Atmospheric carbon dioxide concentration. Note the steady increase in the
concentration of atmospheric cabon dioxide since 1958 when measurement began at
the Mauna Loa Observatory, Hawaii. The fluctuations correspond to the annual
cycles of plants in the Northern Hemisphere: winter (a high level of carbon dioxide),
when plants are not actively growing and absorbing carbon dioxide, and summer
(a low level of carbon dioxide) when they are growing and absorbing carbon dioxide
(David Keeling and Tim Whorf, Scripps Institution of Oceanography, La Jolla,
California quoted by Raven and Berg, 2001)