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Last Updated on Sunday, 02 May 2010 12:26
Published on Friday, 25 February 2005 00:00

On Wednesday August 11th, the awesome phenomenon of a total solar eclipse will briefly touch the lives of millions of people across Europe and Asia. But subtle changes in the Sun may have a much more long-term impact on our planet as Earth's climate responds to changing patterns of solar activity. The day before the eclipse, Professor Nigel Weiss of the University of Cambridge will tell astronomers at the UK National Astronomy and Solar Physics Meetings in Guernsey about growing awareness of the connection between solar activity and the controversial issue of climate change.

Although greenhouse gases released by burning fossil fuel are almost certainly responsible for the rapid current rate of global warming, there have been many previous episodes of climatic change that cannot be explained by such human activity. "Variable behaviour of the Sun is an obvious explanation" says Professor Weiss, "and there is increasing evidence that Earth's climate responds to changing patterns of solar magnetic activity."

The most obvious magnetic features on the Sun are sunspots - temporary dark blemishes on the Sun's yellow disc. They occur where there are strong magnetic fields and are dark because they are cooler than the normal surface of the Sun. When the Sun becomes more magnetically active, increasing numbers of sunspots form. It has long been known that the number of sunspots rises and falls over an average period of eleven years, though these cycles are not strictly regular.

Comparison between the recorded incidence of sunspots and climatic records suggests that the Sun's magnetic activity has been a major influence on climate until recently, when greenhouse gases have taken over as the main agent of change. The "Maunder Minimum" in the seventeenth century, when sunspots almost completely disappeared for 70 years, coincided with the coldest interval of the Little Ice Age. By contrast, the Sun was very active during the Medieval Warm Period, when Greenland was colonised by Norsemen.

"Grand minima", similar to the Maunder Minimum, have recurred over the past 10,000 years at least. The evidence for this is captured in the composition of Earth's polar ice caps. Cosmic ray particles striking the Earth create unstable isotopes such as carbon-14 (used for carbon-dating) and beryllium-10, which accumulate in the polar ice. As solar activity waxes and wanes, there are also slight variations in the amount of energy radiated by the Sun and in the strength of the solar wind. That in turn affects the cosmic ray particles and the formation of the isotopes.

The beryllium-10 and carbon-14 records show long-term changes superimposed on the basic solar cycle, typically taking place over a timescale of 200 years. Both the 11-year cycle and its long-term variability have an impact on Earth's climate - but the mechanism by which this influence works is not yet understood.

So far, there is no compelling evidence that cause and effect are directly linked in a straightforward manner. The total amount of energy radiated by the Sun does not change enough to alter Earth's temperature significantly, and other possible factors, such as the effects of ultraviolet on ozone concentration, or the influence of cosmic rays on cloud formation, are hard to estimate. But scientists know that oscillations can take place in Earth's atmosphere and oceans with many different frequencies. These could be a powerful mechanism acting to amplify the effects of small changes in the Sun's radiation.


Prof. Nigel Weiss
Dept. of Applied Mathematics and Theoretical Physics
University of Cambridge.
Phone: (0)1223 337910 Fax: (0)1223 337918
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