Carbon dioxide levels in Antarctica are the highest they have been in four million years.
There are now no more monitoring stations on Earth measuring an atmospheric carbon dioxide level of less than 400 parts per million (ppm).
On May 23, Antarctica hit the dubious milestone, the US National Oceanic and Atmospheric Administration (NOAA) has announced.
The frozen continent at the world's South Pole was the last place on Earth to reach that level of carbon pollution, NOAA's Global Greenhouse Gas Reference Network Pieter Tans says.
Unlike four million years ago, this time there's "abundant and solid evidence that the carbon dioxide increase is caused entirely by human activities", he says.
Readings recorded by NOAA's greenhouse gas monitoring network at the South Pole from 2014 to present (NOAA / supplied)
"Since emissions from fossil fuel burning have been at a record high during the last several years, the rate of CO2 increase has also been at a record high."
When the monitoring station at New Zealand's Baring Head passed 400ppm, NIWA atmospheric scientist Sara Mikaloff-Fletche said it was a path the world had been on for "a very long time".
"The 400ppm threshold represents an opportunity for people to recognise this landmark and understand that there is only a small amount of time to accomplish change," Dr Mikaloff-Fletche says.
"It is a useful point to recognise that because we have left things so long, there is very limited opportunity to get back on track and stop drastic climate change."
Dr Tans says some of the pollutants will remain in the atmosphere for millennia, and it's almost certain the global average of atmospheric carbon dioxide will be over 400ppm.
"Global CO2 levels will not return to values below 400ppm in our lifetimes, and almost certainly for much longer," he says.