Major impacts on crop plant growth and production will come from changes in temperature, moisture levels, ozone, ultraviolet radiation, carbon dioxide levels, pests, and diseases (Figures 11.2 and 11.3).
The effects of a temperature increase on photosynthetic productivity of crop plants will interact with the current rise in the atmospheric concentration of CO2. Under elevated CO2, the extra carbohydrates produced by increases in photosynthesis result in an increase in grain yield (Horie et al., 1996). Many researchers (Kimball et al., 1995; Samarakoon and Gifford, 1995; Horie et al., 1996; Pinter, 1996; Semenov, Kounina, and Koukhta, 1999) are of the opinion that the actual impact of elevated CO2 on crop growth, and especially on yields, is likely to be significantly less than the estimates that are currently presented. It is suspected that a portion of the increase in grain yield driven by anthropogenic enrichment of the atmosphere may be suppressed by ozone.
On the African continent, global warming is likely to negatively alter the production of major food crops—rice, wheat, corn, beans, and potatoes. The high-altitude farming districts in Africa may have their altitudinal zonation wiped out and be forced to find new forms of agriculture. Wheat and corn associated with the subtropical latitudes may suffer a drop in yield due to increased temperature, and rice may disappear due to higher temperatures in the tropics (Odingo, 1990; Pimentel, 1993; Muchena and Iglesias, 1995). African agriculture is expected to survive and even become stronger where mixed cropping is currently practiced and where tree crops are predominant.
Global warming will affect the scheduling of the cropping season, as well as the duration of the growing period of the crop in all the major crop-producing areas of Asia. In general, areas in mid and high latitudes will experience increases in crop yields, while yields in areas in the lower latitudes will generally decrease (Lou and Lin, 1999).
In China, the yields of major crops are expected to decline due to climate change. The decline in rice yield is due to a shortening of the growth period, decrease in photosynthesis ability, and increase in respiration, demanding more water availability. The area under wheat is likely to expand in northern and western China. Climate change should be advantageous to wheat yield in northeastern China. However, in middle and northern China, high temperatures during later crop stages could result in yield reductions (Wang, 1996). A doubling of atmospheric carbon dioxide levels will substantially increase rice yields and yield stability in northern and north-central Japan (Horie et al., 1996; Rosenzweig and Hillel, 1998).
In India, while the wheat crop is found to be sensitive to an increase in maximum temperature, the rice crop is vulnerable to an increase in minimum temperature. The adverse impacts of likely water shortage on wheat productivity could be minimized to a certain extent under elevated CO2 levels. They would largely be maintained for rice crops, resulting in a net decline in rice yields (Lal et al., 1998). Acute water shortage conditions combined with thermal stress should adversely affect both wheat and, more severely, rice productivity in northwest India, even under the positive effects of elevated CO2 in the future.
The impact of rise in temperature and increases in atmospheric carbon dioxide on rice production in Bangladesh, Indonesia, Malaysia, Myanmar, the Philippines, South Korea, and Thailand suggest that the positive effects of enhanced photosynthesis due to doubling of CO2 are canceled out for increases in temperature beyond 2°C (Matthews et al., 1995).
A study of global climate-change impacts on wheat crops across the Australian wheat belt shows that doubling CO2 alone produced national yield increases of 24 percent in currently cropped areas but with a fall in grain protein content of 9 to 15 percent (Howden, Hall, and Bruget, 1999). However, if rainfall decreases by 20 percent, yields would increase for 1°C
warming but decline for greater warming (CSIRO, 2001). With greater decreases in rainfall there would be much larger negative impacts, with cropping becoming nonviable over many regions, especially in Western Australia.
Banks (1996) estimated the broad impact of greenhouse effects on eastern Australian agriculture. According to these estimates, there will be an increase in summer growing grasses in the far west rangelands, and livestock stocking rates could increase. In the Tablelands, winter cereals will benefit from higher temperatures throughout the growing season and may provide more flexibility in sowing time. The productivity of perennial pastures will take advantage of increased rainfall. Lucernes will increase in importance as a component of pastures. In the coastal regions citrus crops will mature early. However, deciduous fruits that require vernalization with a significant frost period will be forced to higher latitudes.
More damage is expected to fruit crops from insect pests. Sutherst, Collyer, and Yonow (2000) examined the vulnerability of apples, oranges, and pears in Australia to the Queensland fruit fly under climate change. The results revealed that the range of the fruit fly would spread further south and the number of fruit fly generations in the sensitive area would progressively increase. One extra generation was experienced over the whole area with a 2°C rise in temperature. In this scenario, the damage costs due to Queensland fruit fly may further increase by $3.5 million to oranges, $5.6 million to apples, and $2.8 million to pears. Similar damage costs may increase due to light brown apple moth (CSIRO, 2001).
A projected decrease in frost will reduce frost damage to fruits. However, temperate fruits need winter chilling to ensure normal bud burst and fruit set. Warmer winters will reduce chilling duration, leading to lower yields and quality.
In New Zealand, generally drier conditions and reductions in ground water will have substantial impacts on cereal production in the Canterbury wheat and barley production area. Other grain-producing areas are less likely to be affected. Crop phenological responses to warming and increased carbon dioxide are mostly positive, making grain filling slightly earlier and decreasing drought risk. Rising temperatures would make the maize crop less risky in the south, but water availability may become a problem in Canterbury. Climate warming is decreasing frost risk for late-sown crops, extending the season and moving the southern production margin further south. Climate change may have mixed results on horticulture in New Zealand (Hall and McPherson, 1997).
Climatic warming will expand the area of cereals (wheat and maize) cultivation northward (Carter, Saarikko, and Niemi, 1996). For wheat, a temperature rise will lead to a small yield reduction, whereas an increase in CO2 will cause a large yield increase. The net effect of both temperature and CO2 for a moderate climate change is a large yield increase in southern Europe (Harrison and Butterfield, 1996). Maize yield will increase in northern areas and decrease in the southern areas of Europe (Wolf and van Diepen, 1995).
A temperature increase will shorten the length of the growing period and possibly reduce yields of seed crops (Peiris et al., 1996). At the same time, however, the cropping area of the cooler season seed crops will probably expand northward, leading to increased productivity of seed crops there. There will also be a northward expansion of warmer season seed crops. Analysis of the effect of climatic change on soybean yield suggests mainly increases in yield (Wolf, 1999).
The response of vegetable crops to changes in temperature varies among species. For crops such as onions, warming will reduce the duration of crop growth and hence yield, whereas warming stimulates growth and yield in crops such as carrots (Wheeler et al., 1996). For cool-season vegetable crops such as cauliflower, large temperature increases may decrease production during the summer period in southern Europe due to decreased quality (Olesen and Grevsen, 1993).
Potato and other root and tuber crops are expected to show increases in yield in northern Europe and decreases or no change in the rest of Europe (Wolf, 1999). Sugar beet may benefit from both the warming and the increase in CO2 concentration (Davies et al., 1998).
For grapevines there is potential for an expansion of the wine-growing area in Europe and also for an increase in yield. The area suitable for olive cultivation could be enlarged in France, Italy, Croatia, and Greece due to changes in temperature and precipitation patterns (Bindi, Ferrini, and Mig-lietta, 1992).
The impact of global warming and CO2 increase on South American agriculture varies by region and by crop. Crop yield in the Pampas of Argentina and Uruguay is more sensitive to expected variations in temperature than precipitation. Under CO2 doubling, maize, wheat, and sunflower yield variations were inversely related to temperature increments, while soybean would not be affected for temperature increments up to 3°C (Magrin et al., 1999).
Plantation forestry is a major land use in Brazil. Climate change can be expected to reduce silvicultural yields to the extent the climate becomes drier in major plantation states as a result of global warming (Gates et al., 1992; Fearnside, 1999).
Estimates of the impacts of climate change on crops across North America vary widely (U.S. National Assessment, 2000; Brklacich et al., 1997; Rosenzweig, Parry, and Fischer, 1995). Most global climate-change scenarios indicate that higher latitudes in North America would undergo warming that would affect the growing season in this region. Estimates of increases in the frost-free season under climatic change range from a minimum of one week to a maximum of nine weeks (Brklacich et al., 1997).
For the North American prairies, Ontario, and Quebec, most estimates suggest an extension of three to five weeks. Although warmer spring and summer temperatures might be beneficial to crop production in northern latitudes, they may adversely affect crop maturity in regions where summer temperature and water stress limit production (Rosenzweig and Tubiello, 1997).
Drought may increase in the southern prairies, and production areas of corn and soybean may shift northward in Canada (Mills, 1994; Brklacich et al., 1997). Southern regions growing heat-tolerant crops such as citrus fruit and cotton would benefit from a reduced incidence of killing frosts resulting from a change in climate. Production of citrus fruit would shift northward in the southern United States, but yields may decline in southern Florida and Texas due to higher temperatures during the winter (Rosenzweig, Parry, and Fischer, 1995).
Mexican agriculture appears to be particularly vulnerable to climate-induced changes in precipitation, because most of its agricultural land is classified as arid or semiarid. On average, more than 90 percent of losses in Mexican agriculture are due to drought (Appendini and Liverman, 1993). Under the impact of global warming, the area presently suitable for rainfed maize production would shrink in northern and central regions of Mexico (Conde, 1997).
Was this article helpful?