Posted by Peter Gleick on May 10, 2013

The planet has passed a disturbing landmark, a marker on a continuing highway to climate disruption. On May 9th, the NOAA and the Mauna Loa observatory reported that atmospheric CO2 levels touched 400 parts per million. Before humans started burning fossil fuels, they were around 280 parts per million.

Mauna Loa measurements of carbon dioxide. From

The last time atmospheric CO2 was at 400 parts per million was during the ancient Pliocene Era, three to five million years ago, and humans didn’t exist.

Global average temperatures were 3 to 4 degrees C warmer than today (5.4 to 7.2 degrees F).
Polar temperatures were as much as 10 degrees C warmer than today (18 degrees F).
The Arctic was ice free.
Sea level was between five and 40 meters higher (16 to 130 feet) than today.
Coral reefs suffered mass die-offs.
And much more: As Robert Monroe notes: “The extreme speed at which carbon dioxide concentrations are increasing is unprecedented. An increase of 10 parts per million might have needed 1,000 years or more to come to pass during ancient climate change events. Now the planet is poised to reach the 1,000 ppm level in only 100 years if emissions trajectories remain at their present level.”

Here are some scientific links for those wanting to know more:

Trends in atmospheric carbon dioxide:
Details on the Mauna Loa Observatory, and the “Keeling Curve:”
Robert Monroe discusses what the Pliocene Earth looked like, here:
Haywood et al. also review what we know about the Pliocene: Haywood et al. 2009. “Introduction. Pliocene climate, processes and problems. Phil. Trans. R. Soc. A. Vol. 367, No. 1886, pp. 3-17. 13 January 2009 doi:10.1098/rsta.2008.0205
B. Schneider and R. Schneider. 2010. Palaeoclimate: Global warmth with little extra CO2. Nature Geoscience Vol. 3, pp. 6-7. 20 December 2009. doi:10.1038/ngeo736.

In fact, here’s an entire set of papers discussing the Pliocene, current and past CO2 levels and conditions, and more:

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