Global Warming Is Real Many Scientists Agree Skepticism Dies Down as Computers Model Real Changes
Robert C. Cowen, writer of The Christian Science Monitor, The Christian Science Monitor
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 trends continue.
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 doubt."
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. …