Sunspots blamed for much change


SUNSPOTS are relatively dull areas or "spots" on the solar disc, which increase and decrease in both size and number over a recurring cycle that lasts about 11 years. It is known that sunspot activity also reflects a cyclical variation in the radiant energy of the sun, and for years there have been continuing efforts to establish links between this solar cycle and events on Earth.

Some researches have tried to relate sunspots to changes in temperature, pressure or rainfall on the Earth's surface, while others have claimed that they are responsible for plagues in China, for the ups and downs of the fashionable hem line, or for the number of Republicans elected to the US Senate.

First indications that there might be some substance to these theories, at least as far as the weather was concerned, came in 1987 when American meteorologists reported a relationship between sunspot activity and the rise and fall of temperatures 15 miles or so above the polar ice caps. Subsequent work uncovered similar temperature oscillations elsewhere in the upper atmosphere, and indeed it appeared that here and there these temperature anomalies might percolate right down to ground level, causing an unseasonal chill in the US southern states, for example, around the solar maximum.

The first challenge was to find a mechanism by which is relationship might be explained. The total energy output of the sun varies by less than 0.1 per cent between solar maximum and minimum, and this was insufficient to cause the observed effects. Then someone noted that the ultraviolet radiation from the sun varied by much more than this proportion over a solar cycle; further investigation suggested that the 1 per cent increase in ultraviolet solar radiation known to occur at the time of maximum energy output in the 11 year cycle could generate 2 per cent more stratospheric ozone, and that this, in turn, would absorb more sunlight, resulting in a slight warming of the upper atmosphere.

The next step was to discover how, if at all, this warming might affect our weather. Just recently, Dr Joanna Haigh, of the Imperial College, London, had incorporated the cyclical sunspot warming into a computer model of the atmosphere using a method which imagines the world to exist in a perpetual January to avoid the complications of seasonal ups and downs of temperature. The results suggest that the increase in UV radiation near the solar maximum ultimately causes North Atlantic winter storms to follow a more northerly track than usual at those times. So it seems as if sunspots may be a factor in our changeable Irish weather after all.