by William R. Cline, Peterson Institute for International Economics
Op-ed in the Omaha World-Herald
April 13, 2008
© Omaha World-Herald
Global average temperatures rose about 1 degree Fahrenheit during the 20th century, and most scientists attribute the change primarily to man-made global warming.
Concentrations of carbon dioxide, the main greenhouse gas trapping heat and causing global warming, are now about one-fourth higher than their known average in the 19th century.
Under the more plausible scenarios for world growth and energy use, global emissions of carbon dioxide would approximately double by 2050 and quadruple by 2100.
The rise would occur partly because energy is estimated to shift increasingly from natural gas and oil toward coal, which is more abundant and more intensive in carbon dioxide emissions.
The amount of carbon dioxide in the atmosphere is estimated to double from its present level by the end of this century. Average temperatures would be about 5 degrees Fahrenheit higher than today, and about 8 degrees Fahrenheit over land. (The oceans warm more slowly.)
The list of likely damages from global warming is long and includes those from rising sea levels, more intense hurricanes, species loss, a wider reach of malaria, reductions in water supplies, and increased urban pollution. Perhaps the biggest likely risk, however, is to world agriculture.
Higher temperatures speed plants through their development and leave less time for grain filling. Evaporation and loss of water through plant leaves rises more rapidly with temperatures than the increase in rainfall expected from global warming, causing a loss of moisture. Incidence of severe drought, like that in the American Dust Bowl in the 1930s or Australia in recent years, would likely increase.
In a 2007 study (Global Warming and Agriculture: Impact Estimates by Country, published by the Center for Global Development and the Peterson Institute for International Economics), I combined an average of the geographically detailed projections of six leading climate models with models from agricultural economics to project the effects of global warming on world agriculture by late this century (the 2080s).
This time horizon is relevant to any family with a child or grandchild under the age of 5 today. It is necessary to look this far out to begin to see the extent of the more severe impacts of global warming.
The agricultural impact models incorporate results from experimental farm evidence relating yields to temperature, rainfall, and soil quality as well as statistical models relating differences in farmland values to differences in climate.
In applying these models, it is also necessary to decide how much to add to yields from the uncertain effect of “carbon fertilization” in a more carbon-rich atmosphere. (Photosynthesis creates plant material using carbon dioxide as an input, along with water and sunlight.)
By the 2080s, unarrested global warming would reduce world agricultural potential by about 5 to 15 percent from levels it would otherwise reach, depending on whether carbon fertilization is included.
The estimated damages fall disproportionately on the developing countries, which tend to be located closer to the equator, where temperatures are already near crop tolerance levels.
By the 2080s, reductions in productivity would be severe in India (losses of 30 to 40 percent), Africa (about 20 to 30 percent), and Latin America (about 15 to 25 percent).
The overall average for the United States would be plus or minus about 7 percent, depending on carbon fertilization. But there would be a big difference between losses of about 15 to 25 percent in the southern half of the country and gains of about 15 to 30 percent in the northern half.
Losses in Mexico would reach 25 to 35 percent, likely putting pressure on immigration into the United States.
This grim prospect does not have to happen. Most industrial countries, but not the United States, have already begun to take action through the Kyoto Protocol commitment to reduce carbon dioxide emissions to about 5 percent below their 1990 levels. In December, more than 180 nations (including the United States) agreed to negotiate a new agreement to replace Kyoto by 2009.
The politics of global warming in the United States appear to have shifted toward action thanks to Hurricane Katrina, Al Gore's documentary "An Inconvenient Truth" and the strong new report of the Intergovernmental Panel on Climate Change.
The leading US presidential candidates are all calling for stiff reductions below 1990 levels by mid-century, with the proposed cuts ranging from nearly half (John McCain) to 80 percent (Hillary Clinton and Barack Obama).
But restraint by industrial countries will not avoid global warming if developing countries continue rapid expansion of carbon dioxide emissions. China's emissions already exceed those of the European Union and will soon surpass those of the United States.
The agricultural impacts would be most severe in developing countries, and agriculture accounts for a larger share of their economies than it does in industrial countries. Bangladesh and Egypt face significant risk from sea-level rise.
If a meaningful successor agreement to the Kyoto Protocol is to be reached, the United States must provide strong leadership and Brazil, India, and China must commit to future carbon dioxide emissions limits.
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