Climate Change 2001:
Working Group II: Impacts, Adaptation and Vulnerability
Other reports in this collection Crop Production

Major impacts on food production will come from changes in temperature, moisture levels, ultraviolet (UV) radiation, CO2 levels, and pests and diseases. CO2 enrichment increases photosynthetic rates and water-use efficiency (WUE) (see Chapter 5). The direct effects are largest on crops with C3 photosynthetic pathway (wheat, rice, and soybean) compared with C4 crops (maize, sorghum, millet, and sugarcane). Increases in local temperatures may cause expansion of production into higher elevations. The grain filling period may be reduced as higher temperatures accelerate development, but high temperatures may have detrimental effects on sensitive development stages such as flowering, thereby reducing grain yield and quality. Crop water balances may be affected through changes in precipitation and other climatic elements, increased evapotranspiration, and increased WUE resulting from elevated CO2.

Specific examples of impacts on crops are available. Pimentel (1993) notes that global warming is likely to alter production of rice, wheat, corn, beans, and potatoes—staples for millions of people and major food crops in Africa. Staple crops such as wheat and corn that are associated with subtropical latitudes may suffer a drop in yield as a result of increased temperature, and rice may disappear because of higher temperatures in the tropics (Odingo, 1990).

Figure 10-8:
Schematic of forward-linkage approach to integrated assessment of climate change impacts on Egypt, using agricultural sector model (Yates and Strzepek, 1998).

The possible impact of climate change on maize production in Zimbabwe was evaluated by simulating crop production under climate change scenarios generated by GCMs (Muchena and Iglesias, 1995). Temperature increases of 2 or 4°C reduced maize yields at all sites; yields also decreased under GCM climate change scenarios, even when the beneficial effects of CO2 were included. It is suggested that major changes in farming systems can compensate for some yield decreases under climate change, but additional fertilizer, seed supplies, and irrigation will involve an extra cost. The semi-extensive farming zone was particularly sensitive to simulated changes in climate, and farmers in this zone would be further marginalized if risk increases as projected.

Analysis of potential impacts, using dynamic simulation and geographic databases, has been demonstrated for South Africa and the southern Africa region by Schulze et al. (1993) (see also Schulze et al., 1995; Hulme, 1996; Schulze, 2000). Relatively homogenous climate and soil zones were used to run agrohydrological, primary productivity, and crop yield models. The results reaffirm the dependence of production and crop yield on intraseasonal and interannual variation of rainfall.

Impacts on crops need to be integrated with potential changes in the agricultural economy. Yates and Strzepek (1998) describe an integrated analysis for Egypt (see Figure 10-8). Their model is linked to a dynamic global food trade model, which is used to update the Egyptian sector model and includes socioeconomic trends and world market prices of agricultural goods. Impacts of climatic change on water resources, crop yields, and land resources are used as inputs into the economic model. The climate change scenarios generally had minor impacts on aggregated economic welfare (sum of consumer and producer surplus); the largest reduction was approximately 6%. In some climate change scenarios, economic welfare slightly improved or remained unchanged. Despite increased water availability and only moderate yield declines, several climate change scenarios showed producers being negatively affected by climate change. The analysis supports the hypothesis that smaller food-importing countries are at risk of adverse climate change, and impacts could have as much to do with changes in world markets as with changes in local and regional biophysical systems and shifts in the national agricultural economy.

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