5.1 Climate and Agriculture 115

5.2 Global Food Demand 117

5.3 Variation in Crop Yields and Climate Change 121

5.4 Soil Degradation and Climate Change 126

5.5 Soil Carbon Dynamics 130

5.6 Soil Carbon Sequestration and Global

Food Security 135

5.7 Conclusions 136

References 137

Global climate change, global food security, and soil degradation are some of the most important challenges of the 21st century. The latter problem is exacerbated by the depletion of the pool of soil organic carbon (SOC) and an accompanying decline in soil quality. Global climate change, the greenhouse effect, and global warming are three interrelated processes. However, whereas global climate change and the greenhouse effect are natural processes, global warming is driven by anthropogenic activities.

Several natural processes affect the earth's climate, including its orbit around the sun, solar radiation, volcanic activities and their associated emissions, and meteorites. When a meteorite struck the Yucatan Peninsula 65 million years ago, it caused a drastic change in climate and subsequent mass extinction. Eighteen thousand years ago, during the most recent "ice age," a thick ice sheet covered a vast area of the northern hemisphere. The Labrador ice sheet covered the whole of eastern Canada and extended south to Cincinnati and New York. It was about 600 m (2000 ft) thick and covered 16 million km2 (Coleman, 1926; Dawson, 1992). The Mount Pinatubo eruption in 1991 altered the climate of the Southeast Asian region, and increased photosynthesis in deciduous forests for the following 2 to 3 years.

The greenhouse effect is also a natural process. It has made the Earth inhabitable by raising its temperature from a frigid -18°C to a tolerable 15°C. The greenhouse effect is caused by the presence of various trace gases in the Earth's atmosphere, notably carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). They act as a glass does in a greenhouse — they permit shortwave radiation to enter Earth's atmosphere but impede the outward flow of infrared or longwave radiation, which causes a positive radiation balance; hence, the reference to "greenhouse gases" (GHGs). Because of this imbalance in radiation, the Earth — including its atmosphere and ocean — are warmer by about 33°C than they would be in the absence of this natural greenhouse effect. It is an extreme natural greenhouse effect that has caused the extremely high temperatures on Venus and extremely low temperatures on Mars. In contrast, the Earth's present temperature (15°C) is just right to support an incredible diversity of life.

Global warming is caused by acceleration of the natural greenhouse effect because of anthropogenic activities that enrich the atmospheric concentration of GHGs. In 1896, Swante Arrhenius warned that CO2 emitted into the atmosphere from burning fossil fuel would act as a greenhouse gas and increase Earth's temperature. The accelerated greenhouse effect is called "global warming" if the rate of increase in global temperature exceeds 0.1°C per decade or 1°C per century. If the rate of increase in temperature is too rapid, the ecosystems cannot adjust. Each 1°C increase in temperature is likely to lead to a poleward shift of all cereal-growing and vegetation zones by several hundred kilometers (200 to 300 km) (Parry and Carter, 1988a; Kirschbaum et al., 1996). There will also be a poleward retreat of the permafrost boundary.

The observed climate change during the 20th century has already caused an increase in the Earth's temperature by 0.6 ± 0.2°C, and the rate of increase in temperature since 1950 has been 0.17°C per decade for some regions (Intergovernmental Panel on Climate Change [IPCC], 2001). The projected global warming caused by a doubling of the atmospheric concentration of CO2 ranges between 2°C and 4°C (Cheddadi et al., 2001; Prentice and Fung, 1990). There is growing concern that climate change may shift precipitation patterns; change the duration of the growing season; and affect regional agricultural productivity, the output of forests and fisheries, and global food security.

This chapter highlights the interactive effects of climate change, soil quality, and soil carbon dynamics on global food security. The chapter focuses on tropical ecosystems and on land use and soil management options that can enhance the SOC pool, thereby reducing the rate of enrichment of atmospheric CO2 and improving food production by enhancing soil quality.

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