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CO2

Seasonally corrected CO2 levels have been increasing steadily since 1960 when measured at Mauna Loa in Hawaii, the longest direct measurements of CO2 in the atmosphere.  It is measured as the mole fraction in dry air. The last year of data are preliminary. The data are collected at an altitude of 3400 m in the northern subtropics.

In March of 1958, the seasonally corrected mean was 314.69, at the end of 2009 it was 388.15 and in October, 2010 the preliminary mean was 390.58 ppm (1). Although month to month data have not always seen a steady increase, there has been a steady yearly increase averaging about 1.4 ppm per year from 1958 to 2008.

(1) Dr. Pieter Tans, NOAA, ESRL
(http://www.esrl.noaa.gov/gmd/ccgg/trends/).  See site for a fuller description of how data is collected as well as comparison to global values).

Graph: http://en.wikipedia.org/wiki/Co2 (Retreived November 15, 2010).