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Winter 2000

Volcanic eruptions, El Niños mask global warming signal

by Anatta

Tom Wigley. (Photo by Carlye Calvin.)

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 NCAR scientist Tom Wigley (Climate and Global Dynamics Division). With the two influences removed, he says, "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 Chichón in April 1982 and Mt. Pinatubo in June 1991—coincided with significant El Niños, making trend detection more difficult.

In a study published in the 15 December Geophysical Research Letters, 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, steplike warming trend over the past century.

In the unadjusted temperature record, the warming trend during the past two decades is similar in intensity to the warming that occurred from 1910 to 1940. However, "when [El Niño–Southern Oscillation] and volcanic effects are removed," writes Wigley, "the recent warming trend increases to 0.25° Celsius [from 0.18° C] per decade and becomes highly significant compared to the earlier period." The overall result is a long-term warming trend that intensifies by century's end, in sync with increasing emissions of greenhouse gases.

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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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Wed Feb 7 15:34:33 MST 2001