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The Greening of West Africa
Heubes, J., Kuhn, I., Konig, K., Wittig, R., Zizka, G. and Hahn, K. 2011. Modelling biome shifts and tree cover change for 2050 in West Africa. Journal of Biogeography 38: 2248-2258.

The authors write that "Africa is expected to face severe changes in climatic conditions this century (IPCC, 2007), which will affect the spatial distribution of biomes and vegetation characteristics (e.g. tree cover)," and they add that "the continent is also characterized by a fast growing human population (up to 3% per year; FAO, 2007), which is imposing an increased pressure upon ecosystems ranging from tropical evergreen forest to deciduous forest, savanna and grassland."

What was done
In an attempt to get some idea of what the ultimate outcome of these competing forces might be at the midpoint of the current century, Heubes et al. modelled the future spatial distribution of desert, grassland, savanna, deciduous and evergreen forest in West Africa using six bioclimatic models. None of these models, however, took any account of the photosynthetic-enhancing and transpiration-reducing effects of projected increases in atmospheric CO2 concentration, being based solely on the climatic projections of 17 general circulation models of the atmosphere for emissions scenario A2a, as described in the Fourth Assessment Report of the IPCC (2007), which projections were downscaled to 0.1 degree of latitude and longitude as described by Ramirez and Jarvis (2008).

What was learned
The six scientists report finding "a climate-driven greening trend," with "northward spread of grassland into the Sahara and the replacement of savannas by deciduous forest," which results they say "are concordant with results from Cramer et al. (2001), Scholze et al. (2006) and Scheiter and Higgins (2009)," although they add that the latter investigators "attributed the greening to increased CO2 levels." Further to this point, they state that the models they used "indicate that climatic change alone can yield this pattern," where "the expected 'greening' of the Sahara is primarily driven by increasing precipitation," as they note has also been suggested by Hickler et al. (2005).

Using satellite images that reflect the region's current vegetation state, Heubes et al. additionally modeled "real" as opposed to "potential" vegetation, which enabled them to "clearly show," as they describe it, "effects of human activity negatively affecting tree cover, as also demonstrated by other case studies, e.g. in Senegal (Vincke et al., 2010) and Mali (Ruelland et al., 2010). More specifically, they report that in West Africa, "agricultural expansion, sometimes facilitated by other human activities such as wood extraction, has been identified as major drivers of forest loss and degradation," citing Norris et al. (2010).

What it means
"Considering climate change alone," in the words of Heubes et al., "the model results of potential vegetation (biomes) show a 'greening' trend by 2050." However, they say that "the modeled effects of human impact suggest future forest degradation." Hence, it can readily be appreciated that the additional impetus for greening that is provided by the ongoing rise in the air's CO2 content may well spell the difference between better days or sadder days for the biomes of West Africa and the welfare of the region's growing human population in the years and decades ahead.

Cramer, W., Bondeau, A., Woodward, F.I., Prentice, I.C., Betts, R.A., Brovkin, V., Cox, P.M., Fisher, V., Foley, J.A., Friend, A.D., Kucharik, C., Lomas, M.R., Ramankutty, N., Sitch, S., Smith, B., White, A. and Young-Molling, C. 2001. Global response of terrestrial ecosystem structure and function to CO2 and climate change: results from six dynamic global vegetation models. Global Change Biology 7: 357-373.

FAO. 2007. State of the World's Forest. Food and Agriculture Organization, Rome, Italy.

Hickler, T., Eklundh, L., Seaquist, J.W., Smith, B., Ardo, J., Olsson, L., Sykes, M.T. and Sjostrom, M. 2005. Precipitation controls Sahel greening trend. Geophysical Research Letters 32: 10.1029/2005GL024370.

IPCC. 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change, Cambridge, United Kingdom.

Norris, K., Asase, A., Collen, B., Gockowksi, J., Mason, J., Phalan, B. and Wade, A. 2010. Biodiversity in a forest-agriculture mosaic - The changing face of West African rainforests. Biological Conservation 143: 2341-2350.

Ramirez, J. and Jarvis, A. 2008. High Resolution Statistically Downscaled Future Climate Surfaces. International Centre for Tropical Agriculture, Cali, Colombia.

Ruelland, D., Levavasseur, F. and Tribotte, A. 2010. Patterns and dynamics of land-cover changes since the 1960s over three experimental areas in Mali. International Journal of Applied Earth Observation and Geoinformation 12: S11-S17.

Scheiter, S. and Higgins, S.I. 2009. Impacts of climate change on the vegetation of Africa: an adaptive dynamic vegetation modellng approach. Global Change Biology 15: 2224-2246.

Scholze, M., Knorr, W., Arnell, N.W. and Prentice, I.C. 2006. A climate-change risk analysis for world ecosystems. Proceedings of the National Academy of Sciences USA 103: 13,116-13,120.

Vincke, C., Diedhiou, I. and Grouzis, M. 2010. Long term dynamics and structure of woody vegetation in the Ferlo (Senegal). Journal of Arid Environments 74: 268-276.

Reviewed 23 May 2012