The now conventional view on global warming, as stated by the Intergovernmental Panel on Climate Change, is that most of the warming recorded in the past 50 years has been caused by emissions of manmade greenhouse gases. But some scientists argue that the Sun might have a significant influence on changes to the Earth's climate, pointing out that in centuries past there has been a close correlation between global temperatures and solar activity.
According to Svensmark, cosmic rays seed low-lying clouds that reflect some of the Sun's radiation back into space, and the number of cosmic rays reaching the Earth is dependent on the strength of the solar magnetic field. When this magnetic field is stronger (as evidenced by larger numbers of sunspots), more of the rays are deflected, fewer clouds are formed and so the Earth heats up; whereas when the field is weaker, the Earth cools down.
Indeed, Chris Folland, a climate researcher at the UK's Met Office, says it is not clear to what extent cosmic rays could really enhance cloud formation, given the vast numbers of naturally occurring particulates within the atmosphere that could act as cloud-condensing nuclei. He also says that even if there is a noticeable effect on cloudiness, this effect could be either positive or negative, arguing that cosmic rays might be expected to have a larger affect on higher-altitude clouds, which tend to warm the planet by preventing radiation from escaping into space. "Low-level clouds generally cool the surface climate, but it’s not clear why they should be preferentially affected by cosmic rays," he adds, "given that there is some effect on overall cloudiness."
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