Peering into the Past

As climatologists puzzled over current warming or cooling trends, some of their colleagues attempted to reconstruct the record of past climate. This was a difficult task because scientifically accurate meteorological observations taken with high-quality, calibrated instruments had only become available during the previous century. Although scientists are usually uneasy about using proxies, that is, records from which scientific information may be inferred, in this case there were very few options.

Tree rings provided one proxy for climate. The University of Arizona's Laboratory for Tree Ring Research concentrated on reconstructing meteorological data on wet and dry years back to 1600 by looking at trees in the desert Southwest. Trees that live in marginal growing zones are more sensitive than those living in optimal climates. Any climatic changes show up more distinctly in tree ring width—wider in wetter years, narrower in drier years. The use of tree rings to determine past climate, called dendroclimatology, provides information back several hundred years. Another use of tree ring data, which uses radioactive carbon 14, matches tree age with high and low concentration of carbon 14. Carbon 14, an isotope of naturally occurring carbon, increases in concentration when sunspot activity is low and decreases when it is high. Using information from tree cores gathered from Douglas fir trees on Vancouver Island, Canada, and in Oregon and Washington, the University of Washington researchers Minze Stuiver (1929- ) and Paul D. Quay (1949- ) correlated periods of relative cold in Europe with the occurrence of low sunspot activity.

Another proxy, the examination of fossilized pollen found in ancient lakebeds, allowed researchers to track changes in vegetation. Since known plant species tend to grow in warm, temperate, or cold climates, the change in pollen types found in cores taken from lakebeds

Tree rings can be dated and used as a proxy for past climate conditions. (Photo by Gary Braasch)

provided clues to climate change and the speed with which it took place. Similarly, scientists examine deep-sea cores looking for fossilized shells of sea creatures and for changes in the amount of volcanic ash as a way of determining changes in sea surface temperature and the level of volcanic activity through hundreds of thousands of years.

Other proxies are drawn from human records. Managers of medieval manors and their surrounding farmlands kept meticulous records of harvest yields and notes on the appearance of



spring blooms and the time of vine harvests. These records were then used to create a description of the weather for a given year. Scholars also examined paintings for images of cloud types, snow levels on mountains, and positions of glaciers. When dating the image, they had to consider that artists often produce field sketches years before they create the final painting. Scientists also used old almanacs, ships' logs, personal diaries, and newspaper accounts to get a sense of what the weather was like. Balancing this information against scientific data, they could fill in gaps in the record.

In the next decade, scientists would continue to research past climate. Peering back into the climatic past was becoming just as important as trying to see into the climatic future.

Coral cores, like these being taken from the Florida Garden Banks National Marine Sanctuary, provide information about paleoclimate. (USGS)

And yet, at the end of the decade there was still no firm conclusion about whether the climate was warming or cooling. Less than six months after the NAS released its report that concluded that warming was a problem, another scientific team drawn from the United States, Japan, and West Germany announced that there was still solid evidence of a cooling trend. Despite this team's results, the number of scientists making the claim for cooling was starting to shrink. In the next decade the data would cause most scientists to reconsider whether the climate was cooling down or warming up.

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