An air monitoring station in New Zealand has passed a climate change milestone which one scientist describes as "a bit like turning 40 and realising you haven't started saving for your retirement".
Carbon dioxide readings from The Baring Head clean air monitoring station near Wellington this week officially passed 400 parts per million, the National Institute of Water and Atmospheric Research says.
It follows the mark being passed at Hawaii's Mauna Loa station in November and Tasmania's Cape Grim last month.
Most carbon dioxide - the primary greenhouse gas - is generated in the Northern Hemisphere.
However, enough gas has now mixed through the atmosphere to push readings further south above 400ppm, Sara Mikaloff-Fletcher says in a statement.
Baring Head was particularly significant because it measured air originating far to the south of the site, away from human activities.
Scientists knew the level would eventually be reached.
When the mark was passed in Tasmania it prompted Greenpeace to call for laws in Australia to end fossil fuel industries.
It was a bit like turning 40 and realising you haven't started saving for your retirement, Dr Mikaloff-Fletcher said.
"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."
Reducing the levels to below 400ppm would likely take many years of emissions reduction, she said.
