A living planet looks different from a dead one, and nowadays we have the photographs to prove it. Back in the early 1960s, James Lovelock realised there were subtler signs too. An atmosphere like ours, with a mix of gases some way from chemical equilibrium, showed life at work. The gas mix on Mars, where chemical reactions have all moved to their endpoint, meant the place was lifeless.

Nasa, which Lovelock was advising at the time, went on looking for signs of life on Mars anyway. The author took his own insight rather more seriously, and went on to develop a broader view of the influence of life on Earth. The Gaia hypothesis, later elevated to Gaia theory, suggested that life, in the sense of the ensemble of all species, regulates the planetary environment to suit itself. The entire planet is one enormous ecosystem, replete with feedbacks and control loops that keep important things such as the atmosphere, temperature and ocean salinity in its own comfort zone.

Human interest in the global environment has grown in recent decades as our own collective influence (mostly bad) has become inescapable. Our anxiety about this often erases the gap between Gaia as a theory and Gaia as an entity that somehow acts ¨C a gap that Lovelock himself has intermittently ignored ever since naming his idea after the goddess of the Earth.

That interest has also helped to fuel the growth of Earth system science, which has yielded a?wealth of new knowledge about the planet¡¯s past and present since Lovelock first pieced together his ideas. So it is timely to present a?systematic review of how Gaia theory looks in the light of all this new information.

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Not too well, is Toby Tyrrell¡¯s conclusion in this clear summary of the evidence to date. He tells us he began his assessment with an open mind, although I suspect he already knew the answer. However, there is nothing to suggest any preconceptions in his methodical appraisal of the elements of Gaia.

Gaia can be a conveniently fuzzy entity and, as he suggests, it is important to be clear just what hypotheses are under scrutiny. The core claims, in his formulation, are that Earth is a favourable habitat, has remained so due to the environment remaining stable over geological time scales, and that this is partly due to life shaping that environment. He then derives several assertions that support the claims and assesses them, along with two alternative hypotheses. One, which formerly commanded broad assent, has no role for life and suggests that the Earth¡¯s environment is mainly shaped by geology. The other grants life a role ¨C with ecosystems and environment co-evolving ¨C but not a controlling one.

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Tyrrell goes through an impressive range of evidence to evaluate these ideas. On Gaia, his findings are largely negative. Earth is well suited to the life that is found here, of course, but that is because organisms adapt. It could easily be better, though. A warmer planet, and one with more available nitrogen, would probably allow more (and more diverse) life. Lovelock is certainly correct that changes in living things have altered the global environment, but sometimes the changes have been for the worse. And ¨C the past record is perhaps clearest on this point ¨C the environment has been far from stable. It has remained habitable, but that seems unremarkable. If it had not, we would not be here to discuss it.

The author comes down firmly in favour of co-evolution, a?less bold hypothesis than Gaia, but one that ¡°has the benefit of not contravening our observations of how the Earth works, and how it has changed over time¡±. The conclusion is important, he argues, because adherence to Gaia promotes an over-optimistic view of how the planet will respond to humans¡¯ activities. This is a little hard to reconcile with Lovelock¡¯s frequent ¡°epochalyptic¡± pronouncements in his more recent books, but Tyrrell¡¯s target is the international science organisations whose ¡°Amsterdam declaration¡± of 2001 said that the Earth behaves as ¡°a single, self-regulating system¡±.

That, he reckons, is Gaian talk, and encourages ¡°a predisposition to suspect natural feedbacks to be stabilizing¡±. They aren¡¯t, and that should make us more wary of disturbing the atmospheric status quo. Tyrrell¡¯s book is persuasive on that point. It also shows that study of our home planet has progressed since Lovelock began discussing his idea, and that may be his most important contribution. Without Gaia, there would still have been Earth system science, but it would not have developed so far, so fast.