Do sun's variations affect Earth's weather? Link still eludes scientists
Scientists who look for links between sunspots, solar variability, and weather continue to chase a will-o'-the-wisp.
This was evident at the recent meeting on Solar-Terrestrial Influences on Weather and Climate - the second in a series of international conferences which meet at four-year intervals to discuss the subject.
Researchers reported little gain either in proving such linkages exist or explaining how they operate. However, most of them appeared to agree with Jack Eddy of the National Center for Atmospheric Research (NCAR) that a great deal of solid progress has been made in other ways.
He explained that progress has come chiefly in the crucial area of understanding basic solar cycles and how these variations are transmitted in the form of radiation and particles across the 93 million miles of space to Earth's upper atmosphere. He noted that it is primarily in the attempt to track solar influences down through the planet's protective outer layer of gases to the lower atmosphere that the subject becomes murky.
''Critics will say, and some have, that as a discipline, sun-weather studies have never really departed from . . . a search guided as much by hope as by reason . . . ,'' Mr. Eddy remarked. He added that these studies have seemed to lack credibility partly because of the urge of its practitioners to search for evidence for the ''needle of solar influence in the haystack of weather records, '' without going through the painstaking work of understanding the physical processes which might link them.
Those critical of the search for sun-weather links often point out that the variation in solar heat and light output is too small to have a meaningful influence on Earth weather. According to recent observations, that output fluctuates about 0.1 percent. Critics, therefore, tend to dismiss any ''apparent'' relationships as coincidental.
According to sun-weather researchers, the strongest such correlation is the 22-year drought cycle in the southwest United States. This appears to persist in climatic reconstructions from tree rings which extend back to the early 1600s.
A second, and still controversial, example was reported from Australia. Rock outcrops north of Adelaide have sequences of distinctive bands called varves. These vary regularly in width in groups of 9 to 14, with an average value of 11. 2, reminiscent of an 11-year sunspot cycle.
George E. Williams of Broken Hill Proprietary Company, who has studied this banding, said he believes they represent sediment accumulated in a large, shallow glacial lake 600 to 700 million years ago. Each spring, the glacial runoff laid down another layer of sediment. Thus the variations in thickness correspond roughly to the size of the spring melt.
The cycle seems to have an 11-year period. ''Of course, such a judgement is a subjective one, necessarily deduced from the geological setting,'' says Mr. Williams. Skeptics have questioned whether the bands represent yearly deposits. They could represent spacings of anywhere from months to decades, these critics say.
To attribute the Australian varves and the southwest US drought cycle to solar influence, sun-weather scientists explain that atmospheric conditions in certain places and times may somehow amplify tiny variations in solar output. To explore this basic thesis, Kirby Hansen of the National Oceanic and Atmospheric Administration (NOAA) has statistically analyzed temperatures in the US from 1895 to 1980 on a month-by-month and state-by-state basis.
He finds that the strongest evidence for an 11-year cycle occurs in the Great Lakes region. During June, temperatures have tended to be 2 degrees F. colder at sunspot maximum and 2 degrees warmer just before sunspot minimum. Also, he finds that the state of Arizona has had a very strong 23-year temperature cycle for six months out of the year.
Attempts to find a solar signal in temperature records of larger areas have yielded ambiguous results. Tom Wigley of the University of East Anglia, for instance, reported on efforts to find solar cycles from temperatures records spread over the entire northern hemisphere from 1900 to 1982. He said that this effort on the part of colleagues P.M. Kelly and P.D. Jones uncovered no coherent pattern of areas showing 11-year temperature variations.
Despite the modest results that their efforts have produced so far, sun-weather researchers appear to believe that a deeper understanding of this subject may be almost within their grasp. If that is true, any such breakthrough in knowledge could be reported at their next international conference four years hence.