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The Color of Carbon
Biological sequestration of carbon ranks high on many people's lists of potential ways to slow the rate of rise of the air's CO2 concentration and thereby reduce the threat of catastrophic (too great or too rapid) global warming, which is suggested by most computer models of earth's climate to accompany rising levels of atmospheric CO2. Some such strategy would definitely seem to be warranted, particularly if one believes in the validity of the model-generated warming scenarios of the Intergovernmental Panel on Climate Change, since worldwide anthropogenic CO2 emissions grow ever greater with each passing day.

As noted by Wofsy (2001), however, the rate of CO2 accumulation in the atmosphere over the past two decades "has stayed the same or even declined slightly," in spite of the fact that emissions of CO2 from the combustion of fossil fuels have increased almost 40% over the same time period. In consequence of this dichotomy, we could well ask what Wofsy asks in the title of his article, "Where has all the carbon gone?" Since man is not removing it from the air, something else must be; and that something else can only be the process of natural biological carbon sequestration, whereby earth's vegetation automatically rises to the challenge - or opportunity, as we like to say - and grows ever more vigorously in response to the aerial fertilization effect of atmospheric CO2 enrichment as it paints the earth green ... with carbon!

Well, the words make a nice story, and the concept does have solid experimental backing; but is there any real-world evidence that it's truly occurring where it really counts, i.e., in nature? The results of a number of meticulous scientific studies say that it is.

In a paper published several years ago in Science, Phillips and Gentry (1994) reported results of numerous tree biomass assessments made in forty of the world's tropical forests, wherein they demonstrated that these highly productive ecosystems had become even more productive since at least 1960, and that they had become more productive still after 1980, which phenomenon - because of its "consistency and simultaneity of the changes on several continents" - they attributed to "enhanced productivity induced by increased CO2" (Pimm and Sugden, 1994).

Four years later, Phillips et al. (1998) published another paper in Science in which they described the results of similar measurements made "at fifty sites scattered throughout the Amazon and Central America ... of more than a hundred thousand trees over the past thirty years," wherein they continued to see this ongoing acceleration of tree growth. And this time they calculated that it was large enough to account for approximately 40% of the so-called missing carbon of the entire globe! In fact, Laurance (1999) concluded that "in the Amazon basin alone, intact rainforests could be absorbing over one billion tons of carbon dioxide each year."

Fast-forwarding to the present, we find a news item published on the website of the National Geographic Society on 13 December 2001 entitled "Adult Amazon Trees Gain Mass, Puzzle Scientists," wherein the scientists - again headed by Phillips - marvel at the increasing growth rates of large trees there over the past two decades, which finding they describe as "unexpected," because "mature forests were thought to be 'carbon neutral,' giving off as much carbon as they absorb each day." Quite to the contrary, however, the scientists now say "the increased mass of Amazonian forests suggests they are acting as a huge 'carbon sink,' absorbing more carbon from the atmosphere than they release." And Phillips continues to suggest "the Amazon forests have become more massive as a result of increased amounts of carbon dioxide in the atmosphere."

But let us not forget the northern forests of the world. In a paper published in the 16 September 2001 issue of the Journal of Geophysical Research, Zhou et al. (2001) report changes in a satellite-sensed parameter called the normalized difference vegetation index (NDVI) for the period July 1981 to December 1999. Their findings? Much of the vegetated area between 40 and 70 N latitude in Eurasia shows "a persistent increase in growing season NDVI over a broad contiguous swath of land from central Europe through Siberia to the Aldan plateau, where almost 58% is forests and woodlands." In addition, they found increases in season-long NDVI in forests of the southeastern United States. Over the two-decade period studied, the NDVI data - which the authors say "are surrogates of plant photosynthetic activity" - increased in magnitude by 12% in Eurasia and 8% in North America.

Yes, our living planet is clearly "greening up" - as seen so vividly from space - as the air's CO2 content continues to rise. And as it does, the forests of the world are becoming more and more productive, yearly extracting larger and larger amounts of CO2 from the atmosphere and sequestering its carbon in their tissues and the soils in which their trees are rooted. Truly, green is the color of carbon, and trees are nature's remedy for CO2-induced climatic change.

Dr. Sherwood B. Idso Dr. Keith E. Idso

Laurance, W.F. 1999. Gaia's lungs: Are rainforests inhaling earth's excess carbon dioxide? Natural History (April), p. 96.

Phillips, O.L. and Gentry, A.H. 1994. Increasing turnover through time in tropical forests. Science 263: 954-958.

Phillips, O.L., Malhi, Y., Higuchi, N., Laurance, W.F., Nunez, P.V., Vasquez, R.M., Laurance, S.G., Ferreira, L.V., Stern, M., Brown, S. and Grace, J. 1998. Changes in the carbon balance of tropical forests: Evidence from long-term plots. Science 282: 439-442.

Pimm, S.L. and Sugden, A.M. 1994. Tropical diversity and global change. Science 263: 933-934.

Wofsy, S.C. 2001. Where has all the carbon gone? Science 292: 2261-2263.

Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback, D., Shabanov, N.V. and Myneni, R.B. 2001. Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981 to 1999. Journal of Geophysical Research 106: 20.069-20,083.