[R-G] Rice yields plunging due to balmy nights

[Tim Murphy]

29th June 04
New Scientist
www.newscientist.com

--

NewScientist.com news service


Rice yields plunging due to balmy nights

Rice yields are crashing as a result of global warming at twice the rate
predicted by climate modellers, according to the first "real world"
experiment on the impact of rising temperatures.

The detailed study of crop yields and temperatures took place on
long-standing research plots at the International Rice Research Institute at
Los Banos in the Philippines. The results suggest that global rice yields
could potentially fall by a catastrophic 50 per cent during this century.

The study found that on the Los Banos plots, "the 0.70C increase in the mean
daily temperature was associated with a rice yield decrease of 10 per cent -
substantially greater than previous estimates", says Kenneth Cassman from
the University of Nebraska in Lincoln, and one of the research team.

Past predictions of future rice yields have forecast declines of five per
cent for an increase of 0.70C. But Cassman told New Scientist that those
predictions are too low, because they missed the fact that global warming is
most intense at night, when tropical plants need to cool off and respire.

Energy drain

In recent decades, global warming has generally been greater at night than
during the day - a trend climatologists expect to continue.

At IRRI's research site, the mean temperature rose by 0.70C over the past 25
years. But while maximum temperatures rose by 0.350C, night minima have
risen by 1.10C.

The research team, headed by the IRRI's Shaobing Peng, believe that the rice
plants suffered from these higher night temperatures because it dramatically
increases the amount of energy they expend on processes such as respiring
during the hours of darkness. That leaves less energy for daytime
photosynthesis, when their grain grows.

Cassman says the IRRI research site is unique because it has grown the same
rice varieties and used the same growing techniques for many years, while
also collecting detailed temperature records. That means sunlight and
temperature are the only variables to explain changing yields.

Widely eaten

The UN Intergovernmental Panel on Climate Change (IPCC) currently predicts
that without drastic action to halt emissions of greenhouse gases, there
will be a rise of 3.60C in average global temperatures in the coming
century. The new findings suggest that could reduce yields of rice - the
world's most widely eaten food - by half.

However, some people believe that global warming should be good for crop
yields. The veteran British TV botanist and environmentalist David Bellamy
recently grabbed headlines by saying that plants should grow faster in a
greenhouse world because the extra carbon dioxide in the atmosphere will
make photosynthesis more efficient.

Cassman agrees that "the increased CO2 concentration should partly offset
the negative effects of higher night-time temperatures." But he points out
that the Los Banos research plots have been subjected to both higher
temperatures and higher CO2 levels, and the negative effects won.

The IPCC's Third Assessment Report in 2001 foresaw the Los Banos results,
stating: "In the tropics, where some crops are near their maximum
temperature tolerance, yields would decrease generally with even minimal
changes in tremperature."

But the report also noted that the same might not apply at higher latitudes:
"A few degrees of warming will lead to general increases in temperate crop
yields."

Journal reference:
Proceedings of the National Academy of Sciences
(DOI:10.1073/pnas.0403720101)

Fred Pearce

---------

http://www.newscientist.com/news/news.jsp?id=ns99996082

--------