Toby Tyrrell is professor of Earth system science at the National Oceanography Centre Southampton (University of Southampton). His new book On Gaia: A Critical Investigation of the Relationship between Life and Earth publishes at the end of July.
He kindly answered some questions from our UK Director of Publicity Caroline Priday.
Q: Why did you write this book
A: My aim was to determine whether the Gaia hypothesis is a credible explanation of how life and environment interact on Earth.
Q: What is the Gaia hypothesis?
A: James Lovelock’s Gaia hypothesis, first put forward in the 1970’s, proposes that life has played a critical role in shaping the planetary environment and climate over ~3 billion years, in order to keep it habitable or even optimal for life down through the geological ages. Life has not been merely a passive passenger on a fortuitously habitable Earth, it is claimed, but rather has shaped the environment and helped to keep it comfortable.
Q: Why has there been so much interest in the Gaia hypothesis?
A: In part because it suggests answers to some fundamental questions of widespread interest, such as how it is that Earth remained continuously habitable for so long, how did our planet and the life upon it end up the way they are, and how does the Earth system work?
Q: Lots of scientists have considered Gaia before – what is different about this book?
A: Previous books have been mostly reviews of the scientific debates over Gaia, collections of scientific papers, or congratulatory restatements of Gaia by supporters. This book is the first to submit the Gaia hypothesis to detailed sceptical scrutiny, subjecting each of three main arguments put forward in support of Gaia to close analysis, and comparing them to modern evidence collected in the more than 30 years since the Gaia hypothesis was first proposed. It is the first book containing a hard-nosed and thorough examination of the Gaia hypothesis.
Q: What are the three main arguments that have been advanced in support of Gaia?
A: Firstly, that the Earth is very comfortable for life. Secondly, that the presence of life on Earth has profoundly altered the nature of the planetary environment. And thirdly, that the environment has remained fairly stable over geological time.
Q: What is the main conclusion of the book?
A: That the Gaia hypothesis does not “hold up in court”: it is not consistent with modern scientific evidence and understanding and should therefore be rejected.
Q: What are the reasons given for rejecting the Gaia hypothesis?
A: Firstly because there are no facts or phenomena that can be explained only by Gaia (no ‘smoking gun’). Secondly because there is no proven mechanism for Gaia (no accepted reason for why it should emerge out of natural selection). And thirdly because the key lines of argument that Lovelock and others advanced in support of Gaia either give equally strong support to alternative hypotheses or else are mistaken. For instance the planet is not excessively favourable for life: it has been colder than optimal during the ice ages that have occupied the majority of the last few million years, and unnecessary nitrogen starvation is ubiquitous. Its environmental history has not been all that stable: we now have abundant evidence of past environmental instability, from ice age cycles to seawater Mg/Ca variation to Snowball/Slushball Earths.
Q: If life and its environment do not interact in the way suggested by Gaia (life moulding the environment towards its own convenience) then how do they interact?
A: Through ‘coevolution’. Stephen Schneider and Randi Londer put forward the idea of a coevolution between life and its environment: biological processes such as oxygen production by photosynthesis shape the environment, and, clearly, the environment also strongly influences life through evolution of organisms to fit their environments. Coevolution recognises that both affect the other. Unlike Gaia, however, coevolution does not claim any emergent property out of the two-way interaction between life and environment. It is neutral with regards to predictions about the resulting effect on the environment. It does not suggest that the interaction tends to improve living conditions on Earth.
Q: If the Gaia hypothesis is not the reason, then why did the Earth remain habitable for such an enormously long interval of time?
A: This may relate partly to the weak Anthropic Principle, whereby we logically cannot observe any facts that preclude our own existence. So however infrequent it may be in the universe for a planet to remain continuously habitable over billions of years, we happen to be on just such a planet. According to this way of thinking, Earth may just have been lucky, with no sentient observers having evolved on other planets which were not so lucky, i.e. where conditions became sterile at some point. Another possible explanation for extended habitability in the absence of Gaia is a predominantly inorganic thermostat, such as has been suggested for silicate weathering.
Q: Why would people be interested in this book?
A: It considers some of the great questions about the nature of our planet, its history, and how it came to give rise to us. Many fascinating topics are covered, often from little-known corners of the natural world. Examples include: hummingbirds in the High Andes and the similarity of their beaks to the flowers they extract nectar from, the wonderfully-named Walsby’s square archaeon in the Dead Sea, the ever-lasting durability of the waste that coral reefs generate (not everything in nature is recycled), changes in the nature of the saltiness of seawater over geological time, and differences in the way Australian snakes bear young depending on climate (they don’t always lay eggs).
Q: Are there any implications for the current era of global change?
A: Yes, it is suggested that belief in the Gaia hypothesis can lead to excessive complacency about the robustness and resilience of the natural system. Gaia emphasizes stabilising feedbacks and protective mechanisms that keep the environment in check. If Gaia is rejected, however, we are left with a less comforting view of the natural system. Without Gaia it is easier to appreciate that the natural system contains lines of weakness and other susceptibilities. One such line of weakness that has already been demonstrated is the ozone layer depletion by CFC’s. I have argued in the book that there is no over-riding Gaia to protect our planet’s life support system. Maintaining the Earth’s environment is up to us.