Developing the Gaia Theory. A Response to the Criticisms of Kirchner and Volk

Abstract

This paper is a response to the recent criticisms of Gaia theory by Kirchner and Volk, in this journal (Kirchner, 2002; Volk, 2002). As noted by Kirchner, there is a need for more dialogue on this topic, and we write in this spirit with the intention of helping clear a pathway ‘Toward a future for Gaia theory’, to use Volk’s phrase. We agree with Kirchner and Volk on a number of points. Our purpose here is to clarify outstanding differences and suggest how they may be resolved, in the hope that this will help form a basis for developing the Gaia theory. ‘Gaia’ is a particular type of open thermodynamic system, planetary in scale with abundant life tightly coupled to its environment. Planet Earth hosts the only known example, for which the name ‘Gaia’ was introduced. The Gaia theory aims to explain the development and functioning of Gaia systems, and is currently restricted to this one example. Our central point is that the functioning of such systems must be consistent with natural selection occurring within them, but this does not mean that planetary-scale regulation has to be the product of natural selection. We try to move from the more general questions raised by Volk to the more specific issues tackled by Kirchner. We begin by considering what constitutes a reasonable basis for generalising about the behaviour of Gaia systems (Section 2). Then we consider the possible explanations for the persistence of life on Earth (Section 3). We associate the ‘Gaia theory’ with the idea that regulatory feedbacks are a probable outcome on planets with abundant life, but note that if this is proved wrong and the existence of regulatory feedbacks on Earth turns out to be good luck, this still leaves an important research agenda. We briefly review the basic tenets of the Gaia theory (Section 4), and expand on the facts that Gaia systems are expected to contain both positive and negative feedback (Section 5), and both feedback from by-products of selection (Section 6) and feedback involving natural selection (Section 7). Means of distinguishing the results of environmental enhancement and adaptation to the environment are suggested (Section 8). We conclude by discussing how conceptions of Gaia have contributed to the emerging field of ‘Earth system science’ (Section 9).