Atmostpheric carbon dioxide hits 400 ppm for first time in human history.
Carbon Dioxide in Atmosphere Hits 400 PPM for First Time in Human History
Future Tense
The Citizen's Guide to the Future
May 10 2013 2:24 PM

Carbon Dioxide in Atmosphere Hits 400 PPM for First Time in Human History

Before the industrial revolution, the proportion of carbon dioxide in Earth’s atmosphere hovered around 280 parts per million. In 1958, when scientists began measuring average carbon dioxide levels at an observatory on Mauna Loa in Hawaii, that figure was around 320 ppm.


The last time carbon dioxide levels were this high, Australopithecus was sharing the earth with mammoths and saber-tooth tigers.

Damian Carrington at The Guardian and David Biello at Scientific American have more context on the milestone and what it means. My colleague Phil Plait has debunked a ridiculous Wall Street Journal op-ed claiming that carbon dioxide levels are no big deal. All are worth reading.

For now I’ll just present you with this simple chart, which shows our stunning progress from pre-industrial carbon-dioxide levels toward the limit of 450 ppm that the Intergovernmental Panel on Climate Change has warned is necessary to have any significant hope of limiting global warming to 2 degrees Celsius.  

The IPCC has estimated that 450 ppm is the limit beyond which it is likely that the earth will warm by more than 2 degrees celsius, with potentially disastrous results. We're getting there.

Future Tense is a partnership of SlateNew America, and Arizona State University.

Will Oremus is Slate's senior technology writer. Email him at or follow him on Twitter.

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