Climate Change in Africa
Although global warming is likely to have impacts at all levels of the global society, these may be most detrimental in African nations, which traditionally have been vulnerable to existing climatic conditions. This is all the more concerning when one considers that Africa's contribution to past and current greenhouse gas emissions is insignificant in global terms.
Recent Climate Change & Variability in Africa
Annual mean surface temperature in Africa has increased 0.53°C since 1895, with the greatest warming occurring during the Northern Hemisphere Spring (March to May) and summer (June to August). Two distinct warming trends occurred during the 20th century, the first in the 1920s and 1930s, the second since about 1980. Whilst computer models have suggested that the cause of this warming may be due to the rise in greenhouse gas concentrations in the atmosphere, other possible mechanisms may include natural variations in solar irradiance or surface ocean temperatures
Large natural fluctuations in rainfall from one year to the next place stresses on African societies based primarily on agriculture. The 30 year desiccation of the Sahel in Northern Africa, however, provides one of the most striking examples of long term climate change in recent times, and one that may represent the first signs of man-made global warming. Again, this is not to say that increases in greenhouse gas concentrations have single-handedly brought about the long term drought. Models have revealed that human-induced changes to the land-surface characteristics, can, through feedback processes, generate self-sustaining drought. Nevertheless, it is likely that these effects remain secondary to the variability of the wider global climate system.
Rainfall Decline in the Sahel
Simulations performed by the UK Meteorological Office have demonstrated a link between Atlantic sea surface temperatures and Sahelian rainfall. Lower North Atlantic ocean temperatures are associated with reduced precipitation. The physical basis of this relationship may lie in a disturbance to the atmospheric circulation induced by the underlying sea surface temperature pattern. The question can then be raised: has the enhanced greenhouse effect caused a change in the Atlantic surface ocean temperature? Some evidence suggests that the cause may be found within the natural climate system, for example a change to the energy transport of the ocean due to a reduction of northward ocean circulation. Other evidence points to a link with rising greenhouse gas concentrations.
Southern Hemisphere Africa
During the early 1990s, severe drought was experienced in much of the Southern Hemisphere Africa. Over parts of southern Zimbabwe and south-eastern Botswana, rainfall levels were only 10% of the long term average during the rainy season of 1991/92. It is widely excepted that the cause of this drought was largely the result of El Niño phenomenon, the periodic warming of the south-eastern Pacific surface ocean and related shifts in atmospheric circulation.
Future Climate Change in Africa
Using computer models to simulate climatic responses in Africa to changing greenhouse gas concentrations has been shown to be subject to a large degree of uncertainty. That is to say, the range of scenarios generated by models is large. That said, most models predict marked changes in land surface temperature (0.5 to 2°C increase by 2050), whilst increases in the temperature of the surface ocean are somewhat lower. Warming is most pronounced over existing desert regions of the Sahara and Kalahari. Modelling the changes in African rainfall by 2050 is subject to an even higher degree of uncertainty than for temperature changes. In the most optimistic case, rainfall is seen to increase by between 20 and 70% by 2050, whilst in the worst case, extensive drought and desiccation would be experienced in all areas.
Impacts of Future Climate Change in Africa
How Africa responds to future climate change will depend upon the sensitivities of her ecosystems, natural resources and national economies. When addressing the impacts of climate change, it is necessary to place this in the global context of agricultural systems, economic development and population growth. Much of Africa is dependent upon agriculture for both the sustenance of individual livelihoods and national economies. For this reason, any increase in temperature coupled with a fall in rainfall could have potentially drastic consequences on the growing of crops. Higher temperatures will increase the rate of evapo-transpiration, thereby reducing the soil moisture availability. Increases in rainfall may or may not be enough to compensate for increases in surface temperature. Changes in rainfall will also affect reservoir storage, water table level and groundwater recharge. Warmer temperatures would facilitate the increased breeding season of agricultural pests, and in some areas, the over-wintering of pests may be allowed for the first time. It is conceivable, however, that reduced rainfall may decrease pest damage in some areas. A reduction in rainfall and/or an increase in temperature will be detrimental to the quality of drinking water, and to irrigation systems used for agriculture.
The migration of natural species of flora provide a useful yardstick to measure to rate and severity of climatic change. Most climate models used to simulate vegetation movements have revealed an expansion and desertification of existing semi-arid regions such as the Sahel, presumably as a consequence of reduced rainfall and higher temperatures. The existing climate in much of Africa supports the spread of vector-borne disease such as malaria, and hygiene-related illnesses such as cholera. It is fair to say that adequate water supplies would remove much of this threat, nevertheless, climate remains a substantial influence on human health in Africa. Climate impacts models have demonstrated that increases in temperature could enlarge the number of areas equitable for malaria transmission via the mosquito. The most sensitive areas would include those presently experiencing low endemicity where changes in climate would have the most dramatic influence on prevalence.
Whilst natural climatic variability, particularly for rainfall, is large in Africa, an Africa-wide warming is noticeable during the 20th century. At the regional scale, significant climatic changes have been witnessed, particularly the recent desiccation of the Sahel. These changes may have natural causes or they may be a manifestation of the man-made enhancement of the greenhouse effect. Projecting future climate change in Africa is subject to a high degree of uncertainty. Large difference exist between model scenarios, particularly for estimations of future rainfall.
Despite the relative scarcity of impact models for Africa, it seems likely that global warming will have many detrimental effects within the continent, particularly regarding agricultural systems. Traditional systems in Africa have developed strategies to cope with the extremities of existing climatic variability. In many cases, however, these systems are, and will continue, to strain under the pressures of economic development and population growth, as well as from the threats of global warming.