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cooling events, as revealed by "ice-rafting", where ice sheets have transported northern grit south for great distances and deposited it on the Atlantic seabed. The authors link these episodes to times when the cosmic-ray flux was higher, as shown by varying traces of radioactive beryllium-10 in ice cores extracted in Greenland and Antarctica.

Also mentioned is the work of Israeli astrophysicist Nir Shaviv, who has correlated variations in the cosmic-ray flux to the solar system's orbit around the centre of the Galaxy and its passage through the Galaxy's four great spiral arms. In these crowded stellar neighbourhoods (such as the Orion Arm, which is where we currently are) there are more cosmic rays and thus the Earth tends to become cooler.

There is much to fascinate in this book. As well as physics and astronomy, it invokes Medieval and Roman history, describing times when high Alpine passes, such as the Schnidejoch, were accessible in the warmer conditions, as well as a later period called the Little Ice Age, when reduced solar activity (as revealed by lower numbers of observed sunspots) led to a cooling. The book also touches on paleontology, discussing the possibility that birds and feathered dinosaurs evolved as a response to a cooling event in the Early Cretaceous Era.

(There's a bit of fun, too. My favourite image from The Chilling Stars is that of our solar system leaping exuberantly in and out of the galactic plane, like a playful dolphin, as it completes journeys lasting hundreds of millions of years around the Galaxy's core.)

Is Svensmark's hypothesis a convincing alternative to Anthropogenic Global Warming? The SKY cloud-chamber experiment at the Danish National Space Center in 2005 went some way to demonstrate a link between cosmic rays and cloud formation.

Its successor will be the CLOUD experiment at CERN's Large Hadron Collider, which I understand is scheduled for 2010. Perhaps success at CERN will turn the tide in Svensmark's favour.

The jury is still out, I think, although for several reasons, I tend to rate Svensmark's hypothesis over AGW. The main reason I do so is that it is able to explain the connection between sunspot activity (or lack thereof) and cold episodes in history, such as the Maunder Minimum. Also, I find AGW not generally all that convincing, in the face of the mid 20th-century cool period, when atmospheric CO2 was shooting up but temperatures dipped (as has also happened in the last few years.)

However, whether or not Svensmark and Calder are vindicated in 2010, they have produced a very fine and thought-provoking book of popular science, which has stirred up controversy and ruffled a few feathers, and at the same time has inspired a sense of wonder in open-minded readers all over the world.

Learn more about this author, Alex Cull.
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