Within the next couple of weeks - perhaps within ten days - CSIRO scientists and the Bureau of Meteorology (BoM) expect to log a disturbing climate change milestone with implications for the entire planet:  The first atmospheric baseline of 400 parts per million (ppm) of carbon.

On May 6, a Commonwealth Scientific and Industrial Research Organization (CSIRO) team took a reading of 399.9 ppm at the Cape Grim monitoring site in northwestern Tasmania, which it runs with the BoM.  "Once it's over (400 ppm), it won't go back," said retired CSIRO fellow Paul Fraser to Fairfax Media, noting that CO2 levels in the atmosphere has been on a steady climb since the late 1970s.

Levels of 400 ppm have been seen before at the other two main global monitoring stations in Alaska and Hawaii since 2013.  But those went back down as the Northern Hemisphere spring arrived, and all of that blooming vegetation absorbed the carbon.  That's not going to be the case once the Cape Grim reading breaks the barrier, because the Southern Hemisphere has less land mass and thus less vegetation - it's much steadier and a more accurate glimpse of what is actually happening to the world climate.  And it's bad.

Australian Greens deputy leader Larissa Waters said the landmark "should act as a global wake-up call and must shock both Australian big political parties out of their blind coal-obsession which is literally cooking our planet and our Great Barrier Reef."  She added, "Our atmosphere cannot take any new coal mines - both the old parties must stop approving them and revoke their approval of the Adani coal mine (in Queensland) at both the state and federal level."

Labor's shadow environment minister Mark Butler deflected that criticism onto the coalition:  "While the Coalition fights about whether or not the science of climate change is real, pollution is rising," he said, "And it's rising on their watch."