Population Effects on Food Demand

We calculated 2100 food demand (assumed proportional to population growth) and the increase in productivity for each scenario (Table 6.3b). The area under crops in 1990 represents slightly over a third of the land that is theoretically estimated to be suitable for crop production. Although this estimated area may be optimistic (since some land is not well suited to permanent cropping, and other land will be removed from production by degradation), there is evidence that additional food can be generated sus-tainably to match population growth. This is supported by the fact that from 1961 to 1997, when human population doubled, agricultural land increased by only 11 percent. Nevertheless, the higher the food demand, the greater the pressure placed on land for agriculture, and less land may be available for other purposes, including carbon mitigation. This increased food demand could be met either by using more land for agri culture or by increasing cropping intensity and per-area productivity. Increased productivity can be obtained either (1) by increased fertilization and irrigation, subject to water availability, with associated carbon costs (Schlesinger 1999) and increased N2O emissions (Robertson, Chapter 29, this volume); or (2) by technological improvement through breeding or biotechnology.

The A1FI, A1B, A1T, and B1 scenarios require an increase in productivity of 0.30.4 percent y-1 whereas the A2 and B2 scenarios require 0.9—1.1 percent y-1. All these increases are in the range considered achievable within integrated assessment models; 1.5 percent y-1 is assumed by Edmonds (personal communication, 2003). Although there is a limit to how much agricultural productivity can be increased, with yield increases slowing during the past two decades (Amthor 1998; Tilman et al. 2002), there is still capacity. The Food and Agriculture Organization of the United Nations (FAO) projects global aggregate crop production to grow at 1.4 percent y-1 to 2030, down from 2.1 percent y-1 over 1970-2000. Cereal yield growth, the mainstay of crop production growth, is projected to be 1.0 percent y-1 in developing countries, compared with 2.5 percent y-1 for 1961-1999 (Bruinsma 2003). We note also that future impacts of climate change on crop yields can be quite large. Leemans et al. (2002) estimate the effect of CO2 fertilization on crop and biofuel yield and hence on land use demand, suggesting a 15 percent increase in the land use demand without CO2 fertilization.

Increased intensity of cropping can lead to land degradation through salinization and erosion. Despite these environmental risks, irrigation is expected to play an important role in agricultural production growth in developing countries, where an estimated 40 Mha could come under irrigation, an expansion of 33 percent more than the land currently irrigated. Scenarios A2 and B2 require increased productivity close to potential growth rates, sustained until 2100.

In summary, scenarios with slow technology development and low per capita gross domestic product (GDP) will be less able to deliver food requirements on the existing land area and are likely to increase pressure on the land resource.

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