Greening of the Earth: Africa
From ClimateWiki
Ciais et al. (2009) modeled the terrestrial carbon balance of Africa over the past century (1901–2002) using a spatially resolved, process-based vegetation model (ORCHIDEE), which is forced by changing climate, human-induced changes in land use, and a parameterization of natural fires. They found “the African net terrestrial carbon (C) balance increased from a net CO2 source to the atmosphere of 0.14 Pg C per year in the 1980s to a net sink of 0.15 Pg C per year in the 1990s.” In addition, they state the land use flux due to deforestation was “a source of 0.13 Pg C per year,” and “this implies that climatic trends (mainly increasing precipitation) and CO2 increase (the fertilization effect), are causing a sink of 0.28 Pg C per year which offsets the land-use source.”
The five researchers also indicate “the trend of gross primary production is closely matching the trend in satellite observed NDVI,” or Normalized Difference Vegetation Index, and they note their simulated trend in gross primary production “is also consistent with an increased vegetation activity over [the] Sahel reported by Eklundh and Olsson (2003) and Olsson et al. (2005),” while at the continental scale the gross primary production trend can be largely (70 percent) explained by the CO2 fertilization effect. Primarily in response to the ongoing rise in the air’s CO2 content, therefore, it would appear from the results of this study that the African continent is significantly “greening up,” and that it has been doing so recently at a significantly enhanced rate.
Examining what might be next for Africa, Doherty et al. (2010) modeled future changes in land biogeochemistry and biogeography in the region bounded by 12.5°N, 12.5°S, 25°E, and 42.5°E, representing most of East Africa (Kenya, Tanzania, Uganda, Rwanda, Burundi, Ethiopia, and Somalia), and portions of Central Africa (the Democratic Republic of Congo and Southern Sudan). They did this using 18 future climate projections derived from nine general circulation models that figured prominently in the IPCC’s Fourth Assessment Report, employing the projections as input to the Lund-Potsdam-Jena dynamic global vegetation model that simulates changes in vegetation and ecosystem carbon cycling under future climate conditions, based on what they describe as “a coupled photosynthesis-hydrological scheme [that] computes gross primary productivity, plant respiration, and evapotranspiration on a daily time step based on the current climate, atmospheric CO2 concentration, vegetation structure and phenological state, and soil water content.”
Doherty et al. report “all simulations showed future increases in tropical woody vegetation over the region at the expense of grasslands,” noting “regional increases in net primary productivity (18–36%) and total carbon storage (3–13%) by 2080–2099 compared with the present-day were common to all simulations,” and “seven out of nine simulations continued to show an annual net land carbon sink in the final decades of the 21st century because vegetation biomass continued to increase.” The researchers conclude “overall, our model results suggest that East Africa, a populous and economically poor region, is likely to experience some ecosystem service benefits through increased precipitation, river runoff and fresh water availability,” and they state “resulting enhancements in net primary productivity may lead to improved crop yields in some areas.” They specifically state their results “stand in partial contradiction of other studies that suggest possible negative consequences for agriculture, biodiversity and other ecosystem services caused by temperature increases.”
Regarding the continent as a whole, Scheiter and Higgins (2009) write, “recent IPCC projections suggest that Africa will be subject to particularly severe changes in atmospheric conditions” in the decades ahead, and these changes could have severe repercussions for its flora and fauna. However, they say that how the continent’s “grassland-savanna-forest complex will respond to these changes has rarely been investigated,” and “most studies on global carbon cycles use vegetation models that do not adequately account for the complexity of the interactions that shape the distribution of tropical grasslands, savannas and forests.”
In an attempt to overcome these shortcomings, the two scientists developed a new vegetation model—the adaptive dynamic global vegetation model (aDGVM)—that employs established sub-models for photosynthesis, respiration, canopy scaling, competition for water, competition for light, reproduction, and mortality, and which additionally contains the novel elements of dynamic carbon allocation and phenology functions. They also employed a fire model that estimates fire intensity as a function of fuel biomass, fuel moisture, and wind speed and simulates topkill (stem mortality) as a function of individual tree size and fire intensity. All of these phenomena are related to the individual plant’s physiological state and the environmental conditions surrounding it.
Forward simulations to the year 2100 with this model suggest, in the words of the two researchers, that “grasslands will spread into the Sahara and into the horn of Africa, such that the total area covered by deserts or bare soil decreases by 5.7%.” In addition, they write, “it is predicted that 34.6% of today’s grasslands are transformed into savannas” and “45.3% of today’s savannas are transformed into deciduous woodlands.” Hence, “the total biomass stored in each of the biomes increases, with high relative changes in grasslands and savannas (by 256% and 241%, respectively)” and a 102 percent increase in tree biomass.
In conclusiuon, the CO2- and warming-induced greening of the Earth, which has been manifest throughout the world over the past few decades, seems destined to continue through the twenty-first century in Africa with positive results for plant and animal life.
References
Ciais, P., Piao, S.-L., Cadule, P., Friedlingstein, P., and Chedin, A. 2009. Variability and recent trends in the African terrestrial carbon balance. Biogeosciences 6: 1935–1948.
Doherty, R.M., Sitch, S., Smith, B., Lewis, S.L., and Thornton, P.K. 2010. Implications of future climate and atmospheric CO2 content for regional biogeochemistry, biogeography and ecosystem services across East Africa. Global Change Biology 16: 617–640.
Eklundh, L. and Olsson, L. 2003. Vegetation index trends for the African Sahel 1982–1999. Geophysical Research Letters 30: 10.1029/2002GL016772.
Olsson, L., Eklundh, L., and Ardo, J. 2005. A recent greening of the Sahel, trends, patterns and potential causes. Journal of Arid Environments 63: 556–566.
Scheiter, S. and Higgins, S.I. 2009. Impacts of climate change on the vegetation of Africa: an adaptive dynamic vegetation modeling approach. Global Change Biology 15: 2224–2246.
