Deserts are an environmental extreme characterized by low rainfall that is highly variable intra-annually and interannually. Desert air is very dry; incoming solar and outgoing terrestrial radiation are intense, with large daily temperature fluctuations; and potential evaporation is high. Many organisms in the deserts already are near their tolerance limits (IPCC, 1996). The Sahara in north Africa and the Namib desert in southWest Africa are classified as the hottest deserts in the world-with average monthly temperatures above 30°C during the warmest months and extremes above 50°C. The diurnal temperature range often is large; winter nights in the Namib Desert sometimes are as cold as 10°C (IPCC 1996, WG II, Section 3.3.1) or lower. Extreme desert systems already experience wide fluctuations in rainfall and are adapted to coping with sequences of extreme conditions. Initial changes associated with climate change are less likely to create conditions significantly outside present ranges of tolerance; desert biota show very specialized adaptations to aridity and heat, such as obtaining their moisture from fog or dew (IPCC 1996, WG II, Section 3.4.2).
Mountains usually are characterized by sensitive ecosystems and regions of conflicting interests between economic development and environmental conservation. In Africa, most mid-elevation ranges, plateaus, and high-mountain slopes are under considerable pressure from commercial and subsistence farming activities (Rogers, 1993). Mountain environments are potentially vulnerable to the impacts of global warming. This vulnerability has important ramifications for a wide variety of human uses-such as nature conservation, mountain streams, water management, agriculture, and tourism (IPCC 1996, WG II, Section 5.2).
There is a general picture of continuing ice retreat on the mountains. On Mount Kenya, the Lewis and Gregory glaciers have shown recession since the late 19th century (IPCC 1996, WG II, Box 5-3). Changes in climate (as projected in Greco et al., 1994) could reduce the area and volume of seasonal snow, glacier, and periglacial belts-with a corresponding shift in landscape processes. The retreat of some glaciers on Kilimanjaro and Mt. Kenya would have significant impacts on downstream ecosystems, people, and their livelihoods because of moderation of the seasonal flow regimes of rivers upstream. Further reduction of snow cover and glaciers also could reduce the scenic appeal of African high mountain landscapes for tourists and thus have a negative impact on tourism.
Forest fires would increase in places where summers become warmer and drier. Prolonged periods of summer drought would transform areas already sensitive to fire into regions of sustained fire hazard. Mt. Kenya and mountains on the fringes of the Mediterranean Sea already subject to frequent fire episodes could be affected (IPCC 1996, WG II, Section 126.96.36.199).
There is potential for spontaneous and assisted adaptation in Africa. Many options will need to involve a combination of efforts to reduce land degradation and foster sustainable management of resources. This section highlights options for forestry and woodlands, rangelands, and wildlife.
A number of adaptive processes designed to prevent further deterioration of
forest cover already are being implemented to some degree. Some of these measures
involve natural responses when particular tree species develop the ability to
make more efficient use of reduced water and nutrients under elevated CO2 levels.
Other adaptive measures involve human-assisted action programs (such as tree
planting) designed to minimize undesirable impacts. These strategies will include
careful monitoring and microassessment of discreet impacts of climate change
on particular species. Low-latitude forest adaptation options, especially in
west Africa, must include active vegetation and soil management. For example,
Gilbert et al. (1995) have indicated that silvicultural practices, endangered
species habitat management, watershed manipulation, and antidesertification
techniques could be applied given current infrastructure in Cameroon and Ghana.
These adaptive measures will help reduce climate change impacts on forest watersheds
and semi-arid woodlands. Smith and Lenhart (1996) have identified enhancement
of forest seed banks as an adaptation policy option for maintaining access to
a sufficient variety of seeds to allow the original genetic diversity of forests
to be rebred. Genetic diversity also provides an assurance that benefits provided
by forests are not lost forever (Smith and Lenhart, 1996) and is particularly
relevant to the maintenance of the forests in the Sahel and other extremely
sensitive regions of Africa where 20 years of recurrent drought have degraded
the forests. Mwakifwamba (1997) asserts that adaptation strategies or measures
in Tanzania should focus mainly on reducing high deforestation rates, protecting
existing forests, and introducing new species or improving existing species.
|Table 2-4: Hydrological characteristics for the Zambezi and Nile River basins (extracted from Riebsame et al., 1995).|
|Area (km2 x 103)||1,330||2,880||313|
|Flow (109 m3/yr||157||89||53|
|Specific Discharge (I/sec-km2)||3.8||1.0||5.3|
|Runoff (R) (mm)||118||31||168|
|Precipitation (P) (mm)||990||730||784|
|Note: PET = Potential evapotranspiration.|
For rangelands, Milton et al. (1994) present a conceptual model of arid rangeland degradation that suggests that degradation proceeds in steps-increasingly difficult and costly to reverse-and discusses adaptation options (see Box 2-4). Assisted management is a lot harder for wildlife in game reserves than for livestock. Monitoring is required to identify populations at risk (from deforestation), as well as reserved areas that are changing their vegetation types in response to climate, leaving some animals in habitat types that are not suitable. Massive fragmentation of previous forests and woodlands makes it difficult for wildlife to migrate along corridors to areas with more water and foliage. Close monitoring would identify groups of wildlife that are in danger, and steps can be taken to move them to suitable habitat.
At the institutional level, mechanisms need to be created (or improved upon)
to facilitate the flow of scientific results into the decision-making and policy-making
process. Joint planning of projects that would impact cross-boundary catchment
areas will become increasingly important if the climate becomes more variable
and water more scarce for many regions of Africa.
Overuse by a narrow suite of domesticated herbivores has led to progressive loss of secondary productivity and diversity in rangelands. Degraded rangelands may not return to their original state, even when they are rested for decades (Westoby et al., 1989; O'Connor, 1991). Milton et al. (1994) develop the idea that the probability of reversing grazing-induced change may be inversely related to the amount of disturbance involved in the transition. They develop a stepwise model of rangeland degradation and show how the potential for recovery appears to be related to the function of the affected component. Their study stresses the need to recognize and treat degradation early because management inputs and costs increase for every step in the degradation process. Steps and management options are described below.
Similar models can be constructed for climate effects, to conceptualize potential impacts and points of intervention.
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