Sun 'minor player' in climate change

Human action is now affecting the climate more than the Sun

By environment correspondent Alex Kirby

Research into the Sun's role in recent warming of the Earth's atmosphere indicates that it probably plays a relatively small part.

The research, by two Danish meteorologists, suggests another factor is involved - probably human activity.

Those who challenge the consensus view that fossil fuel burning is leading to global warming have argued that increases in the intensity of the Sun is a far likelier cause.

The Danish research, reported in the magazine New Scientist, means it is now harder to absolve humanity of blame for what is happening.

The theory that the Sun was playing a dominant part had rested on the correlation between the sunspot cycle and temperatures in the Northern Hemisphere. This seemed to account for most of the warming seen until 1985.

Centuries of support

In 1991, Knud Lassen of the Danish Meteorological Institute in Copenhagen and his colleague Eigil Friis-Christensen found a strong correlation between the length of the solar cycle and Northern Hemisphere temperature changes.

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At first, they used sunspot and temperature measurements from 1861 to 1989, but they found later that climate records dating back four centuries supported their findings.

The relationship they had identified seemed to account for almost 80% of the measured temperature changes over this period.

But now Lassen and another colleague, Peter Thejll, an astrophysicist, have updated the research.

They found that, while the solar cycle still accounts for about half the temperature rise since 1900, it fails to explain a rise of 0.4 degrees Celsius since 1980.

Greenhouse effect suspected

Peter Thejll said: "The curves diverge after 1980, and it's a startlingly large deviation. Something else is acting on the climate."

He and Dr Lassen suspect that emissions from the burning of fossil fuels are responsible, saying "it has the fingerprints of the greenhouse effect".

Richard Betts, of the United Kingdom's Hadley Centre for Climate Prediction and Research, agreed.

He said: "It sounds like an actual piece of evidence for greenhouse warming. Any natural effect would swamp the small early changes, so you'd expect to see the larger changes more recently."

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But Professor Tom Wigley, of the National Center for Atmospheric Research in Boulder, Colorado, who criticised Dr Lassen's original research, was not convinced by the update.

He said that no-one had provided a convincing physical explanation for the correlation between the sunspot cycle and temperature.

Room for both

And while he accepted that solar effects might have dominated until about 1950, he did not think they did so as late as 1980.

The authors recognise the controversial nature of the subject, and say they hope their findings will move climate researchers towards a more balanced view.

Peter Thejll said: "It became political. We're now seeing that the Sun plays a role, and something in addition to the Sun. Maybe that will help people see there is room for both."