Prophets of global warming are winning new respect from
skeptical scientists. Subtle changes in weather patterns that
conform to predictions of improved computer-climate simulations
strongly suggest that man-made climate change is upon us.
As one former skeptic, Thomas Karl, puts it: "I think there's a
likelihood that the changes we see are not just due to natural
variability. There is a human component."
These changes include such effects as a greater share of
precipitation coming in winter and more precipitation coming in
extreme events, more severe warm-season droughts, an increase in
above-normal temperatures, and a decrease in day-to-day variability
in temperature in parts of the Northern Hemisphere.
The draft of a new assessment by the Intergovernmental Panel on
Climate Change (IPCC) likewise concludes that the observed 0.5
degree C rise in average global temperature over the past 135 years
"is unlikely to be entirely due to natural causes."
Dr. Karl - a senior scientist with the National Climatic Data
Center in Asheville, N.C. - says that right now, we're at a point
where a majority of scientists in the field believe there may be a
global-warming signal. But many are not yet willing to say that it
is an obvious signal. He expects that to change to a near-consensus
in favor of global warming within five years if present weather
Michael MacCracken, with the United States Global Change
Research Program office in Washington, makes a similar point. He
says the evidence is "quite compelling" that heat-trapping
so-called greenhouse gases and the cooling effect of man-made
aerosols are affecting climate. Yet it is so far not enough to
convict human activity of climate change "beyond all reasonable
The most important of these aerosols are sulfates emitted by
such sources as coal- and oil-fired power plants. Like particles
from volcanoes, they block sunshine and cool the lower atmosphere.
Computer models had trouble simulating past climate trends in this
century - let alone predict the future - when they took account
only of the buildup of such greenhouse gases as carbon dioxide and
methane. But when aerosols are included, the simulations become
much more realistic.
For example, this was the key to temperature changes that
puzzled James Hansen, Makiko Sato, and Reto Ruedy at the Goddard
Institute for Space Studies in New York. The average daily maximum
temperature over land rose by 0.28 degrees C between 1951 and 1990.
Average minimum temperature rose 0.84 degrees - three times as
much. Yet computer models with only carbon dioxide forcing
predicted the same warming day and night. To match both the
long-term 0.5 degree C global warming and the observed changes in
daily maximum and minimum temperatures, the researchers had to
include the cooling influences of aerosol pollution and increases
in mid-level cloudiness. …