By Richard Black
Environment correspondent, BBC News website
In February 2007, depending on what newspaper you read, you might have seen an article detailing a "controversial new theory" of global warming.
The idea was that variations in cosmic rays penetrating the Earth's atmosphere would change the amount of cloud cover, in turn changing our planet's reflectivity, and so the temperature at its surface.
This, it was said, could be the reason why temperatures have been seen to be varying so much over the Earth's history, and why they are rising now.
The theory was detailed in a book, The Chilling Stars, written by Danish scientist Henrik Svensmark and British science writer Nigel Calder, which appeared on the shelves a week after the Intergovernmental Panel on Climate Change (IPCC) had published its landmark report concluding it was more than 90% likely that humankind's emissions of greenhouse gases were warming the planet.
In truth, the theory was not new; Dr Svensmark's team had proposed it a decade earlier, while the idea of a cosmic ray influence on weather dates back to 1959 and US researcher Edward Ney.
The bigger question is whether it amounts to a theory of global warming at all.
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http://news.bbc.co.uk/2/hi/science/nature/7092655.stm