmospheric readings from Cape Grim, Tasmania along with two stations in Hawaii and Alaska, are closely watched as they date back decades and closely track a range of pollutants from ozone-depleting chemicals to the various greenhouse gases resulting from burning fossil fuels and clearing forests. Sites in the northern hemisphere exceeded CO2 400 ppm from 2012 onwards. But as the region has greater seasonal variation, mostly because there is more terrestrial vegetation, CO2 concentrations dropped back below that mark each spring. Now Cape Grim, which offers some of the purest air in the world for measuring climate change, has passed CO2 400 ppm, and it will need a huge global effort to curb greenhouse gas emissions to push the level back down.