Tải bản đầy đủ - 0 (trang)
IV. Case Study The Sahel

IV. Case Study The Sahel

Tải bản đầy đủ - 0trang



covers well over 2 million km2 and constitutes significant portions of Senegal,

Gambia, Mauritania, Mali, Burkina Faso, Niger, Chad, and the Sudan. By some

definitions, the Sahel covers a wider latitudinal belt that extends roughly between

10 and 20◦ N into parts of the Ivory Coast, Ghana, Benin, Togo, Nigeria, Cameroon,

and Ethiopia. For our climate change analysis, we utilize the broader designation.

The mean annual temperature of the more broadly defined Sahel region ranges

from 15 to 30◦ C, while rainfall varies from about 100 mm in the north to about

1000 mm in the south (Fig. 4, see color insert). The climatic regime depends on the

excursions of the Intertropical Convergence Zone (ITCZ) and the African jetstream

and is highly variable. The rainstorms are erratic and occasionally violent, and their

variability increases from south to north. The rainy season, lasting 3 to 5 months,

alternates with an extended, unrelieved, dry season. The periodic occurrence of

drought is an inherent feature of this harsh climate and successive years of drought

may be followed by years with torrential rains.

The soils of the Sahel are generally of low fertility, particularly poor in phosphates and nitrogen, structurally unstable, with low humus content and low water

retention. Hardened layers, laterization, and vulnerability to wind and water

erosion are common features. Water for irrigation is available in some places

from streams and rivers (Senegal, Niger, and Chari-Logone), and possibly from

groundwater aquifers, but the area under irrigation is rather small and the irrigation

potential has not been fully developed. The vegetation is a mixed stand of trees,

shrubs, and perennial and annual grasses, typical of savannas and steppes.

In the African Sahel, and similarly in other regions, the establishment and consolidation of European colonial rule in the 19th century brought about fundamental

changes that subsequently were to modify the relation of indigenous societies to

their environment. After an initial period of warfare, the area was stabilized and

security conditions improved. So did medical and veterinary facilities including

vaccination services. These interventions allowed human and animal populations

to increase rapidly during favorable periods. At the same time, traditional patterns

of land utilization and tenure, and of migration and transhumance, were disrupted

by arbitrary boundaries and by imposed political and economic structures.

Although the available historical records are rather meager, they suggest that

similar major droughts, lasting 12–15 years, evidently occurred in the 1680s, the

mid-1700s, the 1820s and 1830s, and the 1910s. In the first half of the 19th century,

the level of Lake Chad apparently declined for 2 or 3 decades, to about where it

was during the drought of the mid-1980s. The geological record shows several

similar falls of the lake level in the past 600 years (Rind et al., 1989). On the other

hand, we know that the Sahel has also gone through much wetter periods in the 9th

through the 13th centuries, and 16th through the 18th centuries; also from 1870 to

1895, and during the 1950s. The area near Timbuktu, which now has only 100 mm

of annual rainfall, was humid enough in the late 19th century for wheat to be




The fortuitous occurrence of favorable weather conditions during most of the

20th century, and particularly during the abnormally wet period of 1950–1965

following the attainment of independence by the region’s states, obscured the

effects of the changes imposed earlier. Given good rains, freshly cleared lands

produced good harvests even in areas that normally would have been considered

ill-suited for cultivation. Instead of deliberately keeping areas underpopulated and

providing for eventual drought, the authorities in some cases encouraged farmers

to move into marginally arable lands. Pastoral tribes were then pushed further

into even more marginal grazing lands, where they were provided with water by

means of mechanically powered tubewells. Inevitably, drought struck. As access

to the wells was free to all, traditional control over management of pastures was

eliminated. The overall result has been an increase in herd numbers, a decrease

in pasture through more widespread cropping, and an abandonment of traditional

range management mechanisms (Hillel, 1992).

The Sahel region seems to have undergone a general decline of rainfall since the

late 1960s (see Fig. 2). There have been several unusually prolonged and severe

droughts since then, in marked contrast with the preceding 20 relatively wet years

(Rind et al., 1989). At each drought, people may remain on the land in the hope that

the rains might soon return, and while waiting, they do what they can to save their

herds of goats, sheep, cattle, and camels. When the grass plays out, they may try

to increase their animals’ intake of browse by lopping trees already weakened by

lack of soil moisture. They also continue to collect firewood from the sparse shrubs

and trees. When many months elapse without rain, the vegetation dies out, while

the soil—desiccated, pulverized, and trampled—begins to blow away in the wind.

And when a sudden rainstorm visits the area, it scours and gullies the erodible

topsoil. Finally, the people are left with no choice but to abandon their traditional

homes and villages and migrate to the cities, where they seek employment or relief


The drought of 1968–1973 highlighted the basic problems that had been too long

ignored. Family and tribal structures and their autonomous traditional practices

of resource management and land tenure had been broken down, so the local

population was now unable to cope with the drought on its own. The plight of

the Sahel was exacerbated by the drought’s recurrence, in even more severe form,

during the early 1980s. Consequently, sections of the region were almost emptied

of inhabitants, as thousands of people migrated from their villages to refugee

camps and overcrowded cities. Semiarid ecosystem degradation has been linked

to migrations that may have displaced 3% of the population of Africa since the

1960s (Westing, 1994).

Some of the Sahel’s problems have been compounded by ill-conceived development efforts. Some planners seem to have misunderstood the logic of traditional production systems, and have underestimated the difficulty of improving

them. They also failed to foresee the potentially negative consequences of intended



improvements brought in under the imprimatur of “technology transfer.” In many

cases, they seem to have neglected the fundamental significance of rainfall variability the probability of drought, and the principle of risk avoidance. Some of

the traditional production systems were based on probable outcomes and were

therefore better able to contend with droughts (though, of course, no production

system can cope with a severe drought prolonged over several successive years).

The population of the western African regions of the Sahel and the regions

lying south of it, called the Sahelo-Sudanian and Sudanian zones, was estimated

at 31 million in 1980. Though the population density is still fairly low throughout,

varying from fewer than 2 per square kilometer in Mauritania to nearly 60 in

Gambia, it has been increasing steadily. In recent decades, population growth rates

have been close to 3% per annum. The area has reached 54 million inhabitants by

the year 2000 (75% more than in 1980, and almost three times as many as in 1961).

The urban population, incidentally, has been swelling at rates exceeding 5% per

year, in large part from the influx of people driven off the land because of drought.

Gonzalez (2001) has measured declines in forest species richness and tree density in the West African Sahel in the last half of the 20th century. Such changes

have apparently shifted vegetation zones in Northwest Senegal towards areas of

higher rainfall at an average annual rate of 500–600 m. Xerophytic Sahel species

have expanded in the north, while mesic Sudan and Guinean species have retracted to the south. Rainfall and temperature are identified as the most significant

factors explaining tree and shrub distribution. The changes have also decreased

human carrying capacity below actual population densities. The rural population

of 45 people per square kilometer exceeded the 1993 carrying capacity of firewood

from shrubs of 13 people per square kilometer. Gonzalez advocates the traditional

practice of regeneration of local species over the planting of exotic species. In

the practice of native regeneration, farmers select small trees in their field, protect

them, and prune them to promote rapid growth of the apical meristem.

The continued destruction of the rural environment is likely to result in further

urbanization. As the demand for food, other agricultural products, and firewood

continues to mount, it is likely to generate greater exploitation of the region’s

meager resources. Policy options include social and educational programs that

foster reduced population growth rates and improvements in rural productivity.

The latter can be achieved by intensifying the use and conservation of favorable

lands, developing the irrigation potential, improving management of range lands,

reforesting marginal lands, and raising the efficiency of household energy use so

as to curtail the burning of firewood. Above all, adequate provision must be made

for the possible occurrence of drought in the future.

Fortunately, the land itself exhibits a remarkable resilience. It had suffered many

droughts in the past, and when the rains subsequently returned, so eventually did

much of the vegetation. In large measure, the recent damage was temporary, and the

land can recover if it is rehabilitated, or at least left undisturbed for a sufficient time.




The techniques of remote sensing have made possible the monitoring of changes

to ecosystems on a regional scale (Fig. 5, see color insert) (e.g., Justice, 1986). Studies based on the remote sensing of the African Sahel were reported by Nicholson

et al. (1998). The authors state that there has been no progressive change of either

the Saharan boundary or of vegetation cover in the Sahel during the 16-year period

of the study, nor has there been a systematic reduction of productivity as assessed

by the water-use efficiency of the vegetation cover.

In principle, statistical criteria designed to test the probability levels of differences (between sites or between successive measurements on the same site)

should not be used to “prove” the opposite, namely that there are no differences.

In this case, absence of evidence of change by one criterion or another is not in

itself evidence of absence of any change. Measurements (partly indirect) made at

various times on large areas may have obscured subtle local changes that may have

occurred in specific sites. Generality may tend to ignore specificity. The authors

themselves report that while their data “showed little change in surface albedo

during the years analyzed, a change in albedo of up to 0.10% since the 1950s is

conceivable.” (The figure 0.10% is apparently a misprint of what may have been

a 1% or a 10% change in albedo).

NDVI is the ratio between the red and near-red infrared reflectance bands,

obtained from advanced high-resolution radiometer data from the polar-orbiting

satellite of the National Oceanic and Atmospheric Administration (Tucker et al.,

1991). In arid and semiarid regions, NDVI evidently correlates with the density of

the vegetative cover and its biomass, as well as with its “leaf area index” (Nicholson

et al., 1998) and photosynthetic activity (Prince, 1991).

Another criticism is in order regarding the use of NDVI (the Normalized Difference Vegetation Index) as a measure of net primary production. That index may

indeed indicate the activity of the vegetative cover at the time of measurement,

but it is oblivious to the amount of vegetation harvested by humans and/or their

animals prior to the time of measurement.

Taken to be a general indicator of the “greenness” of an area, NDVI has also

been conjectured to correlate with biological productivity, but that correlation may

not necessarily hold. In principle, the amount of vegetation present per unit of area

should depend on the amount produced in situ, minus the amount removed from it.

Therefore, the relation between an area’s productivity and its vegetative biomass at

any time must depend on whether the vegetation has been or is being “harvested”

(e.g., grazed by livestock, or cut and carried away by humans). An area could be

quite productive yet relatively bare, if it had been harvested just prior to the NDVI




A more serious caveat is in order: even if there is no discernible change in the

density of an area’s overall vegetative cover, there might well be a considerable

change in the composition of the vegetation (i.e., in its biodiversity, ecological

function, and feed value). For instance, an overgrazed area may exhibit a proliferation of less nutritious plants at the same time that it loses the most palatable

species of grasses and legumes that had contributed to the area’s original carrying

capacity. Evidence of this effect was demonstrated by Gonzalez (2001).

Nicholson et al. (1998) noted that the interannual fluctuations of the desert

boundary, as assessed from NDVI, were indeed considerable, with a displacement

as great as 3◦ latitude (roughly 300 km) back and forth. These fluctuations corresponded to the variations of the region’s rainfall. However, the investigators could

discern no progressive “march” of the desert over West Africa during the period

of their study (1980 to 1995). Furthermore, they reported that the ratio of NDVI to

rainfall, which they took to represent the rain-use efficiency of the vegetation, indicated little interannual variability and no discernible decline during the 13 years

of their analysis.

A criterion used by Tucker et al. (1991) to delineate the boundary between the

Sahara and the Sahel is the mean annual rainfall contour (isohyet) of 200 mm. Malo

and Nicholson (1990) found that this boundary corresponds approximately to an

annually integrated NDVI of 1. However, the density of the vegetative cover must

depend not only on rainfall but also on whether and to what extent that vegetation

is being utilized.

As seen in Fig. 2, the annual precipitation in the Sahel has fluctuated widely,

but the amounts for the last 3 decades of the 20th century are generally lower than

those of the preceding decades. And although the trend in recent years appears

to be an upward one, the annual amounts of rainfall are still low relative to the

century’s earlier decades. Clearly, an analysis based on any particular short period

may be misleading.


Climate in arid and semiarid regions is likely to be even more influenced in the

future by human activity due to the phenomenon known as global climate change.

Emissions of greenhouse gases, among them carbon dioxide (CO2), methane

(CH4), and nitrous oxide (N2O), and aerosols due to human activities are altering

the atmosphere in ways that are expected to warm the climate. The warming trend,

or enhanced greenhouse effect, is attributed to the release into the atmosphere of

radiatively active trace gases, which have the property of trapping a growing proportion of the heat emitted by the earth’s surface. The atmospheric concentration



Table I

Observed and Projected Changes in Extreme Weather and Climate Events Related

to Temperature and Precipitationa

Confidence in observed

changes (latter half of

the 20th century)


Very likely

Very likely


Likely, over many Northern

Hemisphere mid- to

high-latitude land areas

Likely, in a few areas


Changes in phenomenon

Higher maximum temperatures

and more hot days over

nearly all land areas

Higher minimum temperatures,

fewer cold days and frost days

over nearly all land areas

Reduced diurnal temperature

range over most land areas

Increase of heat index over land areas

More intense precipitation events

Increased summer continental drying

and associated risk of drought

Confidence in projected

changes (during the

21st century)

Very likely

Very likely

Very likely

Very likely, over most areas

Very likely, over many areas

Likely, over most mid-latitude

continental interiors (lack of

consistent projections in

other areas)

Source: IPCC WGI (2001).

of CO2 has increased by ∼30% since 1750, mostly due to fossil fuel burning and

partially due to land-use change, especially deforestation. The present CO2 concentration has not been exceeded during the past 420,000 years, and the rate of

increase is unprecedented during the past 20,000 years (IPCC, 2001).

One of the more insidious manifestations of global climate change may be an

increase of climate instability (Rosenzweig and Hillel, 1998). In a warmer world,

climatic phenomena are likely to intensify. Thus, episodes or seasons of anomalously wet conditions (violent rainstorms of great erosive power) may alternate

with severe droughts, in an irregular and unpredictable pattern. Table I presents

the IPCC assessment of confidence in observed changes in extremes of weather and

climate during the latter half of the 20th century and projected changes during the

21st century. Nearly all land areas are very likely to experience higher maximum

and higher minimum temperatures and more intense precipitation events.

A more unstable climatic regime will make it harder to devise and more expensive to implement optimal land use and agricultural production practices,

including drought-contingency provisions. Failure to prepare for such contingencies may exacerbate the consequences of such extreme events as floods and

Tài liệu bạn tìm kiếm đã sẵn sàng tải về

IV. Case Study The Sahel

Tải bản đầy đủ ngay(0 tr)