NEW EVIDENCE FINDS THIS IS WARMEST CENTURY IN 600 YEARS
From the New York
By WILLIAM K. STEVENS
April 28, 1998
New York Times
It is clear, climatologists say, that the earth's surface has warmed since the start of the Industrial Revolution. But is the warmth out of the ordinary? And did people cause it?
Scientists have been unable to provide a definitive answer to these questions because they do not know how much the global climate has varied on its own in the comparatively recent past, say, the last thousand years -- which offers the best basis for comparison with today's climate. Temperature records based on thermometer readings go back only about 150 years.
So investigators are turning to indirect means of measuring past temperatures. These include chemical evidence of climatic change contained in tiny marine fossils, corals and ancient ice, along with fossilized pollen in lake sediments and annual growth rings in trees.
Now, in one of the most comprehensive compilations of this proxy evidence, as it is called, scientists have reconstructed temperature variations in the Northern Hemisphere since about 1400. Combining this evidence with thermometer readings and historical records, they have concluded that the 20th century has been the warmest century in the last 600 years -- and that the warmest years in all of that period were 1990, 1995 and 1997.
Taking the analysis one step further, they examined the multiple forces that determine the earth's temperature. Until the 20th century, they found, a variety of mostly natural factors combined to influence the climate. These included changes in solar radiation, volcanic haze that reflects sunlight and heat-trapping greenhouse gases, mainly carbon dioxide, in the atmosphere.
But in the 20th century, they found, the greenhouse gases have been the dominant influence. Atmospheric concentrations of carbon dioxide have been rising because of the burning of fossil fuels like coal, oil and natural gas.
"Our conclusion was that the warming of the past few decades appears to be closely tied to emission of greenhouse gases by humans and not any of the natural factors," said Dr. Michael E. Mann, a climatologist at the University of Massachusetts at Amherst, the chief author of a report on the new study that appears in the current issue of the journal Nature. The other authors are Dr. Raymond S. Bradley, also of Massachusetts, and Dr. Malcolm K. Hughes of the University of Arizona, in Tucson.
Both are paleoclimatologists, who study ancient climates.
Other kinds of analysis have come to essentially the same conclusions, but the study provides a new, independent line of evidence. The question of global warming is a hotly disputed one, of course, and this analysis, like all others, is surrounded by considerable uncertainty. Most proxy data are inherently imprecise; tree rings, for example, reflect changes in precipitation as well as temperature.
"We do have error bars," said Dr. Mann, referring to the scientists' term for margin of error. "They are somewhat sizable as one gets farther back in time, and there is reasonable uncertainty in any given year. There is quite a bit of work to be done in reducing these uncertainties."
Some experts question whether studies of proxy evidence will ever be good enough to yield reliable information on the question of whether the 20th century warming is unusual and generated by human activity.
"They're making progress, and there is a lot of hard work involved, and I hold them in the highest regard," Dr. Tom Wigley, a climatologist at the National Center for Atmospheric Research in Boulder, Colo., said of Dr. Mann and his colleagues. "But I think there's a limit to how far you can ever go." As for using proxy data to detect a man-made greenhouse effect, he said, "I don't think we're ever going to get to the point where we're going to be totally convincing."
Dr. Wigley is regarded as a leading expert on the issue of detecting the greenhouse signal.
Other experts pointed to other caveats. One, Dr. Philip Jones of the University of East Anglia in England, questioned whether it was valid simply to extend the proxy record by adding the last 150 years of thermometer measurements to it. He said that would be a bit like juxtaposing apples and oranges. The Mann team says that it tested the validity of recent proxy data by comparing it with the thermometer record.
Despite the caveats, Dr. Jones said he thought the study would turn out to be quite important. Its main value, he said, is that it may place some sort of approximate boundaries on the temperature variations of the last six centuries. This in turn, he said, may help determine whether other efforts to detect man-made warming were on or close to target.
One detection method involves the use of computerized models of the atmosphere to predict the ways in which the global temperature should vary by geography and altitude if increasing greenhouse gases were warming the planet. This pattern is then compared with actual observations, and so far the comparison has revealed a reasonable fit.
But the Achilles' heel of this approach is that the models' representation of natural climatic variability is self-generated and cannot be checked against the climatic system's real, natural variability. Studies like that of Dr. Mann and his colleagues might provide such a check.
It seems from the Mann paper, Dr. Jones said, that the models' internally generated variability is "pretty reasonable" when compared with the picture emerging from the proxy evidence.