Climate change is chiefly man-made

Professor Nigel Weiss, an expert in solar magnetic fields, has rebutted claims that a fall in solar activity could somehow compensate for the man-made causes of global warming.

The Cambridge Emeritus Professor in Mathematical Astrophysics stressed today that climate change was still primarily a man-made problem, echoing an announcement by an international panel of scientists.

In a report released in Paris today, the Intergovernmental Panel on Climate Change (IPCC) says that it is "very likely" the climatic changes around the world are caused by humans. The report was also expected to underscore the need for concerted international action to reduce the worst impacts of climate change, stressing that a "business as usual" approach to reducing emissions would lead to unacceptable risks.

A minority of commentators have suggested that solar activity is a more important cause than human, and that a fall in solar activity would lead to cooling that could cancel out the effects of greenhouse gases.

While there have been reports that Professor Weiss backs this view, he stressed today that this was untrue and that the man-made causes of global warming were of grave and far greater concern.

"Although solar activity has an effect on the climate, these changes are small compared to those associated with global warming," he said. "Any global cooling associated with a fall in solar activity would not significantly affect the global warming caused by greenhouse gases.

"This is of course a controversial issue and there is a vocal lobby arguing against the link between anthropogenic gas emissions and climatic change. However I share the view of the majority of the scientific community that the evidence for such a link and thus the occurrence of man-made global warming is significant and a matter of grave concern."

Professor Weiss is an expert on magnetic activity in stars like our own sun, where sunspots are the site of strong magnetic fields. Solar activity varies cyclically with a period of about 11 years and is occasionally interrupted by Grand Minima, like the Maunder Minimum in the 17th century, when scarcely any spots appeared.

From measured variations in abundances of cosmogenic isotopes such as Be-10, which can be measured in polar ice cores, we know that Grand Minima have recurred irregularly for at least the last 50,000 years. For the past 50 years, solar activity has been abnormally high, but such Grand Maxima do not last forever.

Professor Weiss and his colleagues predict that the current boom will be followed by a slump, though they cannot forecast quite when this will happen, or how deep the ensuing Minimum will be. Satellite measurements show that the solar irradiance decreases by 0.1% from sunspot maximum to sunspot minimum, causing a reduction of about 0.1 degrees Celsius in average global temperature. A Grand Minimum might cause a similar reduction but such an effect would be small compared with the rise in temperature caused by man-made drivers of the climate.