Other than pigs, domestic livestock in Africa are concentrated in the arid
and semi-arid zones. This is because the more humid areas were historically
prone to livestock diseases such as nagana (a trypanosome carried by
the tsetse flyFord and Katondo, 1977), typically support grasses of low
digestibility (Scholes, 1990, 1993), and often are densely settled by crop agriculturalists.
The overwhelming majority of these animals feed predominantly off natural grasslands
and savannas, although crop residues are an important supplement during the
dry season. Many urban and rural families also keep poultry.
Domestic livestock play a central role in many African cultures. Cattle and
camels, in particular, have an importance that goes beyond the production of
meat. Their value is based on the full set of services they supply (milk, meat,
blood, hides, draft power), their asset value as a form of savings, and their
cultural symbolism. It would be difficult and damaging for these cultures to
abandon pastoralism in the event that it becomes climatically, environmentally,
or economically unviable.
Although classical concepts of animal carrying capacity may not be very useful
as local management tools in the context of African semi-arid systems with high
interannual variability (Behnke et al., 1993), they remain valid as indicators
of animal production when they are applied over decadal periods and large areas.
Many researchers have demonstrated a strong link between long-term, large-area
herbivore biomass (LHB, kg km-2) in African wildlife and pastoral
systems and mean annual precipitatiation at one site (see also Le Houerou, 1998,
for a similar analysis in west Africa and the Mediterranean basin). Thus, in
broad terms, changes in range-fed livestock numbers in any African region will
be directly proportional to changes in annual precipitation. Given that several
GCMs predict a decrease in MAP on the order of 10-20% in the main semi-arid
zones of Africa, there is a real possibility that climate change will have a
negative impact on pastoral livelihoods. The following additional factors must
The causal chain between rainfall and animal numbers passes through grass production,
which also is approximately linearly related to rainfall (Breman, 1975; Le Houerou
and Hoste, 1977; Rutherford, 1995). The slope of this relationship, which can
be expressed as WUE, is a function of soil nutrient availability (De Ridder
et al., 1982; Scholes, 1993). WUE also is a function of CO2
concentration in the atmosphere (Mooney et al., 1999), especially in
semi-arid regions. Because the CO2 concentration will rise in the
future, its positive impact on WUE (which is on the order of 20-30% for
doubled CO2, even in C4-dominated grasslands such as these)
will help to offset reduction in rainfall of the same magnitude. Simulations
of grassland production in southern Africa indicate an almost exact balancing
of these two effects for that region (Scholes et al., 2000).
About 80% of the grazing lands of Africa are in savannas, a vegetation formation
that consists of a mix of trees and grasses. Grass production in savannas is
strongly depressed by tree cover (see the many references reviewed by Scholes
and Archer, 1997). Because domestic livestock, with the exception of goats,
predominantly eat the grass in these systems, future changes in tree cover are
an important issue from the point of view of carbon sequestration and livestock
production. Tree biomass ultimately is related to climate and soil type, but
the mechanism appears to be via fire frequency and intensity. If fire frequency
and intensity were to decreaseas a result of climate changes (for instance,
an increase in dry-season rainfall) or, more likely, changes in land managementwoody
cover would increase. This has been demonstrated in numerous fire experiments
in Africa (Trapnell et al., 1976; Booysen and Tainton, 1984). Given the
vast area of the savannas, the carbon sequestration potential is substantial
(Scholes and van der Merwe, 1996). In addition, emissions of tropospheric ozone
(O3) precursors would decrease if savanna burning were reduced. The
disadvantage would be a more than proportional decrease in livestock carrying
capacity, as a result of the nonlinear suppressive effect of trees on grass
The bioclimatic limit of savannas in southern Africa is related to winter temperatures.
An increase in temperature of 1-2°Cwell within the range predicted
for next centurywould make the montane grasslands (highveld) of southern
Africa susceptible to invasion by savanna trees (Ellery et al., 1990).
In moister regions, animal productivity is limited not by the gross availability
of fodder but by its protein (nitrogen) content (Ellery et al., 1996).
Increasing the CO2 concentration or the rainfall will not increase
the protein availability; thus, livestock in these regions are likely to be
less responsive to the direct effects of atmospheric and climate change. Under
elevated CO2, the carbon-to-nitrogen ratio of forage will decrease,
but this will not necessarily lead to decreased forage palatability despite
the dilution of protein (Mooney et al., 1999). This may be because in
grasses, the bulk of the excess carbon is stored in the form of starch, which
is readily digestible. Widespread use of protein and micronutrient feed supplements
and new technology for the control of veterinary diseases will have a greater
impact on livestock numbers and productivity, especially in the "miombo"
region of south central Africa.
Domestic livestock, like other animals, have a climate envelope in which they
perform optimally. The limits of the envelope are quite broad and can be extended
by selecting for heat or cold tolerance, feed supplementation, or providing
physical shelter for the animals. African cattle are mostly from the Bos indicus
line, which is more heat-tolerant than the European line of Bos taurus. In extremely
hot areas (mean daily warm-season temperatures greater than body temperature),
even the Bos indicus breeds are beyond their thermal optimum (Robertshaw and
Finch, 1976). Meat and milk production declines, largely because the animals
remain in the shade instead of foraging. There is limited potential for extending
this limit through breeding. Adaptation would require substitution by a species
such as the oryx, which is physiologically equipped for high temperatures and
low water supply.
In the higher altitude and higher latitude regions of Africa, livestock (typically
sheep) currently are exposed to winter temperatures below their optimum. Mortality
often results when cold periods coincide with wet periods, if the animals have
not been herded to shelter. These episodes are likely to decrease in frequency
and extent in the future.
Livestock distribution and productivity could be indirectly influenced via
changes in the distribution of vector-borne livestock diseases, such as nagana
(trypanosomiasis) and the tick-borne East Coast Fever and Corridor Disease (Hulme,
1996). Simulations of changes in the distribution of tsetse fly (Glossina spp.)
indicate that with warming it could extend its range southward in Zimbabwe and
Mozambique, westward in Angola, and northeast in Tanzania, although in all these
simulations there were substantial reductions in the prevalence of tsetse in
some current areas of distribution. The tick Riphicephalus appendiculatus was
predicted to decrease its range in southern and eastern Africa and increase
its range in the central and western part of southern Africa (Hulme, 1996).
One land-use model (IMAGE 2.0Alcamo, 1994) projects that large parts
of Africa will be transformed to pastoral systems during the 21st century. The
model logic that leads to this conclusion is that increasing urbanization and
a rising standard of living typically are associated with a change in dietary
preference toward meat. The area that currently is used for meat production
therefore would need to expand, assuming that meat demand was not met by import
or by increased productivity of existing herds. These are reasonable but untested
assumptions, and their consequences have major implications for biodiversity
conservation and atmospheric composition. The areas indicated as being converted
to pastures (largely the subhumid tropics) already support cattle to some degree.
Increased cattle production would require widespread tree clearing (leading
to conversion of a carbon sink into a carbon source), eradication of key cattle
diseases, and the use of protein and micronutrient feed supplements. The quantity
of fuel consumed by savanna fires would decrease (because it would be grazed),
reducing the release of pyrogenic methane (CH4) and O3
precursors, but the production of methane from enteric fermentation would increase.
Because methane production per unit of grass consumed is higher for enteric
fermentation than for savanna fires (Scholes et al., 1996), the result
is likely to be a net increase in radiative forcing.
10.2.2.5. Impacts of Drought and Floods
Food security in Africa already is affected by extreme events, particularly
droughts and floods (e.g., Kadomura, 1994; Scoones et al., 1996). The
ENSO floods in 1998 in east Africa resulted in human suffering and deaths, as
well as extensive damage to infrastructure and crops in Kenya (Magadza, 2000).
Floods in Mozambique in 2000 and in Kenya in 1997-1998 sparked major emergency
relief as hundreds of people lost their lives and thousands were displaced from
their homes (Brickett et al., 1999; Ngecu and Mathu, 1999; see also <www.reliefweb.int>).
The cost in Kenya alone was estimated at US$1 billion (Ngecu and Mathu, 1999).
Droughts in 1991-1992 and 1997-1998 affected livelihoods and economies
and heightened renewed interest in the impacts of climatic hazards (e.g., Kadomura,
1994; Campbell, 1999). For example, the impacts of the 1991-1992 drought
in Zimbabwe are estimated to have been 9% of GDP (Benson and Clay, 1998).
Such climatic episodes can serve as an analog of climate change. Irrespective
of whether climate change will cause more frequent or more intense extreme events,
it is apparent that many aspects of African economies are still sensitive to
climatic hazards. At the local level, some coping strategies are less reliable
(Jallow, 1995)for instance, Campbell (1999) notes that plants and trees
used as food by pastoralists in southern Kenya declined between 1986 and 1996.
National governments often struggle to provide food security during times of
crisis (Ayalew, 1997; Gundry et al., 1999). For national and international
agencies, the cost of climatic hazardsimpacts, recovery, and rehabilitationmay
result in a shift in expenditure from reducing vulnerability to simply coping
with immediate threats (e.g., Dilley and Heyman, 1995).