Cosmic Rays, Solar Activity Don't Cause Climate Change

By Laura Dattaro
Published: November 11, 2013

The sun is not causing global warming, according to new research. (Hemera Technologies/Getty Images)

Changes in solar activity contribute little to the increasing global temperatures observed in the 20th century, authors of a new study say.

In public discourse about climate change, some have proposed that natural fluctuations in the sun’s activity can account for the observed global warming, in opposition to increasing scientific evidence that carbon dioxide emissions and other human activities drive the changing climate. But new research by physicists Terry Sloan of the University of Lancaster and Arnold Wolfendale at the University of Durham refute this claim, demonstrating that solar activity contributed minimally to Earth’s changing temperatures. 

The sun passes through regular 11-year activity cycles, which affect the rates at which streams of charged particles called cosmic rays hit the Earth. Past theories have proposed that cosmic rays hitting particles in the atmosphere can encourage cloud formation, helping the Earth to cool. In times of increased solar activity, then, fewer clouds would form as fewer cosmic rays made their ways to Earth.

But the new study finds little correlation between sun activity and an increasingly warmer planet. Using two data sets going back to 1955, the study’s authors found that a small correlation between cosmic rays and global temperatures occurred every 22 years, but the rate of change did not line up directly with change in temperature. They concluded that changes to cosmic rays could have contributed about 14 percent of measured changes in temperature since 1960.

“We cannot explain [the correlation] and it might be a lucky coincidence,” Sloan told

The research also reviewed previous studies, including the authors’ own work, and concluded that changing solar activity, either directly or through cosmic ray deflection, contributed 10 percent or less to the total increase in global temperature in the 20th century.

So how did the theory form that cosmic rays and cloud formation caused climate change? Sloan points to an early study that found a decrease in cloud cover at a time of maximum cosmic rays. “They did a back-of-the-envelope calculation to show that all global warming was due to changes in cloud cover,” Sloan said. “Other people as well as us have looked at this hypothesis and found that it does not fit lots of other data. Hence the correlation they saw must have been a coincidence.”

The study was published in the journal Environmental Research Letters.

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