10.2.2.3. 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
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 potatoesstaples 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.