400 ppm

Hourly averages for carbon dioxide concentration at the Mauna Loa observatory reached 400 parts per million for the first time earlier this month.

Update (April 30):  The 400 ppm milestone is the highest concentration in about 3 million years.

Update (May 10):  The daily average has now reached 400 ppm for the first time.  George Monbiot calls it a reflection of a profound failure of politics.  The world was a much different place 3 million years ago.

Update (March 18, 2014):  This year carbon dioxide concentration has already touched 400 ppm with a peak in May expected to bring the first monthly average above 400 ppm.

Update (May 15, 2014):  April was in fact the first month to average 400 ppm.

Update (June 29, 2014):  A study by Raymond Pierrehumbert of the University of Chicago emphasizes the priority of reducing carbon dioxide emissions over other greenhouse gases.

Update (July 1, 2014):  June now makes three months averaging over 400 ppm.

Update (May 6, 2015):  This past March is the first month to average 400 ppm worldwide.  It seems that April 2014 was for the Northern hemisphere.

Update (May 11, 2016):  Soon, the planet will never go below 400 ppm ever again.

Update (September 27, 2016):  September is usually the annual minimum for carbon dioxide in the atmosphere, and this month has not dropped below 400 ppm. We will never go below that threshold ever again.