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Higher Temperatures Seen
Reducing Global Harvests



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Science: Higher Temperatures Seen Reducing Global Harvests

Science 323: 193 January 9, 2009

http://www.sciencemag.org/cgi/content/full/323/5911/193

CLIMATE CHANGE:
Higher Temperatures Seen Reducing Global Harvests

By Constance Holden

Thousands of people died from the heat that baked
western Europe in the summer of 2003. The heat
wave also devastated the region's agricultural
sector: In France, where temperatures were 3.6C
above normal, the country's corn and fruit
harvests fell more than 25%. Thirty-one years
earlier, another very hot summer shrank harvests
in southwest Russia and Ukraine and led to a
tripling in world grain prices.

By the end of the century, two researchers
predict, those summers may seem like cool ones,
and the impact on agriculture will be even
greater.

In a paper appearing on page 240, atmospheric
scientist David Battisti of the University of
Washington, Seattle, and economist Rosamond
Naylor of Stanford University in Palo Alto,
California, apply 23 global climate models used
by the Intergovernmental Panel on Climate Change
to estimate end-of-century temperatures. Their
conclusions with regard to agriculture are
sobering. "In the past, heat waves, drought, and
food shortages have hit particular regions," says
Battisti. But the future will be different:
"Yields are going to be down every place." Heat
will be the main culprit. "If you look at extreme
high temperatures so far observed--basically
since agriculture started--the worst summers on
record have been mostly because of heat," not
drought, he says.

The models predict that by 2090, the average
summer temperature in France will be 3.7C above
the 20th century average. Elevated temperatures
not only cause excess evaporation but also speed
up plant growth with consequent reductions in
crop yields, the authors note. Although rising
temperatures may initially boost food production
in temperate latitudes by prolonging the growing
season, Battisti and Naylor say crops will
eventually suffer unless growers develop
heat-resistant versions that don't need a lot of
water. "You have to go back at least several
million years before you find Š temperatures"
comparable to those being predicted, Battisti
says.

Just as France offered a glimpse of the future in
temperate regions, says Naylor, the Sahel in
Africa shows what life could be like in the
tropics and subtropics, home to half the world's
population. A generation-long drought in the
region lifted in the early 1990s, but higher
temperatures have remained, depressing crop and
livestock production. The authors predict future
production reductions of 20% to 40%, while the
population in tropical regions is expected to
double to 6 billion.

The conclusions of the paper seem "reasonable,"
says plant and soil scientist Peter Smith of the
University of Aberdeen in the United Kingdom, who
also does greenhouse gas modeling. Smith adds
that future pressures on food supplies come not
only from steadily growing populations but also
from changes in food preferences, in particular,
more people eating meat. "Demand for livestock
products in developing countries will greatly
increase over the next few decades," says Smith.
That trend, he says, represents "a switch to less
efficient ways of feeding ourselves."

So developing heat-tolerant crops won't be enough
to solve the problem of rising temperatures, he
says. "We humans also need to change our
behavior."




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