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Chapter 3. Rising Temperatures and Rising Seas: The Crop Yield Effect
Agriculture as it exists today has been shaped by a climate system that has changed little over farming’s 11,000-year history. Crops were developed to maximize yields in this long-standing climatic regime. As the temperature rises, agriculture will be increasingly out of sync with its natural environment. Nowhere is this more evident than in the relationship between temperature and crop yields.
Since crops in many countries are grown at or near their thermal optimum, 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, and the U.S. Corn Belt. 16
Higher temperatures can reduce or even halt photosynthesis, prevent pollination, and lead to crop dehydration. Although the elevated concentrations of atmospheric CO2 that raise temperature can also raise crop yields, the detrimental effect of higher temperatures on yields overrides the CO2 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. 17
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 34 degrees Celsius to near zero at 40 degrees Celsius, leading to crop failure. 18
High temperatures can also dehydrate plants. While it may take a team of scientists to understand how temperature affects rice pollination, anyone can tell that a wilted 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, CO2 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. One of the most comprehensive of these studies was conducted at the International Rice Research Institute (IRRI) in the Philippines. A team of eminent crop scientists using crop yield data from experimental field plots of irrigated rice 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.” 19
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 CO2 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. 20
16. John E. Sheehy, International Rice Research Institute, 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.
17. Mohan K. Wali et al., “Assessing Terrestrial Ecosystem Sustainability,” Nature & Resources, October–December 1999, pp. 21–33.
18. Sheehy, op. cit. note 16; Sanchez, op. cit. note 16.
19. Shaobing Peng et al., “Rice Yields Decline with Higher Night Temperature from Global Warming,” Proceedings of the National Academy of Sciences, 6 July 2004, pp. 9971–75; Proceedings of the National Academy of Sciences, “Warmer Evening Temperatures Lower Rice Yields,” press release (Washington, DC: 29 June 2004).
20. 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; U.N. Population Division, World Population Prospects: The 2006 Revision Population Database, at esa.un.org/unpp, updated 2007.
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