Global warming greater minus El Niños, volcanoes

December 12, 2000

BOULDER--Removing the masking effects of volcanic eruptions and El Niño events from the global mean temperature record reveals a more gradual and yet stronger global warming trend over the last century, according to a new analysis by Tom Wigley, a climate expert at the National Center for Atmospheric Research (NCAR). The analysis supports scientists' claim that human activity is influencing the earth's climate. The findings are published in the December 15 issue of Geophysical Research Letters. NCAR's primary sponsor is the National Science Foundation.

"Once the volcanic and El Niño influences have been removed," says Wigley, "the overall record is more consistent with our current knowledge, which suggests that both natural and anthropogenic influences on climate are important and that anthropogenic influences have become more substantial in recent decades."

Volcanic emissions cool the planet by blocking sunlight, while El Niño events raise global temperatures through warmer ocean waters. Sometimes the two occur simultaneously, muddying evidence of any underlying warming trend. During the past two decades, two massive volcanic eruptions--El ChichÛn in April 1982 and Mt. Pinatubo in June 1991--coincided with significant El Niños, making trend detection more difficult.

Wigley quantified the effects of major volcanic eruptions and El Niño episodes on global mean temperatures. Overall, he found the cooling effect from sun-blocking volcanic emissions was slightly stronger than the warming effect of the coincident El Niños. He then removed both from the temperature record to reveal an intensified, step-like warming trend over the past century.

In the raw temperature record--not adjusted for the influence of volcanic eruptions and El Niño events--the warming trend during the past two decades is similar in intensity to an earlier warming (1910 to 1940). Several decades of slight cooling separate the two warm periods. However, "When ENSO and volcanic effects are removed," writes Wigley, "the recent warming trend increases to 0.25 degree Celsius [from 0.18 degree C] per decade and becomes highly significant compared to the earlier period." (ENSO stands for El Niño-Southern Oscillation, a term that describes interannual changes in both sea surface temperatures and atmospheric pressure across the Pacific basin.)

The overall result is a long-term warming trend that intensifies by century's end, in sync with increasing emissions of greenhouse gases. Using the raw data, greenhouse skeptics have claimed that the earlier warming's similarity to the later one suggests that both were due to natural variations rather than human activity.

Wigley also quantified and removed the warming influence of the 1997- 98 El Niño from the temperature record of the past decade. He found that of the 16 months in 1997-98 announced by the National Climatic Data Center as record breakers, at least six can be attributed to El Niño rather than to a longer-term global warming. "The sequence is still unusual, but no more unusual than 1990-91, when an equal number of records occurred in the ENSO-adjusted data," writes Wigley. Nevertheless, the past decade's warmth is striking in the overall record.
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NCAR is managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.

National Center for Atmospheric Research/University Corporation for Atmospheric Research

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