From Friday's
Globe and Mail
This optimistic view has
been based on hundreds of laboratory experiments showing that crucial crops,
such as wheat, corn and soybeans, when exposed to higher concentrations of
carbon dioxide released through the burning of fossil fuels, respond much like
they've been given an extra dose of fertilizer. The plants experience more
robust growth and have sharply higher yields.
These plant experiments,
conducted mainly during the 1980s, led most researchers to forecast that the
disruption global warming might cause to agriculture through changes in
temperatures and precipitation patterns would be offset by improved crop
yields, as plants thrived in a world with higher levels of carbon dioxide.
But that rosy view, which has been incorporated into projections made
by influential bodies such as the United Nations Intergovernmental Panel on Climate
Change, may be unjustified, says a new research paper in the journal Science
that found the beneficial effect of rising carbon-dioxide levels has been
significantly overestimated.
The paper, based on
experiments using plants grown under actual field conditions rather than in the
more controlled enclosures used for the earlier estimates, found that while
most yields increase as the amount of carbon dioxide in the air rises, the
benefits were only about half the amounts that led to optimism about the impact
of global warming on farm output.
"What we've shown here
is that elevated CO{-2} stimulates crop yields less than previously
thought," says Dr. Elizabeth Ainsworth, co-author of the Science paper and
a plant biologist with the U.S. Department of Agriculture's research service in
Urbana, Ill. "This is a red flag for a world that needs to monitor its
food supply carefully."
In a separate commentary on
the paper in the same edition of Science, David Schimel,
a scientist at the U.S. National Center for Atmospheric Research in
Carbon dioxide, while the
main pollutant driving global warming, is crucial for plants because it is one
of the basic building blocks of photosynthesis, and the ease with which plants
utilize the gas depends on how much is available in the atmosphere.
In the new research,
scientists looked at the effects of higher carbon-dioxide levels in large-scale
trials of major crops, such as rice, wheat, soybeans and corn. The plants were
grown under realistic farm-field conditions, rather than in experiments using
enclosures that have traditionally been employed to maintain constant levels of
the gas around plants.
For the crops, the results
were dramatic. While earlier experiments using enclosures indicated wheat
yields would shoot up 31 per cent if the plants were grown in the higher
carbon-dioxide environment expected to prevail around the middle of this
century, the new research using an open-field test indicated the increase will
be only 13 per cent. For soybeans, the increase dropped from 32 per cent to
only 14 per cent, while for corn, the expected yield rise of 18 per cent in a
high carbon-dioxide world dropped to no yield change at all.
Using piping, spray nozzles
and fast-acting computer controls, the researchers were able to release just
the right amount of carbon dioxide into the air above farm fields to create the
conditions expected to exist around 2050 if aggressive steps to control the
releases of global-warming gas are not taken.
These projected
carbon-dioxide levels, of about 550 parts per million, are about double the
concentration that existed before humans started releasing massive quantities
of the gas into the atmosphere through the burning of fossil fuels and forest
clearing. The current reading is about 380 ppm.
The equipment used in the
experiments was sophisticated enough to maintain constant, elevated
carbon-dioxide levels in open fields from the time crops were sowed until they
were harvested, creating a small version of what the world's farmers will have
to confront at mid-century, when the composition of gases in the atmosphere
will likely reflect more than four additional decades of large-scale
greenhouse-gas releases.
The open-air experimental
plots were spread around the world to test crops under different growing
conditions. Wheat was grown in
The researchers said their
open-field tests had different results from the enclosure studies because the
structures used in experiments produced changes in light, temperature, humidity
and pest levels from what plants experience in realistic growing conditions.
"It is significant
that for some reason the experiments in the field showed different results from
those in chambers," Dr. Ainsworth said.