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Chapter 4. Rising Temperatures and Rising Seas: The Yield Effect
Among the leading economic trends most sensitive to this warming are crop yields. Crops in many countries are grown at or near their thermal optimum, making them vulnerable to any rise in temperature. Even a minor increase--1 or 2 degrees Celsius--during the growing season can reduce the grain harvest in major food-producing regions, such as the North China Plain, the Gangetic plain of India, or the Corn Belt of the United States.8
As noted in Chapter 1, higher temperatures can halt photosynthesis, prevent fertilization, and lead to dehydration. Although the elevated concentrations of atmospheric carbon dioxide (CO2) that raise temperature can also raise crop yields (absent other constraints, such as soil moisture and nutrient availability), the detrimental effect of higher temperatures on yields appears to be overriding the CO2 fertilization effect for the major crops.
In a study of local ecosystem sustainability, Mohan Wali and his colleagues at Ohio State University note that as temperature rises, photosynthetic activity 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), it ceases entirely. At this temperature, the plant is in thermal shock, simply trying to survive.9
The most vulnerable part of the plant's life cycle is the period when fertilization occurs. Each of the three food staples--rice, wheat, and corn--is vulnerable at this stage of development. 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, much as an unfertilized egg moves along the fallopian tube. When temperatures are uncommonly high, the silk strands dry out and quickly turn brown, unable to play their role in the fertilization process.
The effects of temperature on rice fertility have been studied in detail by scientists at the International Rice Research Institute in the Philippines. They report that the fertility of rice falls from 100 percent at 34 degrees Celsius (93 degrees Fahrenheit) to near zero at 40 degrees Celsius, leading to crop failure.10
Higher temperatures can also lead to dehydration. While it may take a team of scientists to understand the effects of temperature on the fertilization of the rice plant, anyone can tell when a corn field is suffering from heat stress and dehydration. 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. The corn plant, which under ideal conditions is so extraordinarily productive, is highly vulnerable to thermal stress.
K. S. Kavi Kumar of the Madras School of Economics and Jyoti Parikh of the Indira Gandhi Institute of Development Research assessed the effect of higher temperatures on wheat and rice yields in India. 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 of 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 incorporated the negative effects of a higher temperature with the positive effects of CO2 fertilization, the decline in yields among the various sites ranged from 8 percent to 38 percent.11
The decline in rice yields was remarkably similar. A separate study in the South Indian state of Kerala, looking at the effect of temperature on rice yields, concluded that for each 1-degree Celsius rise in temperature, rice yields declined 6 percent. These studies are disturbingly relevant given the projected average temperature rise in India of 2.3-4.8 degrees Celsius following the doubling of atmospheric CO2 over pre-industrial levels.12
As rising temperatures become a reality and as the effect of temperature on crop yields becomes clearer, agricultural scientists are becoming concerned. John Sheehy, a crop ecologist and leading researcher on the effects of climate change on crops, offers a scientific rule of thumb for assessing the effect of higher temperature on the yield of rice plants: "For every 1 degree Celsius increase in temperature between 30 and 40 degrees Celsius during flowering, fertility decreases by 10 percent." At 40 degrees, fertility drops to near zero. L. H. Allen, Jr., one of the scientists who is analyzing the temperature-yield relationship at the U.S. Department of Agriculture, concludes that each rise of 2 degrees Fahrenheit (1.1 degrees Celsius) above ideal levels reduces yield by 10 percent.13
A recent study by a team of U.S. scientists at the Carnegie Institution goes further. Based on U.S. corn and soybean yield data from more than 400 counties over the last 17 years, they report that a 1-degree Celsius rise in temperature during the June-August growing season reduces yields of both crops by 17 percent. This may help explain why the record U.S. average corn yield-8.7 tons per hectare in 1994--has not been matched during the eight years since then.14
With the global average temperature for 2002 at a near-record high, it is not surprising that the 2002 harvest in several countries suffered from high temperatures. As the temperature climbs, more countries are likely to suffer crop-withering heat waves. The May 2002 heat wave in India that claimed more than 1,000 lives in Andhra Pradesh also stressed crops. So, too, did the heat wave in neighboring Pakistan.15
In the United States, intense heat in 2002, particularly in the Great Plains states--often 38 degrees Celsius (100 degrees Fahrenheit) or higher--took its toll on the grain harvest. These near-record temperatures at times extended northward into the Great Plains of Canada during the summer, exacerbating drought there and shrinking the wheat harvest. The higher latitudes and continental interiors where the projected temperature rise is to be greatest neatly defines the North American breadbasket--the Great Plains of the United States and Canada and the U.S. Corn Belt.16
The North China Plain, China's principal food-producing region, also suffered from high temperatures in 2002. Even in September, temperatures were soaring into the mid-30s and above. Such high temperatures not only stress crops, they also increase soil moisture evaporation, raising the demand for irrigation water.17
Plant breeders will undoubtedly be able to develop crop strains that are more heat-resistant, but it is doubtful that they can fully offset the effects of rising temperature. And thus far biotechnology has not had any major success in this strategically important area of plant breeding.
If we permit atmospheric CO2 levels to continue rising at recent rates, we will be headed for a world far warmer than any since agriculture began some 11,000 years ago--a world in which farmers will be struggling to adjust to an ever-changing climate. They must think about changing to not only new varieties, as they have always done to boost production, but also to new crops in order to adapt to the changing climate. And virtually all the world's farmers will have to change their farming practices, keeping in mind this is not just a one-time adjustment but a continuing change and a guessing game as to whether shifts are aberrations or a lasting change in the local climate. In the past, farmers could deal with aberrations because they knew that sooner or later conditions would return to normal, but now there is no "normal" to return to.
What changes in cropping patterns lie ahead as the earth becomes warmer? Will the decline in production of drought-tolerant crops, such as sorghum and millet, over the last several decades be reversed as they replace wheat in human diets and in rations for livestock and poultry? Will rice give way to more water-efficient wheat in our diets? Will water shortages lead to wheat eventually edging out rice as the dominant food staple in both India and China?
Another response will be to move agriculture northward into Canada and Siberia. Unfortunately, the soils in these regions are not particularly fertile. There is, for example, a world of difference between the deep fertile soils south of the Great Lakes and those north of them. The U.S. Corn Belt is the world's most productive agricultural region, whereas the thin glaciated soils north of the Great Lakes are far less productive. Despite its vast land area, Canada produces less grain than France does. It is a leading exporter only because its population is so small relative to its vast land area.18
Temperature rises in some areas could easily be double the global average. In other areas, there might be little or no change. Such is the world of uncertainty now facing the world's farmers.19
8. For data on the world's grain production, see U.S. Department of Agriculture (USDA), Production, Supply, and Distribution, electronic database, Washington, DC, updated 13 May 2003.
9. Mohan K. Wali et al., "Assessing Terrestrial Ecosystem Sustainability," Nature & Resources, October-December 1999, pp. 21-33.
10. 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.
11. 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.
12. S. A. Saseendran et al., "Effects of Climate Change on Rice Production in the Tropical Humid Climate of Kerala, India," Climate Change, vol. 44 (2000), pp. 495-514, cited in Kumar and Parikh, op. cit. note 11, p. 278.
13. Sheehy, op. cit. note 10; Allen's research noted in David Elstein et al., "Leading the Way in CO2 Research," Agricultural Research, October 2002, pp. 12-13; see also L. H. Allen, Jr., et al., "Carbon Dioxide and Temperature Effects on Rice," in S. Peng et al., eds., Climate Change and Rice (Berlin: Springer-Verlag, 1995), pp. 258-77.
14. David B. Lobell and Gregory P. Asner, "Climate and Management Contributions to Recent Trends in U.S. Agricultural Yields," Science, 14 February 2003, p. 1032; Erik Stokstad, "Study Shows Richer Harvests Owe Much to Climate," Science, 14 February 2003, p. 997; record yield from USDA, op. cit. note 8.
15. Global average temperature from Hansen, op. cit. note 3.
16. Grain harvest from USDA, op. cit. note 8; near-record temperatures from USDA, National Agricultural Statistics Service, "Weekly Weather and Crop Bulletin," at jan.mannlib.cornell.edu/reports/nassr/field/weather, and from NOAA/ USDA Joint Agricultural Weather Facility, "International Weather and Crop Summary," updated weekly at www. usda.gov/agency/oce/waob/jawf/wwcb/inter.txt; daily temperature reports for the United States and the world from "Weather," Washington Post, daily editions.
17. "Weather," op. cit. note 16.
18. Information on world soils from USDA, Natural Resources Conservation Service, at www.nrcs.usda.gov/technical/ worldsoils; grain production from USDA, op. cit. note 8.
19. Committee on Abrupt Climate Change, National Research Council, Abrupt Climate Change: Inevitable Surprises (Washington, DC: National Academy Press, 2002).
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