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A Parking Lot Effect?
By Mark Clayton
The Christian Science Monitor
Thursday 05 February 2004
Spewing
from factory stacks and car tail pipes, carbon dioxide is the poster
child of "greenhouse gases." Most scientists long ago concluded that
CO2 is the single biggest cause of climate change and that cutting its
output is the best way to slow global warming.
So
why are a tiny but growing number of atmospheric scientists taking a
hard look at parking lots? Because, they say, land-use changes have at
least as much, and perhaps even greater, impact on climate change than
CO2. It's a radical idea that has heated up the scientific community
and is prompting a wider look at the forces behind climate change. The
effect on public policy could be enormous.
Do
massive asphalt and concrete "urban heat islands" like Houston or
Atlanta really help ratchet up the global thermostat? What about huge
tracts of farmland like those that span the Midwest?
Eugenia
Kalnay thinks so. Her research into the impact of land-use changes on
global temperature is getting attention from other scientists, even if
this debate hasn't exactly leaped into the public arena yet.
Earth's
surface temperatures have risen about 1 degree F. in the past century
with faster warming in the past two decades, the National Academy of
Sciences reports. The 20th century's 10 warmest years all occurred in
the last 15 years of the century.
But
according to Dr. Kalnay's study, published in the journal Nature last
spring, urbanization, agriculture, and other human changes to
landscapes in the US - quite aside from CO2 - account for as much as 40
percent of the temperature rise over the past 40 years - much larger
than previously believed. That could make it a contender for CO2's
crown.
Kalnay,
a University of Maryland researcher, was director of environmental
modeling at the National Weather Service from 1987 to 1997. She oversaw
development of computer models for the now ubiquitous three- to
five-day forecasts.
But
it is her recent research that struck a chord with the scientific
community. Kalnay and coauthor Ming Cai have received a huge amount of
both praise and criticism. "We were both taken aback that instead of
the paper going quietly, we got hundreds and hundreds of comments and
questions," she says.
Now
Kalnay's research, joined by the work of a growing number of other
scientists, has intensified debate over the relative strength of
"climate forcing" factors.
Recent
studies show that deforestation in parts of Africa is curbing rainfall
in the once-vital Sahel zone bordering the Sahara desert. Changes in
forest cover have also been shown to affect rainfall and climate far
beyond the Amazon Basin. Still others have shown that planting trees
can actually increase the planet's temperature if done in the wrong
climate zones.
"Impacts
of human-caused land changes on climate are at least as important, and
possibly even more important, than those of carbon dioxide," says Roger
Pielke Sr., professor of atmospheric science at Colorado State
University and past president of the American Association of State
Climatologists. His group voted in 2002 to issue a statement almost
unanimously concurring that climate changes are more complex than CO2
changes and include land use. By contrast, the American Geophysical
Union issued a statement last month maintaining CO2 as the key factor.
Dr.
Pielke and others argue that land-use changes in a region may have
significant effects thousands of miles away - not unlike the El Niño
effect in which warming zones of the Pacific Ocean force droughts and
weather changes worldwide.
That's
still theory, of course. Skeptics point out that only 29 percent of the
earth's surface is land - and only 1 to 2 percent is urbanized. Another
40 percent of land has been modified by agriculture and deforestation,
Pielke says. So can the regional land-use tail really wag the global
dog?
Not
according to Alan Robock, an editor of the Journal of Geophysical
Research: Atmospheres. He's also seen no uptick in studies of land-use
impact on climate in scientific literature he's read.
"Everybody
realizes modifying land surface is important locally," says Dr. Robock.
"If you're talking globally, though, CO2 is the dominant way humans
cause climate change."
Still,
the view that human changes to the landscape are a factor driving
climate, too, is gaining some traction in powerful corners of the
scientific community. A report by the National Academy of Sciences due
later this year will examine the warming effects of non-CO2 agents:
aerosols, solar variability, and land-use changes.
"The
public does not hear too much about this, because all the
climate-change treaties have been focused on CO2," says Daniel Jacob, a
Harvard University professor of atmospheric chemistry who chairs the
panel writing the report. "For a long time it's been really hard to
communicate these other factors to the policymakers, mainly because
it's difficult to find the proper currency for them."
The
impact of such change would begin first with global climate modelers,
like Robert Dickinson, president of the American Geophysical Union. Dr.
Dickinson is working to include more detailed effects from land-surface
changes, aerosol, and soot in his climate model. He says one of his
graduate students is pursuing a surface-temperature study of China. But
like many, he still maintains that CO2 is the dominant force in climate
change.
Kalnay's
research is providing ammunition for some private groups to argue that
global warming is a myth. In an editorial last June, the Center for the
Study of Carbon Dioxide and Global Change argued that Kalnay's work
showed that the impact of CO2 was overstated. "The warming of the past
century or so was nothing more nor less than the natural recovery of
the earth from the global chill of the Little Ice Age," the Tempe,
Ariz., nonprofit reported.
Such
conclusions irk Peter Frumhoff, global environment program director for
the Union of Concerned Scientists in Cambridge, Mass. "Just because new
research shows there are other factors to pay attention to [beside CO2,
that] doesn't mean there's any less reason to pay attention to
greenhouse gases," he says.
Kalnay
is undeterred. Having completed her temperature study of the US, she is
working on a global analysis of 50 years of temperature data. Already,
early results from South America support her conclusions. "Greenhouse
gases are undoubtedly very important," she says. "But the second cause
for climate change is the way we are using the land surface."
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Jump to TO Features for Friday 06 February 2004
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