The world is hurtling towards an era when global concentrations of carbon dioxide never again dip below the 400 parts per million (ppm) milestone, as two important measuring stations sit on the point of no return.

The news comes as one important atmospheric measuring station at Cape Grim in Australia is poised on the verge of 400ppm for the first time. Sitting in a region with stable CO2 concentrations, once that happens, it will never get a reading below 400ppm.