We can cut carbon emissions by one third by replacing fossil fuels with renewable energy sources for electricity and heat production." –Lester R. Brown, Janet Larsen, Jonathan G. Dorn, and Frances Moore, Time for Plan B: Cutting Carbon Emissions 80 Percent by 2020
Chapter 3. Climate Change and the Energy Transition: Rising Temperatures, Falling Yields
Since farming began thousands of years ago, crops have been developed to maximize yields in a relatively stable climatic regime. Now that regime is changing.
Since crops typically 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, or the U.S. Corn Belt. 65
Higher temperatures can halt photosynthesis, prevent pollination, and lead to crop dehydration. Although the elevated concentrations of atmospheric CO2 that raise temperature can also raise crop yields, after a certain point the detrimental effect of higher temperatures on yields overrides the CO2 fertilization effect for the major crops.
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 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 400 million people by mid-century, rising temperatures are a troubling prospect. 66
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 as the temperature climbs until it hits 35 degrees Celsius (95 degrees Fahrenheit), whereupon it begins to decline, until at 40 degrees Celsius (104 degrees Fahrenheit), photosynthesis ceases entirely. 67
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.” 68
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. 69
High temperatures can also dehydrate plants. 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 also reduced, thereby restricting photosynthesis. At elevated temperatures, the corn plant, which under ideal conditions is so extraordinarily productive, goes into thermal shock.
Countless global climate models show that as temperature rises, some parts of the world will become more vulnerable to drought. Among these are the southwestern United States and the Sahelian region of Africa, where heat plus drought can be deadly. The Sahel, a wide savannah-like region that stretches across Africa from Mauritania and Senegal in the west to Sudan, Ethiopia, and Somalia in the east, already suffers devastating periodic droughts and high temperatures. Now the low rainfall in this region is becoming even more sparse. 70
For tens of millions in this region across Africa, lower rainfall and higher temperatures threaten their survival. For them time is running out. Cary Fowler, head of the Global Crop Diversity Trust, says, “If we wait until it’s too hot to grow maize in Chad and Mali, then it will be too late to avoid a disaster that could easily destabilize an entire region and beyond.” 71
65. 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.
66. 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, op. cit. note 59.
67. Mohan K. Wali et al., “Assessing Terrestrial Ecosystem Sustainability,” Nature & Resources, October–December 1999, pp. 21–33.
68. 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. 9,971–75; Proceedings of the National Academy of Sciences, “Warmer Evening Temperatures Lower Rice Yields,” press release (Washington, DC: 29 June 2004).
69. Sheehy, op. cit. note 65; Sanchez, op. cit. note 65.
70. Tim P. Barnett et al., “Human-Induced Changes in the Hydrology of the Western United States,” Science, vol. 319 (22 February 2008); T. M. Shanahan et al., “Atlantic Forcing of Persistent Drought in West Africa,” Science, vol. 324 (17 April 2009); Marshall B. Burke, David B. Lobell, and Luigi Guarino, “Shifts in African Crop Climates by 2050, and the Implications for Crop Improvement and Genetic Resources Conservation,” Global Environmental Change, in press.
71. U.N. Population Division, op. cit. note 59; Burke, Lobell, and Guarino, op. cit. note 70; Marlowe Hood, “Warming May Outstrip Africa’s Ability to Feed Itself: Study,” Agence France-Presse, 17 June 2009.
Copyright © 2009 Earth Policy Institute