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Keywords: Climate change, global warming, science, science policy Title: The Chilling Stars Author: Henrik Svensmark and Nigel Calder Publisher: Icon Books/Totem Books ISBN: 1840468157 Verdict: An interesting but controversial read

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Despite the claims of some politicians and scientists, the science of climate change is far from settled. While the mainstream seems to have accepted that global warming is real and that it is caused mainly by man-made carbon dioxide (CO2) emissions, there are many scientists who dispute what is still a speculative theory that depends heavily on mathematical models rather than solid theoretical understanding and empirical evidence. However, the political climate is such that discussion of these issues is difficult, and those who dispute the official position (as exemplified by the IPCC Summary Reports) are likely to be attacked as 'climate change deniers' (note the deliberate allusion to 'holocaust deniers').

For those who accept that global warming is taking place, there are a number of alternative theories as to the causes. One of the leading alternative theories is that proposed by Danish physicists Henrik Svensmark and Eigil Friis-Christensen, which suggests that it is variations in cloud cover that have a large effect on climate, and that clouds are seeded by cosmic rays, which are in turn influenced by the Sun's magnetic field.

In 'The Chilling Stars', science writer Nigel Calder and Henrik Svensmark present the first popular exposition of the theory. In doing so they present not just the bare bones of the theory, but also show how it can explain both past climate events and the present situation. In particular the authors make sure that a whole range of supporting evidence is used to establish historical climate trends (including events such as the Medieval Warm Period and the Little Ice Age) and to tie this in with records of solar activity.

As well as presenting the correlation between the historical climate record and solar activity, there is also an explanation of the physical process by which cosmic rays kick-start cloud formation. The theory is explained clearly, along with details of the experimental evidence used to support it. There is also a clear explanation of how the electromagnetic activity of the sun controls the rate at which cosmic rays reach the earth.

While it's Svensmark's theory that is at the heart of the book, much of the supporting evidence comes from other researchers and scientists. The work of Nir Shaviv, Jan Veizer and others is also mentioned in some detail, illustrating both the cross-disciplinary nature of the theory - which takes in cosmology, particle physics, paleoclimatology and meteorology - and the fact that there is a degree of support for the theory from a range of academics working in different countries and different fields.

Given the controversial nature of the subject, it would have been easy to write this book as a piece of scientific polemic. But on the whole, the emphasis is more on the merits of the theory - which Svensmark calls cosmoclimatology - than on politics and polemics. In some respects more could have been made of this, but it's clear that the authors were more intent on making a case for the theory rather than in explicitly attacking the 'consensus' view.

For those who believe that the argument about the causes of climate change have been settled may find this a difficult book to read. But those who retain an open mind may find this an interesting read, even if it is only to confirm that the science is far from being settled.