Future Climate Change Scenarios in the Arid and Semi Arid Tropics

The multi-century control integration of atmosphere-ocean global climate models (AOGCMs) unforced by anthropogenic changes in atmosphere composition offer an excellent opportunity to examine the skill of individual models in simulating the present-day climate and its variability on regional scales. Climate change scenarios based on an ensemble of results as inferred from skilled AOGCMs for Asia on annual and seasonal mean basis are discussed in IPCC (2001a). As a result of increases in the atmospheric concentration of GHGs the projected area-averaged annual mean warming is likely to be 1.6 ± 0.2 °C in the 2020s, 3.1 ± 0.3 °C in the 2050s, and 4.6 ± 0.4 °C in the 2080s over land regions of Asia. Under the combined influence of GHGs and sulfate aerosols, surface warming will be restricted to 1.4 ± 0.3 °C in the 2020s, 2.5 ± 0.4 °C in the 2050s and 3.8 ± 0.5 °C in the 2080s.

Lal et al. (1995) suggested that the surface air temperature over the Indian subcontinent (area-averaged for land regions only) is likely to rise from 1.0 ° C (during the monsoon) to 2.0 °C (during the winter) by the middle of the next century. The rise in surface temperature could be quite significant across the semiarid regions of NW India. The increase in surface air temperature simulated by regional climate model (RCM) over central and northern India is not as intense as in the general circulation model (GCM) and does not extend as far south (Hassel and Jones, 1999). These anomalies are linked with changes in surface hydrological variables. Over the south Asia region, a decrease in DTR on an annual mean basis during the winter and a more pronounced decreased in DTR during the summer are projected. The significantly higher decrease in DTR over south Asia during the summer is a result of the presence of monsoon clouds over the region

(Lal et al., 1996). For tropical Asia, Whetton (1994) suggested that warming would be least in the islands and coastal areas throughout Indonesia, the Philippines and coastal south Asia and Indo-China and greatest in inland continental areas of south Asia and Indo-China, except from June to August in south Asia, where reduced warming could occur.

AOGCMs projected an area-averaged annual mean increase in precipitation of 3 ± 1% in the 2020s, 7 ± 2% in the 2050s, and 11 ± 3% in the 2080s, over the land regions of Asia as a result of future increases in the atmospheric concentration of GHGs. Under the combined influence of GHGs and sulphate aerosols, the projected increase in precipitation is limited to 2 ± 1% in the decade 2020s, 3 ± 1% in 2050s, and 7 ± 3% in the 2080s. The models show high uncertainty in projections of future winter and summer precipitation over south Asia (with or without direct aerosol forcings). The effect of sulfate aerosols on Indian summer-monsoon precipitation is to dampen the strength of monsoon compared with that seen with GHGs only (Roeckner et al., 1999). Projected changes in temperature, diurnal temperature range and precipitation over tropical south Asia are presented in Table I.

On an annual mean basis, the area-averaged rainfall over the land regions of the Indian subcontinent is expected to marginally decline by the middle of the next century. No significant change in rainfall is projected during the winter months (January-February). During the monsoon season, a decline of about 0.5 mm day-1 in rainfall over the central plains of India is likely. The simulated decline in monsoon rainfall is due to a decrease in high intensity rainfall events throughout the season and is found to be statistically significant at 90% confidence level. Moreover, a decline in the frequency of heavy rainfall spells (>10 mm day-1) is likely. No significant changes are discernible in the inter-annual variability of monsoon rainfall in enhanced CO2 simulations with respect to those in the control simulation.

Whereas an increase in rainfall is simulated by RCM (Hassel and Jones, 1999) over the eastern region of India, northwestern deserts see a small decrease in the absolute amount of rainfall. Changes in soil moisture broadly follow those in precipitation except in eastern India, where they decrease as a result of enhanced drainage from the soil. The largest reductions (precipitations reduced to <1 mm day-1; 60% decline in soil moisture) are simulated in the arid regions of northwest India and Pakistan. Relatively small climate changes cause large water resource problems in many areas, especially in the semi-arid regions such as NW India. If water availability decreases in this region, it could have significant implications for agriculture for water storage and distribution and for generation of hydroelectric power. For example, under the assumed scenario of a 1 °C to 2 °C temperature increase, coupled with a 10% reduction in precipitation, a 40 to 70% reduction in annual runoff could occur.

Recent observations suggest that there is no appreciable long term variation in the total number of tropical cyclones observed in the north Indian, southwest

TABLE I

Projected changes in surface air temperature (top), diurnal temperature range (middle) and precipitation (bottom) over tropical South Asia as a result of future increases in greenhouse gases

Temperature change (°C)

TABLE I

Projected changes in surface air temperature (top), diurnal temperature range (middle) and precipitation (bottom) over tropical South Asia as a result of future increases in greenhouse gases

Temperature change (°C)

2020

2050

2080

Annual

1.36

2.69

3.89

(1.06)

(1.92)

(2.98)

Winter

1.62

3.25

4.25

(1.19)

(2.08)

(3.25)

Summer

1.13

2.19

3.20

(0.97)

(1.81)

(2.67)

Change in Diurnal Temperature Range (°C)

Annual

-0.27

-0.45

(-0.22)

(-0.31)

Winter

-0.27

-0.46

(0.14)

(-0.31)

Summer

-3.06

-2.89

-(4.97)

(-4.95)

Precipitation change (%)

Annual

2.9

6.8

11.0

(1.0)

(-2.4)

(-0.1)

Winter

2.7

-2.1

5.3

(-10.1)

(-14.8)

(-11.2)

Summer

2.5

6.6

7.9

(2.8)

(0.1)

(2.5)

Note. Numbers in parenthesis are changes when direct effects of sulphate aerosols are included (adapted from IPCC, 2001).

Note. Numbers in parenthesis are changes when direct effects of sulphate aerosols are included (adapted from IPCC, 2001).

Indian, and southwest Pacific Oceans east of 160 °E (Neumann, 1993; Lander and Guard, 1998). Some of these studies (Krishnamurthi et al., 1998; Royer et al., 1998) suggest an increase in tropical storm intensities with carbon dioxide (CO2) induced warming. Some of the most pronounced year-to-year variability in climate features in many parts of Asia including arid and semi-arid tropics has been linked to ENSO. Meehl and Washington (1996) indicate that future seasonal precipitation extremes associated with a given ENSO event are likely to be more intense in the tropical Indian Ocean region; anomalously wet areas could become wetter, and anomalously dry areas could become drier during future ENSO events. Several recent studies (Kitoh et al., 1997; Lal et al., 2000) have confirmed earlier results indicating an increase in interannual variability of daily precipitation in the Asian summer monsoon with increased GHGs. The intensity of extreme rainfall events is projected to be higher in a warmer atmosphere, suggesting a decrease in return period for extreme precipitation events and the possibility of more frequent flash floods in parts of India, Nepal and Bangladesh. However, Lal et al. (1995) found no significant change in the number and intensity of monsoon depressions (which are largely responsible for the observed interannual variability of rainfall in central plains of India) in the Bay of Bengal in a warmer climate.

3.2. AFRICA

In view of the uncertainties in GCMs, it is important to interpret model outputs in the context of their uncertainties and to consider them as potential scenarios of change for use in sensitivity and vulnerability studies. While there is a good degree of certainty regarding future increases in atmospheric CO2 concentrations and there is confidence in the range of projections of global-mean temperature and sea level, there are many uncertainties about regional patterns of precipitation and soil moisture. Much less is known about the frequencies and intensities of extreme events.

With respect to temperature, future annual warming across Africa is projected to range from 0.2 °C per decade to more than 0.5 °C per decade (Hulme et al., 2001). This warming is the greatest over the interior of the semi-arid margins of the Sahara and central southern Africa. Land areas may warm by 2050 by as much as 1.6 °C over the Sahara and semi-arid parts of southern Africa (Hernes et al., 1995; Ringius et al., 1996). Equatorial countries (Cameroon, Uganda, and Kenya) might be about 1.4 °C warmer. Sea-surface temperatures in the open tropical oceans surrounding Africa will rise by less than the global average (i.e., only about 0.6-0.8 °C); the coastal regions of the continent, therefore, will warm more slowly than the continental interior.

Future changes in mean seasonal rainfall in Africa are less well defined. Rainfall changes projected by most GCMs are relatively modest, at least in relation to present-day rainfall variability. Under the two intermediate warming scenarios, significant decreases (10 to 20%) in rainfall during March to November are apparent in North Africa in almost all models by 2050. In southern Africa, decreases of 5-15% in rainfall during the growing season during November to May are projected. Seasonal changes in rainfall are not expected to be large; Joubert and Tyson (1996) found no evidence for a change in rainfall seasonality among a selection of mixed-layer and fully coupled GCMs. Hewitson and Crane (1998) found evidence for slightly extended later summer season rainfall over eastern South Africa (though nowhere else), based on a single mixed-layer model prediction. Great uncertainty exists, however, in relation to regional-scale rainfall changes simulated by GCMs

(Joubert and Hewitson, 1997). Under the most rapid global warming scenario, increasing areas of Africa experience changes in rainfall that exceed one sigma level of natural variability. Parts of the Sahel could experience rainfall increases of as much as 15% over the 1961-1990 average. Equatorial Africa could experience a small (5%) increase in rainfall. These rainfall results are not consistent: different climate models, or different simulations with the same model, yield different patterns. The problem involves determining the character of the climate change signal on African rainfall against a background of large natural variability compounded by the use of imperfect climate models.

Little can be said yet about changes in climate variability or extreme events in Africa. Rainfall may well become more intense, but whether there will be more tropical cyclones or a changed frequency of El Niño events remains largely in the realm of speculation. The combination of higher evapotranspiration and even a small decrease in precipitation could lead to significantly greater drought risks. An increase in precipitation variability would compound temperature effects.

Changes in sea level and climate in Africa might be expected by the year 2050. Hernes et al. (1995) project a sea-level rise of about 25 cm. There will be subregional and local differences around the coast of Africa in this average sea-level rise -depending on ocean currents, atmospheric pressure, and natural land movements -but 25 cm by 2050 is a generally accepted figure (Joubert and Tyson, 1996). For Africa south of the Equator, simulated changes in mean sea-level pressure produced by mixed-layer and fully coupled GCMs are small (~ 1 hPa) - smaller than present-day simulation errors calculated for both types of models (Joubert and Tyson, 1996). Observed sea-level pressure anomalies of the same magnitude as simulated changes are known to accompany major large-scale circulation adjustments associated with extended wet and dry spells over the subcontinent.

The temperature-precipitation-CO2 forcing of seasonal drought probably is less significant than the prospect of large-scale circulation changes that drive continental droughts that occur over several years. A change in the frequency and duration of atmosphere-ocean anomalies, such as the ENSO phenomenon, could force such large-scale changes in Africa's rainfall climatology. However, such scenarios of climate change are not well developed at the global level, much less for Africa.

3.3. LATIN AMERICA

The arid and semi-arid tropics are the ones suffering greatest impacts from climatic fluctuations from adverse phenomena such as the El Niño and La Niña. Government actions should focus on the scenery of these fluctuations, and how to mitigate its possible effects.

The IPCC report (2001a) calls for a troubling situation regarding possible global warming. There are estimates of an increase between 1.4 and 5.8 °C, taking 1990

Figure 5. Climatic risk zoning for coffee crop in Sao Paulo State, Brazil (Pinto et al., 2000). ■ Favourable; □ Temperature restriction; □ Water restriction-irrigation necessary; □ Seasonal frost; CH Unfavourable.

as reference. Even though there are discrepancies about the absolute values for the increase in temperature, everyone agrees that there will be a slight increase in global temperature, and an increase in precipitation as well (Pinto et al., 2001a). Agricultural exploitation in the semi-arid and arid tropic regions will undoubtedly be greatly affected by these temperature increments.

For example, for the coffee crop, largely cultivated in the tropics, it is possible to predict the impact of possible climatic changes and on its cultivation. Pinto et al. (2000) outlined the ecologically viable areas for agricultural exploitation of this crop in the State of Sao Paulo (BR). On the basis of mean historical temperature values and precipitation and water balance, areas suitable for agriculture were determined (Figure 5). Assuming an average increase in temperature of 1.0 °C and an increase in precipitation of approximately 15% and by recalculating the water balance, it was shown that the areas suitable for cultivation were drastically reduced as indicated in Figure 6 (Pinto et al., 2001b). It is important to evaluate these scenarios with care, since some assumptions were made concerning water availability in the soil. Besides, this simulation indicates a trend for the next 100 yr while assuming that the current trends in global warming would continue. It is important to note that no allowance was made for changes in crop and irrigation management and genetic improvements.

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