Earth's climate records feature many fluctuations apparently linked to solar activity but the physical processes at work connecting the Sun and climate are not yet properly understood.

Satellites have measured how the Sun's brightness has changed in the past two decades, and these data can be compared with high precision records of Earth's temperatures over the same period to throw light on the problem.

But it is difficult separating solar effects from other factors influencing our climate over different time-scales, such as major volcanic eruptions and the increase in greenhouse gases in the atmosphere due to human activities.

For example, the most recent increase in the Sun's activity on its regular 11-year cycle corresponded with a rise of 0.1 degree C. By comparison, the eruptions of El Chichon and Pinatubo cooled Earth by 0.2 degree C for a short time.

A new study proposes that, on top of the change due to its 11-year cycle, the Sun has brightened steadily during the past two decades. If true, the suggested trend of 0.05% per decade would account for half or more of the 0.3 degrees warming currently attributed to greenhouse gas increases since 1980.

"This study is very controversial," says Judith Lean. "It relies on splicing together solar irradiance datasets made by different instruments flown on various spacecraft. Because the datasets do not have the same absolute scale, they must be cross-calibrated to construct the long-term record needed for studying climate change.

"Drifts in the instrument sensitivities must be properly clarified as well, to avoid mistaking spurious trends for real solar brightness changes. For this purpose, the recent study used observations previously reported to suffer from known instrumental effects but did not take these effects into account."

Dr Lean then cites another study, which brought together the various solar brightness datasets in a different way and compensated for instrumental drifts.

It concluded that there was no general brightening of the Sun over the past two decades. This result is consistent with what solar physicists would expect from their understanding of the Sun's magnetism.

Sunspots and faculae, both magnetic features on the Sun's surface, respectively reduce and enhance the Sun's overall brightness and independent records of sunspots and faculae show no underlying upward trends during past decades.

The same is true of numerous other indicators of the Sun's behaviour that have been closely monitored. This alternative study concluded that long-term solar brightness changes are not a significant cause of recent global warming.

"Other claims in recent years have also exaggerated the role of the Sun in climate change" warns Judith Lean.

As an example, she quotes a study published in 1991 that reported a close tie between the actual length of the solar cycle (which averages 11 years but varies from 9 to 15 years) and surface temperatures in the Northern Hemisphere throughout the entire twentieth century.

If true, it would mean that human influences, such as increasing concentrations of greenhouse gases, have contributed little or nothing to the approximately 0.8 degree C warming of Earth's surface since 1885.

But subsequent examination of additional data gathered over a longer period of time, including the pre-industrial era, have made the real connection between solar brightness and climate change clearer.

"Temperature changes in concert with solar activity are indeed apparent during the past millennium," reports Dr Lean, "but are typically of order 0.2 to 0.5 degrees C on time scales of hundreds of years. Since 1885, global warming in response to changes in the Sun's brightness is now thought to have been less than 0.25 degrees C."

"To really resolve the controversies, we need longer and more precise monitoring of the solar brightness to determine whether or not there are long-term trends," she concludes. To that end, a new generation of solar radiometers was launched into space in January 2003 on board the Solar Radiation and Climate Experiment (SORCE).

Related Links
UK/Ireland National Astronomy Meeting
Space Research Naval Research Laboratory,
SORCE at NASA
SORCE at Colorado
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