New global warming research confirms link to greenhouse gases

The analysis, by Tom Wigley, a researcher at the National Center for Atmospheric Research (NCAR) in Colorado, and published in the latest issue of the Geophysical Research Letter, supports scientists’ claims that human activity is influencing the earth’s climate.

“Once the volcanic and El Nino influences have been removed, 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,” said Wigley.

Volcanic emissions cool the earth by blocking sunlight, while El Nino events raise global temperatures by warming the oceans. Sometimes the two occur simultaneously, confusing still further the evidence of an underlying trend towards warming. For example, within the past two decades two massive volcanic eruptions – El Chichon in April 1982 and Mount Pinatubo in June 1991- coincided with significant El Nino events.

Mr Wigley’s research quantified the effects of such major eruptions and El Ninos on global mean temperatures. The result showed that the cooling effect from volcanic emissions was slightly stronger than the warming effect of the El Ninos that coincided with them. Removing both from the temperature record revealed an intensified, step-like warming trend over the past century.

In the unadjusted temperature record, the warming trend over the past 20 years is similar in intensity to an earlier warming period, from 1910 to 1940, which was followed by several decades of slight cooling. Using this evidence, sceptics have been able to claim that the 1910-1940 warming was similar enough to the current trend to suggest that both were caused by natural variations in temperature rather than human activity. But when El Nino and volcanic effects were removed from the equation, Mr Wigley found the recent warming trend increased to 0.25C from 0.18C per decade, a significant increase compared to the earlier period.

The adjusted record shows that the world is undergoing a long-term warming trend that intensified by the end of the century, in parallel with increasing greenhouse gas emissions.

In addition, taking out the warming influence of the 1997-98 El Nino from the temperature record showed that of 16 months in 1997-98 with record-breaking temperatures, at least six were due to El Nino rather than 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 [El Nino-Southern Oscillation] data,” said Wigley. Despite this, the increase in warmth over the past decade is still seen as striking in the overall record.

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.