The negative effects of climate change are well documented, from rising sea levels to melting polar ice caps, but rising temperatures might just start to hit us where it really hurts: the world's collective bread basket. A new study by Wolfram Schlenker at Columbia University, New York, and Michael Roberts at North Carolina State University in Raleigh used a high-resolution database of weather patterns from 1950 to 2005 to show how yields of three main U.S. crops--corn, soybeans, and maize--would respond to increasing temperatures.
According to the researchers, "The single best predictor of a year's yield is the amount of time temperatures exceed about 29 °C and the extent to which they do so. Below this warmer temperatures are beneficial for yields, but the damaging effects above 29 °C are staggeringly large," they say.
For every “degree-day,” a measure developed by the team to show how much 29° C the temperature goes and for how long, yields of cotton, soybeans, and maize could drop by about 0.6 percent. Right now, agricultural regions across America spend a average of 57 degree-days above 29° C during the growing season. The model suggests that this number will rise as global warming continues. Even if greenhouse gas emissions are reduced by 50 percent by 2050 relative to 1991 levels (the government's tentative target number), the crop yields could still drop between 30 and 46 percent. Currently, the U.S. is the largest producer and exporter of crops, making up nearly 50 percent of the world's maize and soybean production.
Fairly recently, in 2003, heat waves scorched Western Europe, killing not only people, but also corn, wheat, and fruit. Harvests of these crops fell by a third. A report conducted by scientists at the University of Washington concurs with Schlenker and Wolfram, showing that if the buildup of greenhouse gas emissions isn't slowed significantly the odds are higher than 90 percent that the average growing-season temperatures will be higher than ever recorded worldwide by the end of the century.
Excess heat, even by a few degrees, causes crops like rice, corn and wheat to grow faster but reduces the plants' fertility and grain production. The report shows that if the average growing-season temperatures rise more than 6° F in many places, crop yields will fall 20 to 40 percent, right in line with the Raleigh study.
Those who will suffer most keenly from lower crop yield are, undoubtably, farmers and the poor. During the heat wave in 2003, France and Italy turned to other countries to fuel their food gaps, but in the future, the report predicts that there will be fewer places to turn for help when the crops dry up.
The scientists reached their conclusions by combining climate data with projections from more than 20 global climate models used by the International Panel on Climate Change, winner of the Nobel Peace Prize in 2007. Cary Fowler, director of Rome-based Global Crop Diversity Trust, believes the answer lies in the development of new, heat-resistant crops. Organizations including the Bill & Melinda Gates Foundation are helping Africa to develop hardier crops, and the Global Crop Diversity Trust has launched a program to screen existing seed collections for traits such as resistance to heat and drought.
Stanford University's Program on Food Security and the Environment (FSE) also blames weather as one of the main culprits in the spike in food prices and scarcity. In a paper published in the journal, Science, in January 20009, an FSE research team led by David Lobell looked at the probable effects of climate change on agriculture throughout the developing world.
The team combined data from climate models that simulate future changes in rainfall and precipitation with historical data on climate and agricultural production and found that by 2030 the productions of key crops in the poorest parts of sub-Saharan Africa could decline by as much as 30 percent or more. Somewhat smaller declines were predicted for south and southeast Africa. Evidence also showed that climate change could immediately harm agriculture in the U.S. and other major exporting regions, putting further constraints on the global food supply.
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