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March
15, 2007 Over a span of two
decades, warming temperatures have caused
annual losses of roughly $5 billion for major food crops, according to
a new
study by researchers at the Carnegie Institution and Lawrence Livermore
National Laboratory. From 1981-2002,
warming reduced the combined production of
wheat, corn, and barley—cereal grains that form the
foundation of much of the
world’s diet—by 40 million metric tons per year.
The study, which will be
published March 16 in the online journal Environmental
Research Letters, demonstrates that this decline is due to
human-caused
increases in global temperatures. "Most people tend to
think of climate change as
something that will impact the future," said Christopher Field,
co-author
on the study and director of Carnegie’s Department of Global
Ecology in The study is the
first to estimate how much global food
production has already been affected by climate change. Field and David
Lobell,
lead author of the study and a researcher at Lawrence Livermore
National
Laboratory, compared yield figures from the Food and Agriculture
Organization
with average temperatures and precipitation in the major growing
regions. They found that, on
average, global yields for several of
the crops responded negatively to warmer temperatures, with yields
dropping by
about 3-5 percent for every 1 degree Fahrenheit increase. Average
global
temperatures increased by about 0.7 degrees Fahrenheit during the study
period,
with even larger changes in several regions. "Though the impacts
are relatively small compared to
the technological yield gains over the same period, the results
demonstrate
that negative impacts are already occurring," said Lobell. The researchers
focused on the six most widely grown crops
in the world: wheat, rice, maize (corn), soybeans, barley and
sorghum—a genus
of about 30 species of grass raised for grain. These crops occupy more
than 40
percent of the world’s cropland, and account for at least 55
percent of
non-meat calories consumed by humans. They also contribute more than 70
percent
of the world’s animal feed. The main value of
this study, the authors said, was that it
demonstrates a clear and simple correlation between temperature
increases and
crop yields at the global scale. However, Field and Lobell also used
this
information to further investigate the relationship between observed
warming
trends and agriculture. "We assumed that
farmers have not yet adapted to
climate change—for example, by selecting new crop varieties
to deal with
climate change. If they have been adapting—something that is
very difficult to
measure—then the effects of warming may have been lower,"
explained
Lobell. Most experts believe
that adaptation would lag several years
behind climate trends, because it can be difficult to distinguish
climate
trends from natural variability. "A key moving forward is how well
cropping systems can adapt to a warmer world. Investments in this area
could
potentially save billions of dollars and millions of lives," Lobell
added.
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