The Gaia hypothesis invites us to imagine Earth as an integral living system in order to explore the mechanisms by which life helps create and maintain the conditions for life, such as an oxygenated atmosphere.
“The Gaia theory proposes that organisms contribute to self-regulating feedback mechanisms that have kept the Earth’s surface environment stable and habitable to life” [Lenton 2000: 439]. This theory was developed by James Lovelock, a chemist who observed that Earth’s atmosphere is in a constant state of disequilibrium,
in which highly reactive gases, such as methane and oxygen, exist together at levels that are different by many orders of magnitude from photochemical steady states. Large, biogenic ﬂuxes of gases are involved in maintaining such disequilibrium. This perturbed state is remarkable in that the atmospheric composition is fairly stable over periods of time that are much longer than the residence times of the constituent gases, indicating that life may regulate the composition of the Earth’s atmosphere. This concept became the foundation of Gaia theory [Lenton 2000: 439].
The theory is based on the evolutionary biology concept of natural selection, focusing on traits that alter the environment and the resulting feedback from that environmental change on the organisms with the traits that produced it. Lenton offers a few examples to illustrate such feedbacks, starting with the “Daisyworld” model, where black pigment in daisies confers advantage in an environment with below-optimal temperatures. By absorbing heat, the black daisies grow better than their white counterparts and their population dominates. The global effect of a growing population of individually warm daisies raises the overall temperature of the world. At this point, the population of white daisies begins to rebound and the global temperatures cool again.
Gaia theory aims to be consistent with evolutionary biology and views the evolution of organisms and their material environment as so closely coupled that they form a single, indivisible, process. Organisms possess environment-altering traits because the beneﬁt that these traits confer (to the ﬁtness of the organisms) outweighs the cost in energy to the individual [Lenton 2000: 440].
Some activities that alter the environment are so advantageous (to the organisms carrying out the activities) that they become widespread, fundamental properties of organisms. (An example is photosynthesis, the implications of which have been studied by modelling the Archaean–Proterozoic transition.) Other activities are favorable only under particular environmental conditions and hence are subject to selection. In such cases, it is often changes in one environmental variable that determine whether a trait remains selectively favorable. If the spread of the trait alters this environmental variable, it also alters the forces of selection determining its own value [Lenton 2000: 442].
Furthermore, ecosystems-level environmental feedbacks can be understood in terms of natural selection. For example:
The trees of the Amazon rainforest, through generating a high level of water cycling, maintain the moist environmental conditions in which they can persist (a positive feedback on growth and selection). Nutrients are also effectively retained and recycled. If too much forest is removed, the water-regulation system can collapse, the topsoil is washed away and the region reverts to arid semi-desert, a change that may be difﬁcult to reverse [Lenton 2000: 445].
Lenton explains that while there are geochemical mechanisms involved in regulating the climate, “it is clear that organisms are involved in many environmental feedbacks on Earth, and their effects need to be considered” [Lenton 2000: 441]. For example, acid rain weathers calcium-silicate rocks resulting in the formation of calcium carbonate by removing carbon dioxide from the atmosphere, thus cooling the Earth. Warmer average global temperatures would lead to more rain, thus more weathering and the cooling effects of that negative, self-correcting feedback. “However, geochemical feedbacks [such as this one] operate slowly and are not very responsive to perturbation” [Lenton 2000: 441]. Rock weathering organisms can amplify the weathering effects of the rain, hastening the negative feedback. Thus, there’s an intertwining of processes that regulate the climate.
The significance of this article is that if life has been at least partly responsible for creating and maintaining the habitability of Earth’s climate for the past 3.5 billion years, then it has a key role to play today as we grapple with how to keep global temperatures from rising above 1.5C. Ecosystems are clearly victimized by climate chaos, while also being directly damaged by avoidable human activity, such as land-clearing for development and agriculture and the ubiquitous use of chemical toxins and plastics. Yet if ecosystems are also a driver of climatic conditions, then it is critical to protect them from further harm and to nurture their growth and stability. Humans can become Gaia’s nursing team – we can improve the conditions for her recovery to the point when her own systems kick in and bring her back to health.
Lenton, Timothy M., 1998, Gaia and natural selection, Nature 394, https://www.nature.com/articles/28792.