Carbon Dioxide -- Summary

When the earth was in its infancy, some four-and-a half billion years ago, it is believed that the atmosphere was predominantly composed of carbon dioxide, which would have put its CO2 concentration, in terms of the units most commonly used today, at something on the order of 1,000,000 ppm. Ever since, however, the CO2 content of the air in the mean has been dropping. By 500 million years ago, in fact, the atmosphere's CO2 concentration is estimated to have fallen to only 20 times more than it is today, or something on the order of 7500 ppm; and by 300 million years ago, it had declined to close to the air's current CO2 concentration of 370 ppm, after which it rose to about five times where it now stands at 220 million years before present (Berner 1990, 1992, 1993, 1997; Kasting 1993). Then, during the middle Eocene, some 43 million years ago, the atmospheric CO2 concentration is estimated to have dropped to a mean value of approximately 385 ppm (Pearson and Palmer, 1999); while between 25 to 9 million years ago, it is believed to have varied between 180 and 290 ppm (Pagani et al., 1999). This latter concentration range is essentially the same range over which the air's CO2 concentration oscillated during the 100,000year glacial cycles of the past 420,000 years (Fischer et al., 1999; Petit et al., 1999). With the inception of the Industrial Revolution, however, the air's CO2 content once again began an upward surge that has now taken it to the 370 ppm level, with the promise of significantly higher values still to come.