The Keeling Curve is a graph measuring the increase in the levels of carbon dioxide in the atmosphere since 1958. Measurements taken atop Mauna Loa in Hawaii by Dr. Charles David Keeling show an increase of CO2 in that time span from about 315.0 parts per million by volume to 367 ppmv by the year 2000. The graph shows a cyclical increase and decrease within each year, corresponding to the seasonal change in uptake of CO2 by the world's vegetation. The Keeling Curve has most notably been interpreted as a signal of possible global warming.

Sources:

http://earthguide.ucsd.edu/globalchange/keeling_curve/01.html http://cdrg.ucsd.edu/keeling_curve.html