There's a little black spot on the sun today

Dark patches on the stellar surface may cause freak weather on Earth.

It's been hot, this summer, in Britain. The source of that heat is of course the Sun, but most explanations of why we have had such a long hot summer (and of why the world's climate as a whole seems to be getting warmer) ignore the Sun and look to more down-to-earth causes, such as the increasing concentrations of greenhouse gases in the atmosphere. However, it seems that it may no longer be possible to ignore the Sun itself. Not just the climate generally, but the strength and frequency of thunderstorms, may be affected by variations in the energy output of the Sun.

The Sun is not constant; it is a massive thermonuclear reactor, turbulent on a literally astronomical scale. One of the great unresolved puzzles of meteorology is whether there is a significant link between variations in solar activity and the Earth's weather. The best known aspects of the Sun's variability are the sunspots: dark cool patches with a diameter of several thousand kilometres that appear on the surface of the Sun and last about a week. New research is shedding light on how sunspots could influence the Earth's weather.

For more than 150 years, astronomers have known that there is a regular pattern to the numbers of spots appearing on the Sun. The clearest cycles repeat over intervals of 11, 90 and around 200 years. The fact that similar periods can be found in records of the Earth's weather is ascribed by some meteorologists as clear evidence of a link between solar activity and climate change.

But the majority of meteorologists and climatologists are not convinced. They see the evidence of weather being influenced by sunspots as the product of faulty statistical analysis.

The challenge facing the small vociferous minority of "solar activists" is to explain how small changes on the Sun can have much greater impact on the Earth. Satellite measurements since the late 1970s have shown that the total amount of energy emitted by the Sun rises and falls with the number of sunspots by less than 0.1 per cent. This variation is far too small on its own to explain the changes in the world's climate over the past 100 years or so.

Is there a mechanism whereby variations in the Sun's output can be amplified by the atmosphere? Two possibilities have been proposed. The first relates to the fact that much of the variability in the Sun's output of energy is concentrated in the ultraviolet (UV) region. The second is associated with changes in the Sun's magnetic field.

Although only 1 per cent of the Sun's energy is emitted in the UV range, some 20 per cent of the fluctuation in output is concentrated in this region. This energetic radiation is absorbed high in the atmosphere by oxygen and ozone and is also instrumental in forming ozone at these levels. These photochemical processes may exert a disproportionate influence on the weather.

Joanna Haigh, of the Space and Atmospheric Physics Group at Imperial College, London used a computer model of the atmosphere to calculate how the amount of solar radiation entering the lower atmosphere varies with solar activity, as a result of changes in ozone levels in the stratosphere caused by the changing UV flux. The results show that the amount of radiation reaching the lower atmosphere at middle and high latitudes in winter is less when solar activity is high. This amplification of the sunspot cycle could alter the global circulation patterns that define the movement of winter weather systems. This may help to explain why storm tracks across the North Atlantic for much of this century have been on average 2-5 degrees further south at sunspot maxima than at sunspot minima.

It is not only the variation in the amount of UV reaching the lower atmosphere that matters but also its impact on the photochemistry at these levels. Modelling work by Ralf Toumi at the Department of Physics at Imperial College and Slimane Bekki and Kathy Law at the Department of Chemistry at the University of Cambridge suggests that a chain of reactions resulting from reductions in ozone levels leads to an increase in cloudiness and hence to cooling of the climate. They conclude that perturbations in ozone levels will have a much greater impact than previously thought: yet another means by which solar changes can be amplified by the atmosphere.

Since the beginning of the century it has been known that sunspots travel in pairs across the face of the Sun and the magnetic field associated with these pairs reverses during each successive 11-year cycle. This gives rise to a 22-year "double" sunspot cycle. Interestingly, a periodicity of around 20 years is a common feature in weather records. For instance, it turns up in not only temperature records for central England and in average global figures, but also the drought statistics for the western United States and measurements of snow laid down over the centuries in the Greenland icecap.

The 20-year cycle may be due to solar effects, but there is a rival 18.6- year variation in the lunar tides. It remains an unresolved issue as to which could be affecting the climate. There is no shortage, however, of theories as to why magnetic effects could affect the Earth's weather. The number of high-energy particles emitted by the Sun would change as would the influence of the Sun's magnetic field on the flux of cosmic rays reaching the Earth. Both these effects alter the number of energetic particles streaming down into the atmosphere in polar regions. This could have an impact on the chemistry of the stratosphere. So just as with variations in UV flux, small changes in the Sun's magnetic field may be amplified in the upper atmosphere.

But the Sun's fluctuating magnetic field may affect the strength and frequency of thunderstorms. First proposed by Ralph Markson at the Massachusetts Institute for Technology (MIT) in the late 1970s, this mechanism could allow small changes in the flux of energetic particles entering the atmosphere to have an instantaneous and proportionately much greater impact on the weather by influencing the electric circuit in the Earth's atmosphere that is maintained by thunderstorms around the world.

In 1992, Earle Williams, also at MIT, noted a strong relationship between global warming and worldwide thunderstorm activity. In the tropics, where the majority of thunderstorms occur, the incidence of lightning roughly doubles with every 1 degree Centigrade rise in temperature. By observing a fundamental property of the Earth's electric field, known as the Schumann Resonance, it may be possible to measure not only changes in the global temperature but also whether solar activity modulates the climate.

Solar activity theories underline the complexity of the global climate system. Until we know more about how trace atmospheric constituents, photochemistry and the Earth's electric and magnetic fields influence the climate, we cannot assume that current relatively simple computer models are providing an adequate picture of how the climate may respond to both natural and human perturbations. In the meantime, new evidence may yet show that the solar activists were right all along.