Solar activity and global warming

Print edition : December 13, 2013

A NEW study has found that changes in solar activity contributed no more than 10 per cent to global warming in the 20th century. Terry Sloan at the University of Lancaster and Sir Arnold Wolfendale at the University of Durham found that neither changes in the activity of the sun nor the sun’s impact in blocking cosmic rays could be a significant contributor to global warming. The results have been published in the journal Environmental Research Letters.

Changes in the amount of energy from the sun reaching the earth have previously been proposed as a driver of increasing global temperatures, as has the sun’s ability to block cosmic rays. It has been proposed that cosmic rays may have a role in cooling the earth by encouraging clouds to form, which subsequently reflect the sun’s rays back into space. According to this proposal, in periods of high activity, the sun blocks some cosmic rays from entering the earth’s atmosphere so that fewer clouds form and the earth’s surface temperatures rise. In an attempt to quantify the effect that solar activity —whether directly or through cosmic rays—may have had on global temperatures in the 20th century, Sloan and Wolfendale compared data on the rate of cosmic rays entering the atmosphere, which can be used as a proxy for solar activity, with the record of global temperatures going back to 1955.

They found a small correlation between cosmic rays and global temperatures occurring every 22 years; however, the changing cosmic ray rate lagged behind the change in temperatures by between one and two years, suggesting that the cause might not be down to cosmic rays and cloud formation but might be due to the direct effects of the sun. By comparing the small oscillations in the cosmic ray rate, which were taken from data from two neutron monitors, and temperature with the overall trends in both since 1955, Sloan and Wolfendale found that less than 14 per cent of the global warming seen during this period could be attributable to solar activity.

Furthermore, the researchers reviewed their own previous studies and surveyed the relevant literature to find other evidence, if any, of a link between solar activity and increasing global temperatures. Their findings indicated that overall the contribution of changing solar activity was even less and could not have contributed more than 10 per cent to global warming in the 20th century. Events such as the Chernobyl nuclear disaster and nuclear weapons testing would have been expected to have affected aerosol production in the atmosphere, but no such effects could be seen.