The Big 400

There is considerable media coverage this week discussing how CO2 concentrations at Mauna Loa observatory have eclipsed 400 parts per million for the first time since measurements began in 1958.  This somewhat ill defined but all important milestone is discussed in a press release issued by NOAA yesterday.

Charles David Keeling of the Scripps Institute of Oceanography began collecting CO2 measurements on Mauna Loa in 1958 and the graph illustrating the subsequent increase in CO2 to the present is often referred to as the Keeling Curve.  The graph usually is based on monthly average CO2 concentrations and thus shows both the long term increase in CO2, but also the decline of CO2 in the Northern Hemisphere spring and summer due to plant growth and a rise in the fall due to plant decay.

Source: http://keelingcurve.ucsd.edu/

It is a bit hard to see, but the annual rate of rise has increased from about 0.7 parts per million per year in the late 1950s to about 2.1 parts per million per year in the last decade.  About 70% of the increase over pre-industrial CO2 concentrations has occurred since Keeling began collecting data in the late 1950s and half has occurred since the late 1970s.

I call the 400 parts per million milestone somewhat ill-defined because CO2 does fluctuate.  We hit 400 this week, but the drawdown from plant growth means we will drop below it in the coming months.  It will probably take about 3 years until CO2 is above 400 parts per million for the entire year.

The observatory at Mauna Loa is at high elevation in the central Pacific and one of the better places to measure the "background" CO2 concentrations.  You can find higher CO2 concentrations in areas where emissions are high, especially when the atmosphere is poorly ventilated.  Jim Ehleringer's group at the University of Utah maintains a number of CO2 sensors in the Salt Lake Valley.  This morning, the CO2 concentration in Sugarhouse was as high as 460 parts per million.

Source: http://co2.utah.edu

Going back to 2007, you can see a number of periods, usually characterized by persistent wintertime inversions, where the daily average CO2 has approached or exceeded 500 parts per million.  Perhaps not surprisingly, the most events of this magnitude occurred this past winter when we were plagued by persistent inversions.  

Source: http://co2.utah.edu

Those are daily averages.  My recollection suggests that higher frequency observations show spikes near 700 parts per million during the most intense events.

These high values merely reflect the trapping of CO2 produced primarily by fossil fuel combustion near the ground.  This CO2 eventually mixes into the free atmosphere and contributes to the ongoing increase reflected in the Keeling Curve.