Three-monthly seasonal average atmospheric CO2 concentration recorded at Mauna Loa Observatory, Hawaii, from March 1958 through December 2002 [data from Keeling and Whorf (2003)]. Highest CO2 occurs in the northern hemisphere spring [March, April, May (MAM)]. Because this is peak growing season for northern hemisphere plants, the higher spring CO2 levels may further magnify the impact on plant growth and pollen production. DJF, December, January, February; JJA, June, July, August; SON, September, October, November.