Computer models of Earth's climate have consistently linked long-term, high-magnitude variations in solar output to past climate changes. Now a closer look at earlier studies of the Sun and Sun-like stars casts doubt on the evidence of such cycles, their intensity, and their possible influence on Earth's climate. The findings, by a solar physicist and two climate experts, appear in the October 1 issue of the journal Science.

The authors write, ". . .long-term irradiance variations used in climate models in the past decade may be a factor of 5 [five times] larger than can be justified. The full impact of this changed outlook on attempts to explain past climate variations and estimates of climate sensitivity to external forcing remains to be seen."

Scientists have attributed observed climate changes to a combination of natural variations and human activities. Computer models of global climate reproduced an observed global warming during the first half of the 20th century when two solar influences were combined: a well-documented 11-year sunspot cycle and the decades-long solar cycles now in dispute. A more pronounced warming observed during the century"s late decades is attributed to greenhouse gases accumulating in Earth's atmosphere and is not part of the new study.

"Removing these long-term solar cycles from the input to climate models takes away about a tenth of a degree [Celsius] of the early 20th century warming," says Tom Wigley, a climate expert at the National Center for Atmospheric Research (NCAR) and a co-author of the paper. "Although this makes it harder to explain the warming, the difference falls within the noise range for natural variability. This suggests that other influences on past climate changes may play a greater role than the solar one."

Peter Foukal of Heliophysics Inc., Gerald North of Texas A&M University, and Wigley authored the paper. The National Science Foundation, NCAR's primary sponsor, and NASA funded the research.

Climate models have also linked long-term changes in solar energy to preindustrial climate changes, such as the so-called Little Ice Age of the 17th and 18th centuries.

"Removing or reducing the long-term solar influence is most important for understanding the Little Ice Age," says Wigley. Although this cooling occurred primarily in Europe, it has commonly been attributed to a lethargic Sun.

The authors scrutinize a considerable range of previous evidence related to solar cycles, including:

• brightness variations of selected stars
• whether the selected stars are actually Sun-like
• the long-term relation between stellar brightness and magnetic field
• the magnitude of the supposed solar variations

The 11-year sunspot cycle is not questioned in the Science paper, but its effect alone is "probably too little for a practical influence on climate," the authors write. They also briefly consider possible influences of ultraviolet and cosmic ray fluxes in Earth's climate.

The authors allow that long-term brightness variations of the Sun may exist, but more convincing evidence is needed, they say. New technologies now available can provide better data for understanding Sun-climate relations, they conclude.

Opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.