Loss of water from rivers, lakes, snowmelt, and groundwater means a reduction in drinking water. But it also means a reduction in water for crops.
As discussed in Chapter 4, global warming may cause an increase in droughts in some areas. Even without accounting for such droughts, however, global warming could have a serious effect on world agriculture. This is because hotter weather often means lower crop yields. A study by David Battisti of the University of Washington and Rosamond Naylor of Stanford University suggested that higher heat in Europe could affect food; in 2003, for example, when Europe saw record temperatures, wheat production in France and Italy dropped by a third. Higher temperatures in the tropics could also cut crop yields by 20 percent to 40 percent, according to Battisti and Naylor. Because the tropics is "home to about half the world's population, the human consequences of global climate change could be enormous."18
Scientists are also concerned that higher temperatures will increase the water needed for irrigation, even as water supplies in general are decreasing. According to the Intergovernmental Panel on Climate Change (IPCC), "Higher temperatures and
Following page: California's lengthy drought in the early twenty-first century emptied reservoirs and left irrigation canals dry. David McNew/Getty Images.
increased variability of precipitation would, in general, lead to increased irrigation water demand even if the total precipitation during the growing season remains the same." Increased evaporation at higher temperatures would mean that crops would need more water; irregular rainfall would also require more irrigation to make sure that crops had a steady water supply. Thus, even if rainfall stays the same, more irrigation may be needed for crops. The paper notes that irrigation requirements for China and India, the countries with the largest irrigated areas on the earth, might change by 2 percent to 15 percent for China and by -6 percent to +5 percent in the case of India.19
Some scientists have argued that worries about global crop shortages are exaggerated. In the first place, as noted in Chapter 4, the combination of increased overall rainfall, higher levels of carbon dioxide, and longer growing seasons may actually increase crop yields in many places. Patrick J. Michaels, a climatol-ogist and Senior Fellow at the Cato Institute, asks, "If the defining characteristics of greenhouse warming are warmer winters, more rain, and longer growing seasons, what's so bad about that?"20
Some commentators have also argued that the threats of world crop disaster fail to take into account human adaptation. They maintain that when the world warms, farmers will not just plant the same crops in order to watch them wither in the field. Instead, farmers will adjust, changing what they plant to reflect the new climate. In some cases, the change in climate may even allow farmers to switch to more productive crops. "If the climate changes, you may be worse off if you don't adapt, but you may be better off than you were before if you do," according to Christopher Essex, a professor of mathematics at the University of Western Ontario, and Ross McKitrick, a professor of economics at the University of Guelph.21
One way in which humans might adapt to climate change is by creating new crops. For example, Brazil is worried that a rise in global temperatures could "mean a 10% reduction in Brazil's arable land for coffee by 2020." To meet this threat, Brazilian sci entists are working to develop genetically modified crops that will be able to withstand the coming heat. Such crops take many years to develop, but Brazil has already seen some success with modified soy plants "that respond favorably to dry, hot conditions while thriving in normal weather as well."22 Experiments with coffee have been less successful, however.
Despite the hopeful signs, however, the short-term and medium-term outlook is worrisome, according to a 2008 article by U.S. News & World Report writer Kent Garber. Discussing a U.S. government report on the impact of global warming on crops, Garber notes that, in the United States, "Some crop yields are predicted to drop; growing seasons will get longer and use more water; weeds and shrubs will grow faster and spread into new territory, some of it arable farmland; and insect and crop disease outbreaks will become more frequent."23 Garber notes that government action might mitigate problems in the long term, but over the next 30 years at least, America and the world will have to face the fact that higher temperatures may reduce crop yields.
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