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 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.