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According to current scientific orthodoxy, the main cause of global warming is the large amount of man-made carbon dioxide in Earth's atmosphere. Henrik Svensmark is a scientist who has put forward an alternative hypothesis, however. He proposes that global warming - and global cooling, for that matter - is controlled, not by levels of CO2, but by the way cosmic rays and clouds interact.

That man-made carbon dioxide is causing the Earth to overheat, and there is an urgent need to reduce levels of this trace gas in the atmosphere, is something that national governments all over the world, NGOs, the UN, environmental groups and corporations have been impressing on each and every one of us, over the last two decades.

But what if carbon dioxide was largely irrelevant when it came to determining global temperatures? And what if there was some other factor that controlled the planetary thermostat? This is what the authors of The Chilling Stars are attempting to demonstrate in this extremely readable and controversial book.

Henrik Svensmark is a physicist at the Danish National Space Center, and Nigel Calder is an experienced science writer, and former editor of New Scientist magazine. The hypothesis they present is a direct challenge to the supremacy of AGW, or Anthropogenic Global Warming.

I will not be able to do justice to the theory in this short review, but here it is, in a nutshell. Our Galaxy is teeming with stars, many of which end their days in colossal stellar explosions. These detonations create vast amounts of cosmic radiation, which are floods of charged particles (mostly protons.) When these particles encounter the Earth's atmosphere, they tend (according to this theory) to seed clouds, especially low-level clouds below the 3000-metre mark.

The more cosmic rays there are, the cloudier the Earth gets, and thus the cooler it becomes. However, if something (for example, the Sun's magnetic field) acts to shield the Earth from cosmic rays, the fewer low-level clouds there are and the warmer Earth becomes.

Over the billions of years since the planet was formed, it has veered from one extreme to the other. At times it has been in a torrid "hothouse" state, with no ice at the poles and with sea levels much higher than they are now. At other times, however, the planet has been in an "icehouse" condition - or even a "Snowball Earth" state, with ice sheets reaching down as far as the Equator. We are currently in an icehouse phase, incidentally.

There have been numerous

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