National Science Foundation (NSF)-funded climatologists at the University of Massachusetts (U. Mass.) at Amherst have reconstructed global temperature over the past 600 years and determined that 1997, 1995 and 1990 were the warmest years since at least 1400 A.D. The study, conducted by Michael Mann and Raymond Bradley of U. Mass., along with University of Arizona colleague Malcolm Hughes, is detailed in the current issue of the journal Nature.

"This study adds solid information to the growing base of data which points to the warming of our planet by human-related activities," says Herman Zimmerman, program director in NSF's division of atmospheric sciences, which funded the research. "The balance of evidence now firmly supports an important human influence on the global climate system. This is a serious problem for people everywhere, and it needs to be addressed at all levels of government."

The researchers were able to estimate temperatures over more than half the surface of the globe, pinpointing northern hemisphere yearly temperatures to a fraction of a degree back to 1400 A.D. The study places in a new context the long-standing controversy over the relative roles of human and natural changes in the climate of past centuries. Scientists were particularly interested in natural "forcings," that is, factors that can affect climate significantly, but which are not part of the climate system itself. Based on statistical comparisons of reconstructed northern hemisphere temperatures, the best estimates indicate that natural changes in the brightness of the sun and volcanic emissions both played an important role in governing climate variations over the period studied.

However, over the past few decades, greenhouse gases produced by human activities appear to have had an increasing influence on temperatures. "The anomalous warmth of several recent years appears likely to be related to human influences on climate," said Mann.

The study bears out concerns voiced by scientists in recent years regarding global warming, Bradley said. It is known that industrialization during the past century has increased levels of carbon dioxide in the earth's atmosphere by more than 25 percent over its pre-industrial level. If the amount of carbon dioxide in the atmosphere were to continue to increase at its current rate, it could rise to double its pre-industrial level during the next century, leading to a magnification of the already observed warming. For example, melting ice caps could raise sea levels, threatening coastal regions with more frequent flooding. The planet as a whole might expect to see frequent extreme weather events, Mann said. "Heat waves and droughts could become more common, and more intense."

Climatologists are also concerned about the degrees of uncertainty surrounding increased or accelerated global warming. "We have a sense of what might happen to the planet as a whole, but the fact is, we don't really know what the regional impacts might be," said Mann.

Researchers' studies of certain individual years were particularly intriguing. For example, historical documents from 1791 suggested conditions consistent with a strong El Nino event that year; the reconstructed temperature pattern bore out these suspicions. The weather was much cooler than usual over most of the globe in 1816 following the eruption of the Indonesian volcano, Tambora, the year before. Warming observed in certain regions, however, was consistent with changes in atmospheric circulation also expected to result from a strong volcanic eruption.

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