Each spring, when the Northern Hemisphere's vegetation awakens from the dormancy of winter and begins to grow again, it removes enough carbon dioxide from the atmosphere to reduce the air's CO2 content by several parts per million. Then, in the fall, when much of this vegetation dies and decays, it releases huge quantities of carbon dioxide back to the atmosphere, raising the air's CO2 content by a small amount. Together, these two phenomena produce a seasonal oscillation that is superimposed upon the yearly incremental rise in the air's mean CO2 concentration; and the greater the yearly growth of the planet's vegetation, the greater are the yearly down- and up-swings in the amount of carbon dioxide in the air. Consequently, the amplitude of the atmosphere's seasonal CO2 oscillation serves as a good relative measure of the planet's total vegetative productivity in any given year.
Detailed measurements of this phenomenon at Mauna Loa, Hawaii have revealed that the biosphere's seasonal CO2 "inhalations" and "exhalations" are growing more and more pronounced each year, as the average CO2 content of the atmosphere rises with time. The increasing vigor of this phenomenon is revealed in the accompanying figure, where the yearly differences between the high and low points of the seasonal CO2 cycle are plotted as a function of time.
As can be seen from these data, over the period 1958-1999 the "breath" of the biosphere has been considerably enhanced. The 19.5% increase in the strength of this phenomenon is primarily a direct result of atmospheric CO2 fertilization (Pearman and Hyson, 1981; Cleveland et al., 1983; Bacastow et al., 1985; Enting, 1987; Kohlmaier et al., 1989; Keeling et al., 1996), nitrogen-induced increases in the growth rates of earth's ecosystems (Shindler and Bayley, 1993; Hudson et al., 1994; Galloway et al., 1995), and CO2-induced expansions in some of their ranges (Idso, 1995). A slight temperature increase reported in some Northern Hemisphere land areas over this time period may also be a contributing factor (Keeling et al., 1996; Myneni et al., 1997). Together, these phenomena combine to produce the results shown in the graph above, which stands as a strong testament to the reality of the ubiquitous "greening of the earth" (Idso, 1986) that is currently in progress.
To access a file of the data from which the graph was constructed, click here.
References
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Cleveland, W.S., Frenny, A.E. and Graedel, T.E. 1983. The seasonal component of atmospheric CO2: Information from new approaches to the decomposition of seasonal time-series. Journal of Geophysical Research 88: 10,934-10,940.
Enting, I.G. 1987. The interannual variation of carbon dioxide concentration at Mauna Loa. Journal of Geophysical Research 92: 5497-5504.
Galloway, J.N., Schlesinger, W.H., Levy II, H., Michaels, A. and Schnoor, J.L. 1995. Nitrogen fixation: Anthropogenic enhancement -- environmental response. Global Biogeochemical Cycles 9: 235-252.
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Idso, S.B. 1986. Industrial age leading to the greening of the Earth? Nature 320: 22.
Idso, S.B. 1995. CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution. Special Publication. Department of Soil, Water and Climate, University of Minnesota, St. Paul, MN.
Keeling, C.D., Chin, J.F.S. and Whorf, T.P. 1996. Increased activity of northern hemispheric vegetation inferred from atmospheric CO2 measurements. Nature 382: 146-149.
Kohlmaier, G.H., Sire, E.O., Janecek, A., Keeling, C.D., Piper, S.C. and Revelle, R. 1989. Modeling the seasonal contribution of a CO2-fertilization effect of the terrestrial vegetation to the amplitude increase in atmospheric CO2 at Mauna Loa Observatory. Tellus Series B 41: 487-510.
Myneni, R.B., Keeling, C.D., Tucker, C.J., Asrar, G. and Nemani, R.R. 1997. Increased plant growth in the northern high latitudes from 1981 to 1991. Nature 386: 698-702.
Pearman, G.I. and Hyson, P. 1981. The annual variation of atmospheric CO2 concentration observed in the northern hemisphere. Journal of Geophysical Research 86: 9839-9843.
Schindler, D.W. and Bayley, S.E. 1993. The biosphere as an increasing sink for atmospheric carbon: Estimates from increased nitrogen deposition. Global Biogeochemical Cycles 7: 717-734.