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Chapter 4. Rising Temperatures and Rising Seas: The Crop Yield Effect
One of the economic trends most sensitive to higher temperatures is crop yields. Crops in many countries are grown at or near their thermal optimum, making them vulnerable to any rise in temperature. Even a relatively minor increase during the growing season of 1 or 2 degrees Celsius can shrink the grain harvest in major food-producing regions, such as the North China Plain, the Gangetic Plain of India, or the U.S. Corn Belt. 20
Higher temperatures can reduce or even halt photosynthesis, prevent pollination, and lead to crop dehydration. Although the elevated concentrations of atmospheric carbon dioxide that raise temperature can also raise crop yields, the detrimental effect of higher temperatures on yields overrides the CO 2 fertilization effect for the major crops.
In a study of local ecosystem sustainability, Mohan Wali and his colleagues at Ohio State University noted that as temperature rises, photosynthetic activity in plants increases until the temperature reaches 20 degrees Celsius (68 degrees Fahrenheit). The rate of photosynthesis then plateaus until the temperature hits 35 degrees Celsius (95 degrees Fahrenheit), whereupon it begins to decline, until at 40 degrees Celsius (104 degrees Fahrenheit), photosynthesis ceases entirely. 21
The most vulnerable part of a plant’s life cycle is the pollination period. Of the world’s three food staples—rice, wheat, and corn—corn is particularly vulnerable. In order for corn to reproduce, pollen must fall from the tassel to the strands of silk that emerge from the end of each ear of corn. Each of these silk strands is attached to a kernel site on the cob. If the kernel is to develop, a grain of pollen must fall on the silk strand and then journey to the kernel site. When temperatures are uncommonly high, the silk strands quickly dry out and turn brown, unable to play their role in the fertilization process.
The effects of temperature on rice pollination have been studied in detail in the Philippines. Scientists there report that the pollination of rice falls from 100 percent at 93 degrees Fahrenheit (34 degrees Celsius) to near zero at 104 degrees Fahrenheit, leading to crop failure. 22
High temperatures can also dehydrate plants. While it may take a team of scientists to understand how temperature affects rice pollination, anyone can tell when a cornfield is suffering from heat stress. When a corn plant curls its leaves to reduce exposure to the sun, photosynthesis is reduced. And when the stomata on the underside of the leaves close to reduce moisture loss, CO 2 intake is reduced, thereby restricting photosynthesis. At elevated temperatures, the corn plant, which under ideal conditions is so extraordinarily productive, goes into thermal shock.
Within the last few years, crop ecologists in several countries have been focusing on the precise relationship between temperature and crop yields. In an age of rising temperatures, their findings are disturbing. One of the most comprehensive of these studies was conducted at the International Rice Research Institute (IRRI) in the Philippines, the world’s premier rice research organization. The team of eminent crop scientists there noted that from 1979 to 2003, the annual mean temperature at the research site rose by roughly 0.75 degrees Celsius. 23
Using crop yield data from the experimental field plots for irrigated rice under optimal management practices for the years 1992–2003, the team’s finding confirmed the rule of thumb emerging among crop ecologists—that a 1-degree-Celsius rise in temperature above the norm lowers wheat, rice, and corn yields by 10 percent. The IRRI finding was consistent with those of other recent research projects. The scientists concluded that “temperature increases due to global warming will make it increasingly difficult to feed Earth’s growing population.” 24
While this study analyzing rice yields was under way, an empirical historical analysis of the effect of temperature on corn and soybean yields was being conducted in the United States. It concluded that higher temperatures had an even greater effect on yields of these crops. Using data for 1982–98 from 618 counties for corn and 444 counties for soybeans, David Lobell and Gregory Asner concluded that for each 1-degree Celsius rise in temperature, yields declined by 17 percent. Given the projected temperature increases in the U.S. Corn Belt, where a large share of the world’s corn and soybeans is produced, these findings should be of concern to those responsible for world food security. 25
Two scientists in India, K.S. Kavi Kumar and Jyoti Parikh, assessed the effect of higher temperatures on wheat and rice yields. Basing their model on data from 10 sites, they concluded that in north India a 1-degree Celsius rise in mean temperature did not meaningfully reduce wheat yields, but a 2-degree rise lowered yields at almost all the sites. When they looked at temperature change alone, a 2-degree Celsius rise led to a decline in irrigated wheat yields ranging from 37 percent to 58 percent. When they combined the negative effects of higher temperature with the positive effects of CO 2 fertilization, the decline in yields among the various sites ranged from 8 percent to 38 percent. For a country projected to add 500 million people by mid-century, this is a troubling prospect. 26
20. John E. Sheehy, International Rice Research Institute, Philippines, e-mail to Janet Larsen, Earth Policy Institute, 1 October 2002; Pedro Sanchez, “The Climate Change–Soil Fertility–Food Security Nexus,” speech, Sustainable Food Security for All by 2020, Bonn, Germany, 4–6 September 2002; USDA, op. cit. note 3.
21. Mohan K. Wali et al., “Assessing Terrestrial Ecosystem Sustainability,” Nature & Resources, October-December 1999, pp. 21–33.
22. Sheehy, op. cit. note 20; Sanchez, op. cit. note 20.
23. Peng et al., op. cit. note 13.
24. Ibid.; Proceedings of the National Academy of Sciences, “Warmer Evening Temperatures Lower Rice Yields,” press release (Washington, DC: 29 June 2004).
25. David B. Lobell and Gregory P. Asner, “Climate and Management Contributions to Recent Trends in U.S. Agricultural Yields,” Science, vol. 299 (14 February 2003), p. 1032.
26. K. S. Kavi Kumar and Jyoti Parikh, “Socio-Economic Impacts of Climate Change on Indian Agriculture,” International Review for Environmental Strategies, vol. 2, no. 2 (2001), pp. 277–93; United Nations, World Population Prospects: The 2004 Revision (New York: February 2005).
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