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2003-51 FOR IMMEDIATE RELEASE: December 11, 2003

Sun Gives Way to Greenhouse Gases as Major Influence on Earth’s Changing Climate


David Hosansky
UCAR Communications
Telephone: (303) 497-8611
E-mail: hosansky@ucar.edu
AGU Press Phone: 415-905-1007

Cheryl Dybas, NSF
Washington, D.C.
Telephone: 703-292-7734
E-mail: cdybas@nsf.gov

SAN FRANCISCO—Natural variations in solar energy and volcanic emissions do not explain the climate changes of the last few decades of the 20th century, according to scientists at the American Geophysical Union’s annual meeting in San Francisco this week.

Increasing greenhouse gases in Earth’s atmosphere are now the dominant influence behind rising global temperatures, said Caspar Ammann of the National Center for Atmospheric Research (NCAR) at an AGU press briefing today. Ammann used improved climate reconstructions and advanced climate model results to show recent climate warming clearly exceeds the range of natural climate variations over the past millennium. Ammann’s research is funded by the National Science Foundation, NCAR’s primary sponsor.

Scientists have thought that changes in the Sun’s activity have been responsible for some part of past climatic variations. However, useful measurements of solar energy are limited to the last 25 years of satellite data. This record is not long enough to confirm potential trends in changes over time. To estimate solar energy over a longer period, scientists have used tentative connections between the measured solar activity and sunspots or the production of certain particles in the Earth’s atmosphere (such as carbon-14 and beryllium-10).

When Ammann applied the solar estimates in simulations by the NCAR coupled Ocean-Atmosphere General Circulation computer model, he found that the climate system contained a clearly detectable signal from the Sun. However, smaller rather than larger background trends in the Sun’s emitted energy are in better agreement with the long-term climate history, as obtained from proxy climate records, such as tree ring data.

The influence of the Sun on Earth’s climate shifted from being one of the major natural regulators of past climate to a much more minor role during the predominantly human-modified climate of the last few decades.

“When we subtracted human activities in the form of increasing greenhouse gases in the 20th century, we found the greatest discrepancies between the model results and observations over the past millennium increased dramatically at the end of the century,” said Ammann.

Drew Shindell of NASA's Goddard Institute for Space Studies and Thomas Crowley of Duke University also spoke at the press briefing.

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