54 Piscium is a sun-like star that dimmed a little. Photo: Space Telescope Science Institute

A programme to monitor large numbers of stars resembling the Sun could give a new perspective on solar effects on the Earth's climate, according to Eugene Parker of the University of Chicago. He was summing up the conference on "The Solar Cycle and Terrestrial Climate" held last week on the Spanish island of Tenerife. Two hundred solar and climate scientists from many countries part in the meeting, which was sponsored by the European Union and the European Space Agency.

We know from observations of a few sunlike stars that one of them lost 0.4 per cent of its luminosity in only a few years," Parker said. "If the Sun did that, it could quickly reproduce the cold conditions of 300 years ago, when solar activity was much reduced in the interval we call the Maunder Minimum. To find out what the Sun might do one day, we should set up an automated system to watch a thousand sunlike stars."

As the father of the theory of the solar wind, which has guided interplanetary space research for 40 years, Parker was given the difficult task of drawing threads together from more than forty papers at the Tenerife conference. Their subjects ranged from the origin of the Sun's magnetic field to computer modelling of the response of rainfall to climate change.

Parker commended new efforts to understand in greater detail the possible mechanisms of solar effects on climate. These include the variations in ultraviolet, visible and infrared radiation, and also in cosmic rays from the Galaxy, which are reduced when the Sun is most active and the solar wind is gusty. In the atmosphere, cosmic rays may influence the occurrence of thunderstorms, and perhaps also the formation of low-level clouds.

"I am very pleased to hear about the proposed experiment at CERN in Geneva, to investigate the cosmic-ray effects," Parker said. "We need laboratory physics to make sense of cloud formation and other consequences of what we observe in space and on the Earth."

Parker agreed with the opinion of some speakers that increasing activity in the Sun contributed to the global warming in the early part of the 20th Century, whilst changes in recent decades could not be explained by continuing solar changes. Effects due to manmade greenhouse gases and aerosols remained the likely cause of recent global warming.

Commenting on the current computer projections of future climate change, Parker noted that the models used for this purpose were probably not yet rich enough to capture all the essential complexities of weather and climate. "Ultimately we need a computer model that can give us a reliable estimate of the consequences of a specified addition of greenhouse gases or a given change in solar brightness," Parker said.