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VI. Future Climatic Variability and Change

VI. Future Climatic Variability and Change

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HILLEL AND ROSENZWEIG

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)

Likely



Very likely



Very likely

Likely

Likely, over many Northern

Hemisphere mid- to

high-latitude land areas

Likely, in a few areas



a



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



DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE



23



droughts, to the effect of worsening land degradation and periods of severe food

shortages.

Working Group I of the Intergovernmental Panel on Climate Change (IPCC) has

found that an increasing body of observations reveals that warming at the global

scale is already underway (IPCC, 2001). The global average surface temperature

has increased over the 20th century by 0.6◦ C +/−0.2◦ C. Most of the warming

has occurred during two periods: 1910–1945 and 1976–2000. Since 1975, the

Sahelian region has experienced warming of up to 1.5◦ C (Fig. 6, see color insert).

The IPCC further finds that the frequency and the intensity of droughts in parts of

Africa have increased in recent decades; in particular, there has been a decrease in

rainfall over large portions of the Sahel (IPCC, 2001).

Working Group II of the Intergovernmental Panel on Climate Change on Impacts, Adaptation, and Vulnerability finds that Africa is highly vulnerable to climate

change (IPCC WGII, 2001). Sectors of concern include water resources, food security, natural resources and biodiversity, human health, and desertification (Table II).

Global climate models (GCMs) are mathematical formulations of the processes

that comprise the climate system, including radiation, energy transfer by winds,

cloud formation, evaporation and precipitation of water, and transport of heat

by ocean currents (Fig. 7). GCMs are used to simulate climate by solving the

fundamental equations for conservation of mass, momentum, energy, and water.

For boundary conditions relevant to the Earth’s geographic features and with the

relevant parameters, the equations of the GCMs are solved for the atmosphere, land

surface, and oceans over the entire globe. GCMs project global climate responses

at relatively coarse-scaled resolutions (2.5 to 3.75◦ latitude by ∼3.75◦ longitude).



Table II

Sectors Vulnerable to Climate Change in Africaa

Sector

Water resources

Food security

Natural resources

and biodiversity

Human health

Desertification



a



Projected impacts

Dominant impact is predicted to be a reduction in soil moisture in

the subhumid zones and a reduction in runoff.

There is wide consensus that climate change, through increased

extremes, will worsen food security in Africa.

Climate change is projected to exacerbate risks to already

threatened plant and animal species, and fuelwood.

Vector-borne and water-borne diseases are likely to increase,

especially in areas with inadequate health infrastructure.

Changes in rainfall, increased evaporation, and intensified land use

may put additional stresses on arid, semiarid, and dry ubhumid

ecosystems.



Source: IPCC WG II (2001).



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HILLEL AND ROSENZWEIG



Figure 7



The climate system. (Source: WMO, 1985).



The models are used to simulate the climate system’s future responses to additional greenhouse gases and sulfate aerosols emitted into the atmosphere by human

activities.

Temperature and precipitation changes for the Sahel region of Africa in the

2050s projected by two global climate models (GCMs) are shown in Figs. 8 and 9

(see color inserts). The global climate models are the United Kingdom Hadley

Centre (HC) and the Canadian Centre for Climate Modeling and Analysis (CC)

(Flato et al., 1997; Johns et al., 1997).

There are two types of scenarios for each GCM: the first accounts for the effects

of greenhouse gases on the climate (GG), and the second accounts for the effect

of greenhouse gases and sulfate aerosols (GS). The GCM simulations for the

21st century are forced with a 1% per year increase of equivalent carbon dioxide

(CO2) concentration in the atmosphere. These simulations are based on “businessas-usual” greenhouse gas emission scenarios of the Intergovernmental Panel on

Climate Change and account for changes in other greenhouse gases besides CO2

(IPCC, 1996). Sulfate aerosols are emitted as by-products of industrial activities

and create a cooling effect as they reflect and scatter solar radiation. Thus, the

scenarios that incorporate both greenhouse gases and sulfate aerosols tend to be

somewhat cooler than those with greenhouse gas forcing alone. Simulated annual

temperature and precipitation were linearly interpolated across the GCM gridboxes

in the Sahel region.



DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE



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The scenarios vary in the magnitude of the projected temperature changes, but

they all project a warming trend for the Sahel region. The GCM models project

temperature changes ranging from 2 to 7◦ C in the 2050s. The Canadian Centre

for Climate Modeling and Analysis (CC) scenario consistently projects higher

temperatures for the region than the United Kingdom Hadley Centre (HC), while

the scenarios that combine greenhouse gases and sulfate aerosols (GS) are consistently cooler than those with the greenhouse gases alone (GG). Precipitation

projections of the two global climate models show different patterns for the 2050s,

indicating uncertainty regarding future hydrological conditions. Changes in precipitation range from −40% to +40% in the 2050s.

At three sites across the Sahel (Fig. 10), an analysis was done to project the

potential for future drought in the region. Mean monthly temperature and precipitation for Bamako, Mali; Kano, Nigeria; and Kosti, Sudan for the period of record

are shown in Fig. 11. (Data were available from 1945 to 1988 in Bamako, Mali;

from 1947 to 1965 in Kano, Nigeria; and from 1943 to 1979 in Kosti, Sudan).

Mean annual temperature is 28.2, 26.3, and 27.3◦ C for Bamako, Kano, and Kosti,

respectively. Mean annual precipitation is low at the Mali (1014 mm y−1) and

Nigeria (859 mm y−1) sites, and extremely low at the Sudan site (400 mm y−1),

with highest rainfall occurring in August.



Figure 10 Study sites for analysis of future droughts in the Sahel.



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HILLEL AND ROSENZWEIG



Figure 11 Monthly mean temperature and precipitation for Sahel study sites (Bamako, Mali

1945–1988; Kano, Nigeria 1947–1965; and Kosti, Sudan 1943–1979) (Source: NASA GISS).



For the coming decades, both GCMs project significant warming at all three

sites (between ∼4 and 8◦ C by the 2080s) (Fig. 12). Precipitation projections, on

the other hand, are mixed, with the Hadley Centre GCM simulating declines up

to 30% in Bamako, Mali, in the 2080s, and increases of more than 20% in Kano,

Nigeria, in the 2050s (Fig. 13).

We explored the potential for drought in the Sahelian region further by calculating potential evaporation (PET) with the Penman–Monteith (Monteith, 1980)

equation and the Thornthwaite (1948) equation and then using these formulas to

calculate the Palmer Drought Stress Index (Palmer, 1965). The PDSI compares

anomalous dry and wet years to normal years and is used to identify relative

droughts and floods at particular places (Table III). It uses a water balance approach

to calculate infiltration, runoff, and potential and actual evaporation. Inputs are

monthly mean temperature and precipitation, soil water capacities, and Thornthwaite (1948) parameters, which are a function of the mean temperature and

latitude.



DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE



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Figure 12 Projected annual change in temperature for the Sahel study sites for the Hadley Centre

(HC) and Canadian Centre (CC) climate change scenarios with greenhouse gases alone (GG) and with

greenhouse gases and sulfate aerosols (GS).



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HILLEL AND ROSENZWEIG



Figure 13 Projected annual change in precipitation for the Sahel study sites for the Hadley Centre

(HC) and Canadian Centre (CC) climate change scenarios with greenhouse gases alone (GG) and with

greenhouse gases and sulfate aerosols (GS).



DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE



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Utilizing the Penman–Monteith equation, the base PET is higher than the Thornthwaite (5.89 compared to 4.85 mm day−1, respectively) and the projected changes

in PET are smaller (∼10–15% increases calculated with Penman–Monteith compared to ∼20–25% increases calculated with Thornthwaite) (Fig. 14).

When these PET formulations are used to calculate projected changes in

the PDSI, the Thornthwaite PDSI projected greater changes than the Penman–

Monteith PDSI (∼−6 compared to ∼−4) for the 2080s (Fig. 15). According to

the definition of PDSI classes, indices = /<−4 are classified as extreme drought

conditions (Table III).

Finally, we utilized the Hadley Centre GCM results to see how other climate variables besides temperature and precipitation that affect potential evaporation may



Figure 14 Projected annual change in potential evapotranspiration (PET) calculated with the

Thornthwaite and Penman–Monteith formulas for Bamako, Mali for the Hadley Centre (HC) and

Canadian Centre (CC) climate change scenarios with greenhouse gases alone (GG) and with greenhouse

gases and sulfate aerosols (GS).



Table III

PDSI Classes for Wet and Dry Periodsa

Class



Description



>4.00

3.00–3.99

2.00–2.99

1.00–1.99

0.50–0.99

0.49– −0.49

−0.50– −0.99

−1.00– −1.99

−2.00– −2.99

−3.00– −3.99

<−4.00



Extremely wet

Very wet

Moderately wet

Slightly wet

Incipient wet spell

Near normal

Incipient drought

Mild drought

Moderate drought

Severe drought

Extreme drought



a



Palmer (1965).



Figure 15 Projected change in Palmer Drought Stress Index (PDSI) calculated with the Thornthwaite and Penman–Monteith potential evapotranspiration (PET) formulas for Bamako, Mali for the

Hadley Centre (HC) and Canadian Centre (CC) climate change scenarios with greenhouse gases alone

(GG) and with greenhouse gases and sulfate aerosols (GS).



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Table IV

Annual Changes in Climate Variables (a) and Penman–Monteith Potential Evapotranspiration

(PET) (b) for the Sahel Region Utilizing Projections of the Hadley Centre GCM with

Greenhouse Gases Alone (HCGG)

2020s

(a) Climate variable (%)

Solar radiation

Windspeed

Relative humidity



.6

1.5

−4.2



(b) Potential evapotranspiration

(mm day−1) (%)

PET (temperature)a

PET (temperature, solar radiation,

windspeed, relative humidity)

a



2050s



1.9

3.1

−12.7



6.16 (4.6)

6.27 (6.5)



6.43 (9.2)

6.79 (15.3)



2080s



3.1

6.2

−19.6



6.74 (14.4)

7.35 (24.8)



Base PET (temperature) = 5.89 mm day−1.



change, namely, solar radiation, windspeed, and relative humidity. For each of

these variables, as projected by the Hadley Centre GCM, direction of change is toward greater potential evaporation. Solar radiation and wind speed increase, while

relative humidity declines (up to ∼20% in the 2080s). When the Penman–Monteith

PET is calculated with GCM projections for the four variables (temperature, solar

radiation, wind speed, and relative humidity), the percentage of change in PET

increased in all decades compared to PET changes calculated with temperature

projections alone (Table IV).



VII. PROSPECTS

Climate change appears likely to cause further semiarid ecosystem degradation through alteration of spatial and temporal patterns in temperature, rainfall,

solar insolation, winds, and humidity. Our analyses point toward a prolongation and worsening of drought conditions in the Sahel under climate change

conditions. The global climate models, potential evapotranspiration formulations, and the suite of variables tested all project the potential for exacerbated

drought in the region. In turn, desertification may aggravate CO2-induced climate change through the release of CO2 from cleared, burned, and dead vegetation, and the reduction of the carbon sequestration potential of degraded semiarid

lands.



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HILLEL AND ROSENZWEIG



Projecting climate change due to anthropogenic greenhouse gas forcing at regional scales is an exploratory, albeit important, exercise. There is still considerable uncertainty regarding how fast and by how much the climate of the Sahel may

change in the future, on how different subregions within the Sahel may experience

the change, or on how the variability (as well as the mean values) of climatic parameters may change. Gradual changes may be punctuated by increasing frequencies

of intense precipitation events.

It is still difficult, if not impossible, to ascribe probabilities to any of the various

climate change scenarios, owing to uncertainties regarding future emissions of

radiatively active trace gases and tropospheric aerosols and the potential response

of the climate system to those emissions. Scenarios are also uncertain because

global climate models lack realism in their simulation of current climate processes,

especially regional hydrology.

Regardless of future climate change, the pressures generated by growing populations and intensified land use are evidently causing a progressive degradation of

arid and semiarid ecosystems in the Sahel and elsewhere. To define and quantify the

nature, degree, and extent of the degradation, national and international programs

are working to build consistent monitoring systems of monitoring. These consist not only of remote sensing from above but also of quantitative ground-based

observations in both “natural” (relatively undisturbed) and managed ecosystems.

Both national and international efforts should be strengthened, especially in such

vulnerable regions as the Sahel. In these programs, the interacting influences of

human interventions and of potential climate-change effects should be defined,

since the possibility exists that the two factors may interact, especially given the

current trends of warming and drying in the region.

To redress or rehabilitate degraded ecosystems, vulnerable countries are beginning to institute appropriate policies and programs. These include keeping reserve

areas to protect biodiversity, avoiding over-grazing on managed lands, reseeding of

pastures, implementing soil and water conservation measures, and—in the social

arena—land tenure, family planning, and contingencies for droughts. Determining

the availability of fresh water resources (surface water, renewable groundwater,

and nonrenewable groundwater) and planning their careful utilization are important components of such programs. Inappropriate patterns of management may

lead to a downward spiral of semi-arid ecosystem degradation, as illustrated in

Figs. 16 and 17, whereas appropriate measures of soil and water conservation

hold the promise of sustainable development in the contexts of both rainfed and

irrigated systems.

Given the range and magnitude of development constraints and challenges facing

many countries in Africa and other nations with large areas of semiarid ecosystems, vulnerability to the intertwined effects of continued semiarid ecosystem

degradation and to climate change appears to be high and the overall capacity to

adapt low. However, the prospect of a changing global climate also offers some



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