Climate change is a contentious issue among the public. One of the main arguments made by people who claim that climate change is not caused by humans states that recent global warming is a result of changes in solar activity. Indeed, a 2007 broadcast on Channel4 titled "The Great Global Warming Swindle" tried to make exactly that case.

©Ron Almog

This case is based on the idea that changes in solar activity influence cloud formation, which influences the climate. The original concept dates back at least as far as a 1975 paper published by the American Meteorological Society, but it's recently been revived as an alternative explanation to the recent rise in global temperatures. This idea has been pushed by papers that Henrick Svensmark has published in the last decade.

The basic sun-climate argument centers on the impact that cosmic rays have on the earth's atmosphere. These cosmic rays lead to ionization that, in turn, leads to an increase in highly reflective cloud cover, which lowers the average global temperature. Because increased solar activity causes a decrease in the number of cosmic rays that reach the earth, when solar activity increases, global temperatures increase and vice-versa. Numerous studies have been published on this relationship, some supporting it, others refuting it. The relationship between solar activity and cosmic ray intensity has been clearly documented, but the relationship between cosmic ray intensity and cloud cover is widely debated.

To test the hypothesis that changes in cosmic rays due to solar activity are the cause of recent global warming, a paper published in the Institute of Physics' Environmental Research Letters compared cloud cover data with solar activity data for the past 22 years (two 11 year sunspot cycles). They find that fits for the entire 22 years are very poor, but fits for cycle 22 (1985-1996) suggest that solar activity explains greater than 30 percent of the variation in cloud cover. Of course, we all know that correlation does not imply causality, so they dug a little deeper.

To investigate whether the relationship during cycle 22 was causal, they looked at variations in the correlation with latitude. At higher latitudes, the earth's magnetic field deflects fewer cosmic rays than it does at low latitudes. Thus, if the correlation between solar activity and cloud cover is due to cosmic ray caused ionization, this correlation should be greater at higher latitudes.

The researchers found that there was almost no correlation between latitude and the correlation between cloud cover and solar activity. This suggests that, while there may be a relationship between solar activity and cloud cover during cycle 22, it is not due to cosmic rays.

The study also looks at two other tests of the cosmic ray hypothesis and found it lacking. Those that are interested are encouraged to read the entire paper (open access).