On Gaia A Critical Investigation of the Relationship between Life and Earth

James Lovelock is probably best remembered for the Gaia hypothesis: the notion that the Earth is a giant self-regulating system that maintains conditions suitable for life on the planet. It has gained a certain respectability in academic circles over the decades, but how justified is this? In On Gaia, Earth system scientist Toby Tyrrell gives a thorough and dispassionate overview of the scientific evidence and whether it supports Gaia.

Given the holistic, top-down nature of the Gaia hypothesis, any critique of it will have to cover a lot of ground and straddle many disciplines. Tyrrell tackles this challenge in several ways. The book is well structured with good use of numbered sections to keep information organised while relegating digressions and details to a fascinating collection of endnotes. Furthermore, the book is self-contained and you do not strictly need to read Lovelock's work. Lastly, by going back to first principles, On Gaia is a terrific primer on palaeoclimatology and Earth system science.

Additional challenges in evaluating Gaia are that Lovelock both modified the idea with time and was not very clear in his definitions. Even so, the different formulations have three assertions in common: 1) the environment is hospitable to life (i.e. its biosphere), 2) life can change the environment, and 3) the environment has been constant through time. Evaluating these claims is at the heart of this book and takes half of the book. Tyrrell furthermore compares Gaia to two alternative explanations dubbed the geological and coevolutionary hypotheses. The former says that Earth's environment is primarily shaped by geological forces. The latter only says that life and the planet influence each other but remains neutral on whether the outcome will be favourable for life or not.

Before delving into the meat of the book, Tyrrell discusses the question of mechanism. How could something like Gaia evolve? Lovelock argued for an extreme form of group selection at the level of the whole planet. Tyrrell outlines why this is considered unlikely by evolutionary biologists: it requires cooperation amongst very distantly related organisms which is simply not what we observe in the real world. There are admittedly limited circumstances where natural selection leads to regulation of local environments and Lovelock made a lot out of them. However, both Gaia-as-physiological-homeostasis and Gaia-as-superorganism work as crude analogies but "any suggestion of a deeper resemblance is very misleading" (p. 30). Similarly, though Lovelock's Daisyworld model showed that planetary regulation could theoretically emerge from individual actions, it is not a realistic description of how the climate and biosphere interact.

The core of the book compares the three abovementioned assertions with the data. Lovelock's first assertion is that the environment is well-suited to life. Here, Lovelock gets the arrow of causation backwards: adaptation to one's environment via natural selection is a sufficient explanation, even if it can give "the impression that all environments have been made to be uncannily well suited to their inhabitants" (p. 202). Moreover, this is not even always true. Lovelock has suggested that Gaia likes to keep the planet cool. Tyrrell fleshes this out in two further chapters by looking at the biological effects of temperature. Biological processes proceed faster at higher temperatures and the overall tendency is for physiological rates, biomass, and biodiversity to increase with higher temperatures. Furthermore, after considering a range of factors, Tyrrell makes the point that ice ages are unfavourable: the planet is simply too cold for life to thrive.

The evidence for Lovelock's third assertion, that the environment has been held constant by Gaia, also crumbles under the weight of the evidence against it. This is a chapter that showcases the marvellous array of proxies that palaeoclimatologists have developed. By cross-referencing multiple such indirect lines of evidence they have reconstructed an increasingly detailed picture of Earth's past climate. Tyrrell here considers evidence from stratigraphy, ocean chemistry and acidity, temperature, greenhouse gases, and erratic climatic episodes such as Snowball Earth and more recent ice ages to convincingly show that our planet's climate has been anything but constant.

Only Lovelock's second assertion, that life can influence the planetary environment, is supported by the data. He explores in more detail two factors that are under biological control: the composition of Earth's atmosphere and the nitrogen-to-phosphorus ratio in seawater. Indeed, the fact that Earth's atmosphere is far from chemical equilibrium and contains short-lived gases such as oxygen and methane was one of Lovelock's original observations that sent him down this path.

The above is but a brief summary of some of the key arguments that Tyrrell makes. There are various other interesting points I have not touched on here and the whole book, including its extensive endnotes, deserves close reading. Tyrrell pulls no punches in his conclusion: "in my view Gaia is a fascinating but a flawed hypothesis [and] a dead end" (pp. 208–209). Many of Lovelock's claims are not supported once carefully evaluated and there is nothing that can be explained exclusively by the Gaia hypothesis. Circling back to the other two hypotheses, the geological one also has its flaws. Instead, the data support the coevolutionary hypothesis: life and the planet influence each other, though not always to life's benefit.

Tyrrell's book-length argumentation and evaluation to arrive at his conclusion is, in my opinion, calm, reasoned, fair, factual, and still relevant despite the book now being ten years old. You might wonder if it matters whether the Gaia hypothesis is right or wrong. In closing the book, Tyrrell argues that, in the face of climate change, having a true picture of how the Earth system functions has never been more important. The risk of the Gaia hypothesis is that it fosters complacency: "there is no Gaian safety net to come to the rescue if we mismanage it" (p. 218).

In conclusion, On Gaia has confirmed my suspicions while pointing out many other flaws I had not even considered yet. Even so, Tyrrell praises Lovelock for his originality and breadth of vision, recognizing that "his audacious concept has helped to stimulate many new ideas about the Earth, and to champion a holistic approach to studying it" (p. 209). I found this a phenomenally interesting book and highly recommend it. Though he builds on previous edited collections, Tyrrell has done readers a tremendous service in writing such a focused and in-depth review of the Gaia hypothesis and presenting it in such an accessible manner.