Danes Link Sunspot Intensity to Global Temperature Rise
By WILLIAM K. STEVENS
Published: November 5, 1991
ARE changes in the intensity of the Sun's radiation partly responsible for the rise in global temperature that has been observed over the last century and especially over the last few years? If so, it would mean that heat-trapping atmospheric gases like carbon dioxide played a smaller role in the warming than some scientists have believed likely.
The possibility has been raised anew by Danish geophysicists. They report in the current issue of the journal Science that they have established a close statistical correlation between variations in the length of the sunspot cycle -- which they say are an indicator of changes in the intensity of solar radiation -- and fluctuations in the world's surface temperature over the last 130 years.
The analysis was made by Dr. Eigil Friis-Christensen and Dr. Knud Lassen of the Danish Meteorological Institute in Copenhagen. Bush's Cautious Approach
Their finding is one of a number of new pieces of information being considered by the Intergovernmental Panel on Climate Change, an international group of scientists convened by the United Nations to make a continuing assessment of the global warming problem as new evidence develops.
In international talks on a treaty to deal with the threat of global warming, the Bush Administration has taken a cautious approach on the question of whether to reduce carbon dioxide emissions. It argues that significant reductions would be economically damaging and that not enough is known about the magnitude of the threat. Earlier suggestions that changes in solar radiation might affect the climate as much or more than the heat-trapping gases have played to the Administration's hand.
Some scientists say that if the findings of the Danish scientists are borne out by further investigation, carbon dioxide's suspected role in the warming of the last century would have to be downgraded. But they emphasize that even then, no conclusions could be drawn as to how the climate would be affected by changes in solar radiation over the next century, since the length and intensity of solar cycles are unpredictable. Meanwhile, atmospheric carbon dioxide is expected to increase steadily as a result of the burning of fossil fuels.
To cancel out the warming that many scientists expect to come from carbon dioxide would require a decrease of 2 percent in the intensity of solar radiation, "and I don't know of anyone who believes there is that large a variability" in solar activity over a century's time scale, said Dr. James E. Hansen of the Goddard Institute for Space Studies in New York. Dr. Hansen is a foremost proponent of the view that global warming caused by humans is taking place.
He and others, including the authors of the new study, point out that the new finding is based on a statistical correlation and that no physical mechanism for the supposed variations in solar radiation has been demonstrated. A Remarkably Close Fit
Dr. Friis-Christensen and Dr. Lassen analyzed the lengths of sunspot cycles, which vary from 9 to 13 years with an average of 11 years. Shorter cycles, they said, are associated statistically with more intense solar activity. The variations in cycle length, the scientists found, almost precisely match the fluctuations in Northern Hemisphere temperatures from the mid-1850's to the mid-1980's.
The years immediately before 1985, when temperatures were rising, coincided neatly with a period of shorter solar cycles lasting about nine and a half years. By contrast, the years from 1945 to 1970, when the climate cooled, coincided with a period of longer cycles.
While the correlation established by Dr. Friis-Christensen and Dr. Lassen falls short of definite proof, a number of scientists nevertheless called it remarkable in its close fit between the solar and temperature trends. The findings were termed "a major advance" by Dr. Robert Jastrow, a professor of earth sciences at Dartmouth College who is also the president of the George C. Marshall Institute in Washington. A 'Gut Feeling'
In 1989, the institute issued a report, cited favorably by Bush Administration officials, which said that the last century's warming could have been caused partly by solar variability. It also said that natural cooling factors could partly or largely offset any temperature increase caused by carbon dioxide in the next century.
Dr. Hansen said that while the Danish researchers' statistical correlation may or may not hold up, "my gut feeling, nevertheless, is that they're at least partly right." If the findings turn out to be true, he said, the variations described by the Danes "could substantially moderate or enhance" climate changes brought about by heat-trapping gases.
If the findings are ultimately borne out by physical evidence, they would advance the effort to separate from a welter of natural variations whatever amount of warming humans have contributed to a rise of about 1 degree Fahrenheit in the average surface temperature of the globe over the last century. The Danish scientists said that human-induced warming could have taken place but might have been counteracted by air-polluting sulfates produced by industrial activity. These are believed to exert a cooling effect on climate.
It is also possible, Dr. Friis-Christensen said, that some of the observed warming might be attributable to human factors and some of it to increased solar radiation.