Professor Nigel Weiss, 2007 winner of the Royal Astronomical Society (RAS) Gold medal, is rebutting claims that a fall in solar activity could cancel out the effects of man-made global warming. In a lecture at the RAS-sponsored National Astronomy Meeting in Preston, Professor Weiss, who is Emeritus Professor in Mathematical Astrophysics at the University of Cambridge, described how solar activity was an important factor in past climate change but that current global warming is very much driven by human activity – specifically the emission of greenhouse gases.
Solar magnetic activity manifests itself in sunspots, flares and coronal mass ejections, which give rise to magnetic storms on earth. The incidence of sunspots, which are the sites of strong magnetic fields, varies cyclically with a period of about 11 years. This cyclic pattern is occasionally interrupted by grand minima, like the Maunder Minimum in the 17th century, when scarcely any spots appeared. From variations in Carbon-14 (which is preserved in trees) and Beryllium-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 in fact been abnormally high, but such grand maxima do not last forever. The current boom will inevitably be followed by a slump, though it is impossible to forecast quite when this will happen, or how deep the ensuing grand minimum will be.
Although sunspots are themselves dark, they are accompanied by bright faculae. Satellite observations show that the solar output of radiation (irradiance) is actually greater at sunspot maximum than at sunspot minimum, though the change of only 0.1% is slight, corresponding to a variation of 0.1 degrees Celsius in average global temperature. A grand minimum might lead to a similar reduction in irradiance.
Of course, the effects of solar variability on the earth's climate, which is a very complex system, could be amplified by other processes. For instance, the Sun's ultra-violet emission doubles from sunspot minimum to maximum, and ultra-violet radiation affects the ozone content of the stratosphere, which is coupled to the troposphere below it and so influences the overall climate. Again, it has been suggested that solar modulation of the flux of galactic cosmic rays affects cloud formation, altering the amount of radiation the Earth reflects back into space and affecting climate (though this hypothesis is very shaky). There might also be a coupling between variations in solar activity and natural oscillations in the atmosphere or ocean.
The extent of any such climatic modulation can be estimated from the long-term record of global temperatures. Until the beginning of the last century, variations in solar activity, along with aerosol emission from volcanoes, dominated climatic variability. There is persuasive evidence that grand minima were indeed associated with colder periods and grand maxima with warm periods. During the past millennium, there were several such maxima and minima, with associated fluctuations of around 0.3 degrees Celsius in global temperature. But these changes are significantly smaller than the increase of almost one degree over the last hundred years so it follows that solar activity is not a major contributor to current global warming.
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 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.”
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