Climate Change Scenarios

In the ADAPT project, climate change projections for the periods 2010-2039 and 2070-2099 under two IPCC scenarios are selected for comparison with the baseline period of 1961-1990. From the many global models producing climate change projection data, we selected to use the SRES A2 and B2 projections by the model from the Hadley Centre for Climate Prediction and Research in the UK, referred to as HadCM3. For the baseline period 1961-1990, temperature and precipitation data from the Climatic Research Unit were used. For more information see Chapter 2.

Comparison of climate data during 1961-1990 in different data sets

For the comparison between current climate and projected climate, we looked at the differences in average temperature and precipitation at both basin level and regional

Table 8.2. Projection of population and food demand under scenarios A2 and B2.

Food demand 2000

Growth rate

Food

(% per year)

Population

demand (106 t)

Population

2000-

2025-

(106)

Sub-basin

2000 (106)

(million t)

2025

2085

2025

2085

2025

2085

Scenario A2

China

9.6

2.9

1.2

0.9

12.9

22.1

3.9

6.6

Myanmar

0.9

0.3

1.1

0.8

1.2

1.9

0.4

0.6

Laos

4.9

1.5

2

1.5

8.0

19.6

2.4

5.9

Thailand

23.1

6.9

1.2

0.9

31.1

53.3

9.3

16.0

Cambodia

9.8

2.9

1.6

1.6

14.6

37.8

4.4

11.3

Vietnam

16.9

5.1

1.6

1.2

25.1

51.4

7.5

15.4

Total

65.2

19.6

93.0

186.2

27.9

55.8

Ratio to 2000

1.4

2.9

1.4

2.9

Scenario B2

China

9.6

2.9

0.9

0.6

12.0

17.2

3.6

5.2

Myanmar

0.9

0.3

0.8

0.5

1.1

1.5

0.3

0.4

Laos

4.9

1.5

1.5

1

7.1

12.9

2.1

3.9

Thailand

23.1

6.9

0.9

0.6

28g.9

41.4

8.7

12.4

Cambodia

9.8

2.9

1.6

1.1

14.6

28.1

4.4

8.4

Vietnam

16.9

5.1

1.2

0.8

22.8

36.7

6.8

11.0

Total

65.2

19.6

86.5

137.8

25.9

41.3

Ratio to 2000

1.3

2.1

1.3

2.1

level. For the latter, the MRB is divided into 13 sub-basins (Fig. 8.4). A comparison of the SRES and MRCS datasets reveals the following.

Temperature

For small sub-basins such as the Mun Chi (4175 km2), sub-basin temperature from SRES matches quite well with the average monthly temperature at Ubon station. For the larger sub-basins such as the Chi (55,985 km2) and the Mekong 1 (156,509 km2), the sub-basin temperature from SRES typically is lower than the average temperature at representative stations, Khon Kaen and Nong Khai, respectively.

Precipitation

Unlike temperature, precipitation in the MRB has a clear spatial distribution pattern: for example, high precipitation in the sub-basin Mekong 1 and low precipitation in the Chi and Mun. Taking into account such a distribution pattern, we selected one station with a long record as representative station of each sub-basin. Although the variation of SRES precipitation during that period matches quite well with the observed variation, the values are higher or lower depending on the size and location of sub-basin. Therefore we have to apply an adjustment coefficient to bring the SRES sub-basin average during 1961-1990 close to the observed values at representative

Se Kong

Sre Pok

Mekong 1

Mekong 2

Mekong 3

Fig. 8.4. Thirteen sub-basins and stream network in the SLURP model for MRB (Kite, 2000).

Se Kong

Sre Pok

Mekong 1

Mekong 2

Mekong 3

Fig. 8.4. Thirteen sub-basins and stream network in the SLURP model for MRB (Kite, 2000).

stations. The adjustment coefficient was calculated as the ratio between the average observed precipitation and the SRES sub-basin average (see downscaling method 2, Chapter 2). The values of adjustment coefficient, from about 0.5 to 1.6, reflect the need for correction of the global model data for a specific basin. The same adjustment coefficients are applied for the future scenarios A2 and B2.

Climate in the Mekong Basin under different climate scenarios

We now analyse the changes in temperature and precipitation in the adjusted SRES data.

Temperature

The mean temperature in the whole MRB will increase from 24.3°C in 1961-1990 to 25.3°C during 2010-2039 in both scenarios A2 and B2, and to 28.3°C and 27.2°C during 2070-2099 in scenarios A2 and B2, respectively (Table 8.3). Trends of changes in mean temperature from 1961-1990 to 2010-2039 and 2070-2099 in the MRB under scenarios A2 and B2 are shown in Fig. 8.5. Although the trend of mean temperature in A2 is higher than in B2, the variation in B2 is larger.

Although variations in maximum and minimum temperatures are slightly different in the A2 and B2 scenarios, the common trend is an increase of about 5°C from 1961-1990 to the end of 21st century. In many months during 2070-2099, the

Table 8.3. Comparison of temperature and precipitation data from CRU and SRES data sets.

CRU SRES A2 A2 B2 B2

1961-1990 1961-1990 2010-2039 2070-2099 2010-2039 2070-2099

Item

Mean temperature 25.3 24.3 25.3 28.3 25.3 27.2

Mean precipitation 1713 1563 1534 1623 1530 1546 (mm/year)

sd (mm) 142 135 156 191 145 180

maximum monthly temperature will be over 40°C and the minimum temperature will be over 25°C, and the number of these months is higher in A2 than in B2.

During 2010-2039 in scenario A2, mean temperature in every sub-basin will increase about 3.7-4% compared with the baseline 1961-1990. The same increase is found during 2010-2039 in scenario B2. During 2070-2099, the increments in mean temperature in scenario A2 by sub-basin vary from 14.0% (Delta) to 21.8% (Nam Ou) compared with the baseline 1961-1990. However, in this period, the increments in mean temperature in scenario B2 are lower, from 10.0% (Delta) to 15.7% (Nam Ou).

A2 (1961-2099)

E 25

A2 (1961-2099)

E 25

1961 1971 1981 1991 2001 2011 2021 2031 2041 2051 2061 2071 2081 2091

Year

1961 1971 1981 1991 2001 2011 2021 2031 2041 2051 2061 2071 2081 2091

Year

B2 (1961-2099)

1961 1971 1981 1991 2001 2011 2021 2031 2041 2051 2061 2071 2081 2091

Year

Fig. 8.5. Variations and trend of mean monthly temperature of the MRB under scenarios A2 and B2.

Precipitation

Changes of mean precipitation from 1961-1990 to 2010-2039 and 2070-2099 in Mekong River sub-basins under scenarios A2 and B2 are shown in Fig. 8.6. Overall precipitation in the basin increases, although the change in precipitation varies from one sub-basin to another. Compared with the baseline 1961-1990, during 2010-2039, mean precipitation (from here onwards, the mean precipitation refers to the value after adjustment) in different sub-basins varies from about —6% to +6% in both scenarios A2 andB2. However, forthe MRB as awhole, mean precipitation during 2010-39 only varies by +0.2% and —0.2% in scenarios A2 and B2, respectively. During 2070-2099, mean precipitation in different sub-basins varies from about — 12% to +32% in both scenarios A2 and B2. The positive variations occur in nine sub-basins with a higherper-centage compared to negative variations in four sub-basins. These variations lead to an increase of 9.8% and 9.4% in the MRB as awhole in scenarios A2 and B2, respectively.

Regarding other factors that affect crop production and water balance, there are three different trends from 1961-1990 to 2010-2039 and 2070-2099: (i) minor change in solar radiation; (ii) increase in wind speed; and (iii) decrease in relative humidity. In all cases, the variations in A2 are larger than in B2.

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