Human activities have resulted in increased atmospheric levels of carbon dioxide and many other greenhouse gases. This increase and mankind's continued activity have the potential to warm the earth's climate over the next century (Houghton et al., 1996). As human activities result in continued potential perturbations in climate, the increasing world population also puts pressure on agriculturists and sets new challenges for crop scientists to help to meet the larger population food and fibre needs. The world's population is forecast to increase from 5.3 billion in 1990 to 8.1 billion by the year 2025, with about 84% of this growth expected in the developing countries (Bos et al., 1995). By the year 2050, experts believe that 12.4 billion people will inhabit the earth. Since there is essentially no new arable land that can be cultivated to meet the demand, the increased food supply must come primarily from more intensive cultivation of existing arable land.

As agriculture becomes more intensive, soil degradation will become a major concern. The world's water resource is also finite, and changing climate and increasing population demands will result in the availability of less water for agriculture. Metropolitan and urban communities nearly always have higher priorities than agricultural production for a scarce resource such as water. In many highly populated countries, food and fibre needs are being met by irrigating up to 75% of the arable land (Hoffman et al., 1990). If a major climate change occurs as is forecast, agriculturalists will face a daunting challenge to produce, in an environmentally sustainable manner, enough food and fibre to satisfy the world's increasing population.

Cotton belongs to the genus Gossypium of the Malvaceae family. Of the 39 species of that genus, which are diverse in habitat, only four produce commercial lint and are grown commercially throughout the world. The

©CAB International 2000. Climate Change and Global Crop Productivity

Upland and Acala varieties belong to G. hirsutum L. and the extra-long staple Pima and Sea island or Egyptian varieties belong to G. barbadense L. Upland cotton is grown on more than 5 Mha in the USA and more than 34 Mha worldwide (USDA, 1989). Most of the world's production is in arid and semi-arid climates and must be irrigated for commercial production. Major cotton-producing countries during the 1997/98 market year (million bales) were: USA, 18.8; China, 18.5; India, 12.5; Pakistan, 7.5; Uzbekistan, 5.4; Turkey, 3.3; and Australia, 2.9 (USDA, 1998).

Cotton is grown worldwide, but in a relatively narrow temperature range compared with many other species. The minimum temperature for growth and development of cotton is 12—15°C, optimum temperature is 26-28°C, and maximum temperature depends on the duration of exposure (K.R. Reddy et al., 1997b). Even short periods of above-optimum canopy temperatures may cause injury to young fruit. Canopy temperatures do not necessarily follow air temperature closely, however, and in well-watered, low-humidity environments, cotton crop canopy temperatures may be several degrees cooler than air temperature (e.g. Idso et al., 1987; Kimball et al., 1992a).

Climatic conditions in the middle or latter part of the 21st century are expected to be different from those of today. Currently, atmospheric carbon dioxide concentration [CO2] is about 360 |mmol mol-1 (Keeling and Whorf, 1994) and there is general agreement among climatic and atmospheric scientists that [CO2] could be in the range of 510-760 |mmol mol-1 some time in the middle or latter part of the 21st century (Rotty and Marland, 1986; Trabalka et al., 1986). Other greenhouse trace gases are also increasing rapidly and will contribute to climatic change. These changes are predicted to warm the earth by 2-5°C. Since plant growth and crop production are controlled by weather, it is important to understand the implications of such weather changes on agriculture.

Temperatures that routinely occur in many cotton-producing areas strongly limit many growth and developmental processes (K.R. Reddy et al., 1992a, 1997a,b). Elevated [CO2] generally enhances leaf and canopy CO2 assimilation rates in plants because CO2 is the substrate for photosynthesis, and also is a competitive inhibitor for photorespiration. Both of these factors result in increased growth and productivity (Kimball, 1983a,b, 1986; Bowes, 1993; K.R. Reddy et al., 1995c, 1996a). Elevated [CO2] often reduces stomatal aperture (e.g. Morison, 1987) and increases the ratio of CO2 assimilated relative to water transpired. Some studies have also found that elevated [CO2] causes altered partitioning of photoassimilate among plant organs (Rogers et al., 1994).

This chapter will help to quantify these processes and their effects on crops and develop ways to manage crops effectively with the available climatic resources. In particular, we need to summarize and integrate knowledge of crop responses to weather factors and link those responses appropriately with management factors. In this chapter, we review cotton responses to global climate change.

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