For more than 60 years, readings have been in the 300s, except in urban areas, where levels are skewed. The burning of fossil fuels, such as coal for electricity and oil for gasoline, has caused the overwhelming bulk of the man-made increase in carbon in the air, scientists say.
It's been at least 800,000 years — probably more — since Earth saw carbon dioxide levels in the 400s, Butler and other climate scientists said.
Readings are coming in at 400 and higher all over the Arctic. They've been recorded in Alaska, Greenland, Norway, Iceland and even Mongolia. But levels change with the seasons and will drop a bit in the summer, when plants suck up carbon dioxide, NOAA scientists said.
So the yearly average for those northern stations likely will be lower and so will the global number.
Globally, the average carbon dioxide level is about 395 parts per million but will pass the 400 mark within a few years, scientists said.
The Arctic is the leading indicator in global warming, both in carbon dioxide in the air and effects, said Pieter Tans, a senior NOAA scientist.
"This is the first time the entire Arctic is that high," he said.
Tans called reaching the 400 number "depressing," and Butler said it was "a troubling milestone."