Embargoed until 6 P.M. EDT
NSF PR 98-22 - April 22, 1998
This material is available primarily for archival
purposes. Telephone numbers or other contact information
may be out of date; please see current contact information
at media
contacts.
Scientists Find Further Global Warming Evidence in
Temperature Reconstruction Study
Years 1997, 1995, 1990 Are the
Warmest since 1400 A.D.
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 Niño 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.
|