Global Results Versus Regional Projections

In Table 12.4 a summary is presented with the increases in yield and production in cereals for the year 2030. These are projections according to: (i) field-scale modelling presented in Chapter 3; (ii) projections by FAO; and (iii) model simulations by IMPACT.

Table 12.4. Changes in cereal yields and production towards 2030.

Global

Field scale

FAO

impact

Average

Increase

Increase

Increase

cereal

in total

in total

in total

yield in

yield in

production in

production in

1998

2030

2030 - FAO

2025 - IMPACT

(t/ha)

(%)

(%)

(%)

Volta

0.7

72

152

195

Zayandeh Rud

4.5

19

66

41

Walawe

3.5

56

47

83

Mekong

2.7

43

80

111

Syr Darya

2.3

36

78

25

Rhine

5.5

34

24

15

Sacramento

5.6

12

25

Droogers and van Dam (2004) applied the SWAP (Soil-Water-Atmosphere-Plant) model to analyse the effects of climate change, including CO2 enhancement, on the crop yields for the seven ADAPT catchments (see Chapter 3). The main conclusion of their study is that in almost all catchments negative effects of prolonged droughts or extreme wet periods are offset by the positive effects of higher CO2 concentrations. Due to climate change, average crop yields will increase by about 20% in the period 2010-2039 and by about 30% in the period 2070-2099. These yield increases are not yet included in the analysis by FAO and the global analysis, which means that their production levels are on the safe side.

For the ADAPT project, crop production tables have been prepared for 1998, 2030 and 2050 based on official FAO projections (see, for example, Table 12.4). These tables have been prepared for all basins and contain information for all major crops in the basins on harvested land, crop yield and crop production. For the basins in developing countries a distinction has been made between agricultural production coming from irrigated and rain-fed land. For the Rhine and the Sacramento Basins, this distinction could not be made.

One of the constraints in the global IMPACT analysis is the restriction of specifying basin boundaries. The IMPACT models are currently hardwired for a set of 69 basins, referred to as the IMPACT basins/regions. The seven ADAPT case study basins were mapped to the IMPACT basins/regions. The basin representation is more acceptable in regions that are fairly homogeneous and small. Some basins were split over more than one region, for example the Volta was split between Northern Africa and Central and Western Africa.

Table 12.5. Global IMPACT results: climate change impacts on cereal production in 2025 relative to current situation.

Cereal production in 2025 relative to 1995 (%)

Table 12.5. Global IMPACT results: climate change impacts on cereal production in 2025 relative to current situation.

Climate scenario

Sacramento

Rhine

Syr Darya

Volta

Zayandeh Rud

Walawe

Mekong

Current climate

144

115

125

295

141

183

211

MaxPl B2 2020

170

133

126

268

111

181

204

MaxPl B2 2080

178

135

119

259

101

181

200

HadAI 2020

184

142

130

264

113

182

198

HadAI 2080

178

144

108

258

89

176

189

HadB2 2020

186

136

122

274

116

181

205

HadB2 2080

184

142

119

267

107

182

202

In the FAO (2002) study, for the base year (1998) the total crop yields for cereals vary from 0.7 t/ha in the Volta to 5.6 t/ha in the USA. In the Volta Basin, the major cultivated cereal is sorghum under rain-fed conditions with an average yield of only 0.8 t/ha. The only crop that is cultivated under irrigation is rice, with an average yield of 2.4 t/ha. In the Sacramento Basin, rice is also being cultivated, but with average yields of 6.5 t/ha. In the case of the USA, maize is the cereal that is responsible for the high average cereal yields, with a yield of 8.3 t/ha, which is very high if compared with the 1.3 ton/ha that is obtained in the Volta Basin.

From Tables 12.4 and 12.5 it can be concluded that total production generally exceeds the increase in yields. This means that more land will be harvested (either due to expansion of agricultural land or due to an increase in cropping intensity). Only in the case of the Rhine Basin, the yield increase is higher than the increase in production, which implies that agricultural land will be taken out of production. This is not surprising, since the pressure of other sectors on agriculture in this part of the world is very high and the population is practically not growing anymore.

Table 12.5 further shows that in all basins an increase in yield is expected. The highest percentage increase is expected in the Volta Basin (72%) and the lowest in the USA (12%). Climate change has not been taken into account explicitly in these expected yield increases from the FAO (2002). They expected that the overall effects of climate-induced changes in crop production factors and yields are small compared to those stemming from economic and technological growth.

At the latitudes where most of the ADAPT basins are situated, changes in temperature and precipitation are generally expected to have a negative impact on crop yields. However, the rise in atmospheric concentration of CO2 can also be a positive factor in crop production. It stimulates photosynthesis and it improves the water use efficiency of the crop. Over the last decades an enormous amount of research has been done on the impact of changes in CO2 concentrations on crop growth. An extensive review of these experiments has been provided by Jones and Curtis (2000) and the Center for the Study of Carbon Dioxide and Global Change (CSCDGC, 2002). They show that in case of doubled CO2 concentrations under water-limiting conditions, crop production increases by 35-40% for C3 crops, and by about 25% for C4 crops.

The far right column of Table 12.4 lists the changes in cereal production for 2025 as projected by IFPRI/IMWI's IMPACT model used in the global analysis of the ADAPT project. The values for IFPRI/IWMI are for entire economic regions in which the basins fall and do not correspond exactly with the regions used by the FAO. However, the general trends of the estimates are similar, with an R2 of 0.7 for a linear fit of the two data sets.

Table 12.5 shows the results of the ADAPT global climate change analysis on cereal production in 2025 with six different climate scenarios superimposed on the Business As Usual socio-economic scenario for technology, income and population to 2025. The results show that for the Sacramento (USA) and Rhine Basins, cereal production is enhanced by climate change under all climate change scenarios by as much as 40%. For the Volta, Zayandeh Rud, Walawe and Mekong Basins, all scenarios result in a decrease of cereal production compared to the current climate scenario. However, for the Walawe and Mekong Basins the magnitude of decreases are not large, while for the Volta and Zayandeh Rud Basins the impacts are significant, with up to 55% reduction in the Zayandeh Rud Basin. The Syr Darya Basin results show two scenarios enhancing production up to 5% and four scenarios show decreasing production up to 17%.

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