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
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Contact: Cheryl Dybas
cdybas@nsf.gov
703-306-1070
National Science Foundation
22-Apr-1998