REPORTS - SPECIAL REPORTS

The Regional Impacts of Climate Change


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2.3.1.6. Deserts

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 30C during the warmest months and extremes above 50C. The diurnal temperature range often is large; winter nights in the Namib Desert sometimes are as cold as 10C (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).

2.3.1.7. Mountain Regions

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 5.2.2.3).

2.3.1.8. Adaptation and Vulnerability

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).

Parameter Zambezi Nile Blue Nile

Length (km) 2,600 6,500 1,000
Area (km2 x 103) 1,330 2,880 313
Flow (m3/sec) 4,990 2,832 1,666
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
R/P 0.12 0.04 0.21
PET/P 2.50 5.50 1.80

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.


Box 2-4. A Conceptual Model of Arid Rangeland Degradation

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.


Steps and management options for arid rangeland degradation.

Stepwise degradation of arid or semi-arid rangelands. Symptoms describe the state of plant and animal assemblages; management options refer to actions that a manager could take to improve the condition of the range; and management level refers to the system (level of the food chain) on which management should be focused.

Step 0  
Description: Biomass and composition of vegetation varies with climatic cycles and stochastic events (e.g., droughts, diseases, hail, frost, fire)
Symptoms: Perennial vegetation varies with weather
Management Option: Adaptive management, involving timely manipulations of livestock densities
Management Level: Secondary producers (i.e., grazers and herbivores)
   
Step 1  
Description: Herbivory reduces reestablishment of palatable plants, allowing populations of unpalatable species to grow
Symptoms: Demography of plant population changes (age-structural changes)
Management Option: Strict grazing controls
Management Level: Secondary producers
   
Step 2  
Description: Plant species that fail to establish are lost, as are their specialized predators and symbionts
Symptoms: Plant and animal losses, reduced capacity to support herbivores
Management Option: Manage vegetation (e.g., add seed, remove plants)
Management Level: Primary producers (i.e., vegetation)
   
Step 3  
Description: Biomass and productivity of vegetation fluctuates as ephemerals and weed species benefit from loss of cover from perennial plants
Symptoms: Perennial biomass reduced (short-lived plants and instability increase), resident birds decrease, nomadic bird species
Management Option: Manage soil cover (e.g., mulching, erosion barriers, roughen soil surface)
Management Level: Physical environment (soil)
   
Step 4  
Description: Denudation and desertification involve changes in soil function and soil microbe activity
Symptoms: Vegetation cover completely lost, erosion accelerated; soil salinization, aridification
Management Option: Difficult to address; costs of restoration or rehabilitation too high; nonpastoral use of land only economic option
Management Level: Difficult to identify

 


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