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It’s such a nice sunny day. Why can’t I draw a breath? 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 and suffocation by all life forms.
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 Niņo 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." ------- |
