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Carbon Sequestration in Africa: A Well-Kept Secret
We hear a lot nowadays about the tremendous rates of carbon sequestration occurring in North America (Fan et al., 1998; Pacala et al., 2001), Europe (Zhou et al., 2001; Myneni et al., 2001), Asia (Fang et al., 2001 ), and Central and South America (Phillips et al., 1998; Laurance, 1999).  But what about Africa?

At first blush, prospects for capturing and storing carbon in Africa do not look very promising.  In a recent analysis of the continent's potential to do so, Cao et al. (2001) remind us of the significant warming trend experienced there over the past century or so, noting that warmer years in Africa often result in decreased precipitation.  They also note that drought and warming typically lead to land degradation and desertification, all of which changes - individually and in concert - are known to have large negative impacts on carbon sequestration in vegetation and soils.

With respect to vegetation, Cao et al. report that "in sub-tropical and tropical Africa, where temperatures are often higher than the optimum for plant growth, warming may cause declines in plant net primary production."  In addition, they report that "because water availability limits plant growth in most parts of Africa, drying has reduced plant productivity."  With respect to soils, they similarly note that "warming enhances microbial activity and thus can cause declines in the carbon stocks of soils."  Therefore - and in light of what climate alarmists call the unprecedented warming of the last quarter-century, which has coincided with "a long-term reduction in rainfall ... on the order of 20 to 40% in parts of the Sahel" (Nicholson, 2001) - the African continent would appear to have very little going for it in the way of carbon sequestration potential.

Appearances, however, can often be deceiving, especially when the putative cause (the rising atmospheric CO2 concentration) of the hypothetical problem (global warming) is like a soothing balm for earth's vegetation during naturally-occurring periods of heat and drought.  Always operative, for example, is the aerial fertilization effect of atmospheric CO2 enrichment, which directly and dramatically stimulates plant productivity (Idso, 2001).  Higher levels of atmospheric CO2 also enhance plant water use efficiency (Idso and Idso, 1994), making vegetation better able to cope with the debilitating effects of drought.  In addition, elevated CO2 concentrations tend to raise both the optimum temperature for photosynthesis in plants and the temperature at which heat-induced tissue death occurs (Idso, 1995), enabling vegetation to better withstand the deleterious and potentially fatal effects of uncharacteristically rapid increases in temperature.

In light of these important rising-CO2-induced remedies for too much heat and too little water, it is only to be expected that we could be in for some surprises if the biological consequences of atmospheric CO2 enrichment were included in a reconstruction of Africa's carbon sequestration history, which is precisely what Cao et al. did in simulating what they describe as "the dynamic variations in the carbon fluxes and stocks of African ecosystems caused by changes in climate and atmospheric CO2 from 1901 to 1995."  So what did they find?  In the authors' own words, "the combined effect of the changes in climate and atmospheric CO2 caused a total carbon sequestration of 15.5 Gt C from 1901 to 1995," including a sequestration of 4.8 Gt C over the extremely stressful (hot and dry) period of 1981 to 1995.

Clearly, this is an important "win" for CO2 over climate, in reversing what would otherwise have been a significant loss of carbon from the continent due to warming and drying.  "Nevertheless," as Cao et al. continue, "Africa is not necessarily a significant carbon sink, because a large part of the carbon sequestration is offset by the carbon release arising from land use changes," such as deforestation, which is the scourge of many a continent.  The silver lining of this observation, however, is that if these destructive land use changes can be corrected, Africa stands to dramatically increase its sequestration of carbon, as a continent-wide release of 6.6 Gt C due to land-use changes has been estimated to have occurred in the 1980s alone (Gaston et al., 1998).

The bottom line, therefore, is that the potential for carbon sequestration everywhere is likely much greater than most people have long believed, and that it is positively amenable to human manipulation, which suggests that the planet may not be committed to nearly as great a warming as is suggested by the infamous "scenarios" of the IPCC, which is, again, more good news.

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

References
Cao, M., Zhang, Q. and Shugart, H.H.  2001.  Dynamic responses of African ecosystem carbon cycling to climate change.  Climate Research 17: 183-193.

Fan, S., Gloor, M., Mahlman, J., Pacala, S., Sarmiento, J., Takahashi, T. and Tans, P.  1998.  A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models.  Science 282: 442-446.

Fang, J., Chen, A., Peng, C., Zhao, S. and Ci. L.  2001.  Changes in forest biomass carbon storage in China between 1949 and 1998.  Science 292: 2320-2322.

Gaston, G., Brown, S., Lorenzini, M. and Singh, K.D.  1998.  State and change in carbon pools in the forests of tropical Africa.  Global Change Biology 4: 97-114.

Idso, C.D.  2001.  Earth's rising atmospheric CO2 concentration: Impacts on the biosphere.  Energy & Environment 12: 287-310.

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.

Idso, S.B.  1995.  CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution.  Kuehnast Lecture Series.  Department of Soil, Water & Climate, University of Minnesota, St. Paul, MN.

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

Myneni, R.B., Dong, J., Tucker, C.J., Kaufmann, R.K., Kauppi, P.E., Liski, J., Zhou, L., Alexeyev, V. and Hughes, M.K.  2001.  A large carbon sink in the woody biomass of Northern forests.  Proceedings of the National Academy of Sciences, USA: 98: 14,784-14,789.

Nicholson, S.E.  2001.  Climatic and environmental change in Africa during the last two centuries.  Climate Research 17: 123-144.

Pacala, S.W., Hurtt, G.C., Baker, D., Peylin, P., Houghton, R.A., Birdsey, R.A., Heath, L., Sundquist, E.T., Stallard, R.F., Ciais, P., Moorcroft, P., Caspersen, J.P., Shevliakova, E., Moore, B., Kohlmaier, G., Holland, E., Gloor, M., Harmon, M.E., Fan, S.-M., Sarmiento, J.L., Goodale, C.L., Schimel, D. and Field, C.B.  2001.  Consistent land- and atmosphere-based U.S. carbon sink estimates.  Science 292: 2316-1320.

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.

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.