SCIENTISTS concerned about future climate change are looking back to what they call the Little Ice Age for clues to what may happen.
That was a run of cooler- and wetter-than-normal weather in Western Europe from around 1500 to 1850. It also showed up in some other Northern Hemisphere areas at various times.
Raymond S. Bradley, a geographer at the University of Massachusetts at Amherst, explains that "our human-induced impact on the climate will be superimposed on some natural background variability." He adds: "It is incumbent on us to understand what that natural variability may be."
That involves studying what John Eddy, a geophysicist with the Consortium for International Earth Science Information Network in University Center, Mich., calls "the most recent of the reported major changes in climate that have gripped the earth."
Dr. Eddy notes that the Little Ice Age has another "bearing on a very important topic of today." This is the climatic effects of changes in the Sun's energy output. Some global-warming skeptics are arguing that solar variability is such a strong influence for climate change that it is misleading to base public policy on computer forecasts of the effect of man-made pollution from carbon dioxide, methane, and other "greenhouse" gases.
Eddy explains that the Little Ice Age provides a good case study. It coincided with an unusual absence of sunspots and related solar activity.
He points out that research to date - including computer-based simulations - indicates that solar variability has "a trimming effect" on global climate. It could diminish anticipated greenhouse warming by perhaps half a degree Celsius out of a projected 2- to 3-degree planetary temperature rise. However, Eddy warns, "it could equally well increase the warming." He says it would be "socially irresponsible" to use uncertainty about solar influence as a reason to delay action to curb greenhouse-gas pollutio n.
MEANWHILE, geophysicists need a clearer picture of what actually happened in the Little Ice Age. There are no instrumental temperature or precipitation records to go on. The Little Ice Age period has been delineated mainly by studies of glacial advances and retreats and by anecdotal stories of climatic decline. Flemish painters depicted the harsh winter conditions of the time.
But these sorts of clues tell little about the global extent of the Little Ice Age or its year-to-year variability. Earth scientists now have new types of data to study these questions. Deep bore holes tap rocks that retain a "memory" of past temperature trends. Tree rings and cores from ice sheets contain a detailed year-by-year climatic record.
Malcolm Hughes of the University of Arizona's Laboratory for Tree Ring Research in Tucson says that "it is not possible to give strong support to the idea of a global Little Ice Age from what are some of the best available [tree ring] records of the last thousand years."
Geologist Henry N. Pollack at the University of Michigan at Ann Arbor says residual temperatures in deep bore holes "confirm cooling took place in Greenland, Alaska, France, and Canada. They also show that the Little Ice Age was not a globally uniform event in terms of onset, duration, or amplitude."
On the other hand, Paul A. Mayewski, a glaciologist at the University of New Hampshire at Durham, says that ice cores - especially one from Greenland that extends back 42,000 years - show "there was a cold signal ... experienced throughout much of the world." They also show that the Little Ice Age "is one of several events of this character throughout the last many thousand years," he says.
Summing up the evidence, Raymond Bradley concludes that we can interpret today's climate either as natural recovery from the Little Ice Age or as warming strengthened by human activity. "I think the long-term records, at this point, provide food for argument in both camps," he says.
Malcolm Hughes warns, though, that to use this uncertainty as a science-based excuse to avoid concern about possible greenhouse warming "would be thoroughly dishonest."