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 100,000-year 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.
In addition to its variation over geologic time, the atmosphere's CO2 concentration exhibits a strong seasonal variation. It declines when the terrestrial vegetation of the Northern Hemisphere awakens from the dormancy of winter and begins to grow in the spring, thereby extracting great quantities of CO2 from the air; and it rises in the fall and winter, when much of the biomass produced over the summer dies and decomposes, releasing great quantities of CO2 back to the atmosphere. Over the past four decades that this phenomenon has been accurately measured, it has been observed that this yearly "breath of the biosphere" has risen in strength by approximately 20%, due primarily to the aerial fertilization effect of the ongoing rise in the mean value of the air's CO2 concentration (Idso et al., 1999), but influenced by a number of other factors as well (Zimov et al., 1999).
The air's CO2 content also varies spatially over the surface of the earth. Most spectacular in this regard are the local concentration enhancements observed over large metropolitan areas due to high levels of vehicular traffic and commercial activities. Idso et al. (1998a, b), for example, measured CO2 concentrations near the center of Phoenix, Arizona that were 50% greater than those measured over surrounding rural areas. Significant enhancements of the air's CO2 concentration may also be observed in the vicinity of burning coal seams and naturally occurring high-CO2 springs. In Italy, the CO2-enriched air near such springs has enabled oak trees to transpire less water and thus maintain a better internal leaf water status in the face of drought than similar trees growing in ambient air a short distance away from the springs (Tognetti et al., 1998); and in Venezuela, it has allowed herbs and trees growing near a high-CO2 spring to continue to sequester carbon during dry periods of the year when plants exposed to normal air just tens of meters away actually lose carbon (Fernandez et al., 1998). In fact, Schwanz and Polle (1998) have observed that naturally-CO2-enriched trees appear to experience less stress of all kinds than trees growing in ambient air.
It is interesting to note, in this regard, that some naturally-occurring high-CO2 springs produce very high CO2 concentrations in their immediate vicinity; and it is therefore only natural to wonder if such high concentrations might be detrimental to vegetation. Apparently, they are not; for studies carried out at 10,000 ppm CO2 have produced positive responses in plants (Gouk et al., 1999; Louche-Tessandier et al., 1999), as have experiments conducted at 35,000 ppm (Fernandez et al., 1998). Also of interest within this plant health context is the fact that atmospheric CO2 enrichment has been observed to have little effect on the growth of the noxious bracken weed (Caporn et al., 1999) and that it has helped oat plants infected with barley yellow dwarf virus considerably more than it has helped uninfected plants (Malmstrom and Field, 1997). Hence, even very high CO2 concentrations - some as much as 100 times greater than those of the past century - appear to benefit earth's vegetation.
A final concern related to the ongoing rise in the air's CO2 concentration is the worry that it may lead to catastrophic global warming. There is little reason to believe that such will ever occur, however, for several observations of historical changes in atmospheric CO2 concentration and air temperature suggest that it is climate change that drives changes in the air's CO2 content and not vice versa. In a study of the global warmings that signaled the demise of the last three ice ages, for example, Fischer et al. (1999) found that air temperature always rose first, followed by an increase in atmospheric CO2 some 400 to 1000 years later. Likewise, Petit et al. (1999) found that for all of the glacial inceptions of the past half-million years, air temperature consistently dropped before the air's CO2 content did, and that the CO2 decreases lagged the temperature decreases by several thousand years. In addition, the multiple-degree-Centigrade rapid warmings and subsequent slower coolings that occurred over the course of the start-and-stop demise of the last great ice age are typically credited with causing the minor CO2 concentration changes that followed them (Staufer et al., 1998); and there are a number of other studies that demonstrate a complete uncoupling of atmospheric CO2 and air temperature during periods of significant climate change (Cheddadi et al., 1998; Gagan et al., 1998; Raymo et al., 1998; Indermuhle et al., 1999). Hence, there are no historical analogues for CO2-induced climate change; but there are many examples of climate change-induced CO2 variations.
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