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 esearchers at the University of Massachusetts who
study global warm- ing have released a report strongly suggesting
that the 1990s were the warmest decade of the millennium, with
1998 the warmest year so far. Researchers also have found that
the warming in the 20th century counters a 1,000-year-long cooling
trend.
 The
study, by Michael Mann and Raymond Bradley of UMass, along with
Malcolm Hughes of The University of Arizona in Tucson, appears
in the March 15 issue of Geophysical Research Letters. The research
was funded by the U.S. Department of Energy and the National
Science Foundation. "Temperatures in the latter half of
the 20th century were unprecedented," said Bradley.
The UMass/University
of Arizona study involved a close examination of natural archives,
such as tree rings and ice cores, which record climate variations
each year. These natural archives are called "proxy indicators"
by scientists, and allow researchers to consider the short instrumental
record of climate in a longer-term perspective. Using proxy information
gathered by scientists around the world during the past few decades,
the team used sophisticated computer analysis and statistics
to reconstruct yearly temperatures and their statistical uncertainties,
going back to the year AD 1000.
Specifically,
the team relied on three sets of 1,000-year-long tree-ring records
from North America, plus tree rings from northern Scandinavia,
northern Russia, Tasmania, Argentina, Morocco, and France. Additionally,
the team studied ice cores from Greenland and the Andes mountains.
"As you
go back farther in time, the data becomes sketchier. One can't
quite pin things down as well," noted Mann, "but, our
results do reveal that significant changes have occurred, and
temperatures in the latter 20th century have been exceptionally
warm compared to the preceding 900 years. Though substantial
uncertainties exist in the estimates, these are nonetheless startling
revelations."
Research published
by the same team last year reconstructed yearly global surface
temperature patterns going back 600 years. That study and the
current report both relied on natural archives, and determined
that human-induced greenhouse gases were a major factor in 20th
century global warming.
The year 1998
was found to be the warmest year on record in separate reports
released by the National Aeronautics and Space Administration
(NASA) and the National Oceanic and Atmospheric Administration
(NOAA). The reports were issued in January 1999, reviewing climatic
conditions for the previous year, but the studies examined records
only going back 120 years. The UMass/University of Arizona report
puts the conclusions from NASA and NOAA in a much longer perspective.
According
to the UMass researchers, the 1,000-year reconstruction reveals
that temperatures dropped an average of 0.02 degrees Celsius
per century prior to the 20th century. This trend is consistent
with the "astronomical theory" of climate change, which
considers the effects of long-term changes in the nature of the
Earth's orbit relative to the sun, which influence the distribution
of solar energy at the Earth's surface over many millennia.
"If temperatures
change slowly, society and the environment have time to adjust,"
said Mann. "The slow, moderate, long-term cooling trend
that we found makes the abrupt warming of the late 20th century
even more dramatic. The cooling trend of over 900 years was dramatically
reversed in less than a century. The abruptness of the recent
warming is key, and it is a potential cause for concern."
The latest
reconstruction supports earlier theories that temperatures in
medieval times were relatively warm, but "even the warmer
intervals in the reconstruction pale in comparison with mid-to-late
20th-century temperatures," said Hughes. 
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journ.umass.edu; Harvey Leifert, public information manager, American Geophysical Union, (202) 939-3212, hleifert