Water Gaia: The Earth as Aquifer

Inside the FiberMax Center for Discovery in Lubbock, Texas, a central room full of shiny interactive displays depicts scenes from the extraction, conveyance, and conservation of water. One panel proclaims: “Earth’s water is never really lost. It merely changes location, form or quality.” Coming across with the confident ring of scientific fact, this statement actually shoplifts an aura of veracity by mapping itself onto the first law of thermodynamics, also known as the law of the conservation of energy. This physical axiom states that energy “is never really lost,” that is, cannot be created or destroyed, only converted from one form of energy to another. But of course, water is not energy, it is matter. So the question regarding water’s absolute conservation is another issue altogether, one more properly submitted to the laws of chemistry, and also, as I will suggest here, to the contingencies of biology.

Water is indeed the indispensable material condition and chemical medium for a planetary environment in which our kind of life is possible. However, our planetary aquifer is not at all a forever thing. Rather, let us explore the idea that the better part of the water ever held on Earth is still sequestered, here on this planet, thanks to life’s active role in keeping the planet fit — for life. Water can indeed be lost, but here on Earth, it enjoys its current abundance because it has been actively retained rather than lost — thanks to the presence and agency of life over geological time. Gaia theorists Stephan Harding and Lynn Margulis put forward this claim in their article Water Gaia: 3.5 Thousand Million Years of Wetness on Planet Earth:

We suggest that without life’s involvement in complex geological, atmospheric, and metabolic processes, Earth would long ago have lost its water, becoming a dry and barren world much like Mars and Venus.

According to these authors, since attaining its Gaian form, life in the aggregate has actively participated with geological processes to retain Earth’s aqueous endowment, to maintain the surfaces of this planet continuously wet or moist and precisely as such, fit for life. Our water is still here, this thesis runs, because Gaia — the planetary system that comprehends and coordinates the operational couplings of biological and geological systems — has kept it here. Liquid water makes life possible, so life appears to have worked out ways to retain the water within the Gaian system, keeping oceans of this priceless molecule available over geological time for its own purposes of self-maintenance and differential regeneration, or evolution.

For contrast, let’s consider our neighbors Venus and Mars. Planetary scientists are largely agreed that as young planets, both Venus and Mars had relatively abundant water. Like Earth’s water, it likely came from the so-called Heavy Bombardments by water-bearing asteroids that punctuated the final accretion of the rocky planets out of the solar disk some four billion years ago. Although the young sun was fainter, perhaps 30% less luminous than now, Mercury has probably always been too close to the Sun for life to gain hold. However, Venus, Earth, and Mars seem to have had comparable endowments of pre-life chemistry and opportunities for vital origins.

For most of its tenure, Venus has been within the margins of our solar system’s habitable zone, as it’s called, and for life to once have emerged there seems modestly plausible. To remain friendly to life, however, Venus would have needed active cooling. It could possibly have achieved and sustained such a temperate state by establishing a high albedo or index of reflectivity, sending solar energy back into space before it reaches the surface, while minimizing the greenhouse gases that hold heat in. At the other end of the habitable zone, to incubate nascent life, Mars would have needed active heating. For this the Red Planet could have done the opposite of Venus — work up a strong greenhouse atmosphere to retain surface heat and hold its albedo low so that it converted as much of its sunlight as possible into planetary warming.

Plausible scenarios can thus be built for the origin and development of life on both Venus and Mars. However, if life got going on either of these worlds, it now seems to be gone. Moreover, both of our neighboring planets are now desiccated in the extreme. The Water Gaia thesis suggests that a major factor in both planets’ drift toward their present parched state has been either the absence of life altogether or the unhappy loss of the nascent life needed to set up the negative or regulatory Gaian feedbacks within their atmospheric regimes that allow for habitable climates sustained over geological time.

For the cosmic evolution of Venus, a likely scenario is that an intense atmospheric greenhouse emerged that fell into a positive feedback regime of runaway heating, and over hundreds of millions of years Venus’s water simply cooked itself away like a stew left too long on the burner, boiling with enough force to expel the bulk of its water out of that planetary system. The significantly smaller planet Mars had different problems: its lower gravity was less able to counteract hydrogen loss to space produced by photodissociation of water molecules in the upper atmosphere. As the hydrogen faded away, the leftover oxygen oxidized elsewhere, as we can see in the rusty red that shows through its nearly cloudless skies. In either case, life was either never present to be a factor or was itself a victim of abiotic cosmological constraints, dying out before any potential Gaian system could coalesce to maintain anti-entropic counterforces to the unutterable dryness of space.

As far as we know, life is astronomically rare, a cosmic diamond in the infinite rough. Moreover, its very survival and subsequent proliferation appear to depend on a fortuitously rapid burst of initial growth yielding conducive planetary modifications. In The Case for a Gaian Bottleneck: The Biology of Habitability, astrobiologists Aditya Chopra and Charles H. Lineweaver offer a biological rationale for the current state of non-evidence regarding life beyond Earth:

If life emerges on a planet, it only rarely evolves quickly enough to regulate greenhouse gases and albedo, thereby maintaining surface temperatures compatible with liquid water and habitability. Such a Gaian bottleneck suggests that (i) extinction is the cosmic default for most life that has ever emerged on the surfaces of wet rocky planets in the Universe and (ii) rocky planets need to be inhabited to remain habitable. In the Gaian bottleneck model, the maintenance of planetary habitability is a property more associated with an unusually rapid evolution of biological regulation of surface volatiles than with the luminosity and distance to the host star.

We see that when it comes to cosmological perseverance, it’s not enough just for life to get going. Like infants everywhere, nascent life is the most fragile. Even as our planet evolved geologically as a rocky world, early life’s own effluents and accumulated deposits reworked its conditions, inevitably altering its chemistry and its terrain. Proliferating species of early life had to evolve along particular pathways that furthered their own continuation and that of their neighbors. One crucial strategy was the formation of symbioses. On Earth, mutualistic relations among different living forms, all microbial at the outset, arose to cultivate and exploit the creative multiplicity of autopoietic possibilities. Gaia theory posits that this inherently diversifying life works to enhance its scenes of habitation, for itself and its ecological neighbors, and the life that increases the general fertility of a community niche tends to persist.

The most unexpected but telling component of the Water Gaia thesis may be its sketch for the plausible involvement of water retention in the origin and perpetuation of the Earth’s system of plate tectonics. A map of the plate boundaries makes it clear that these fault lines primarily occur running down the middle ridges and at the edges of oceans, where the continents well up from the seas of water. It is also good to remember that plate tectonics only emerged as a broadly accepted geological super-theory during the same period of the later 20th century in which the Gaia concept also made its appearance. The Water Gaia thesis absorbs the seemingly abiotic dynamics of plate tectonics into a fully Gaian description.

In a recent memoir of his collaboration with Margulis titled “Gaia and the Water of Life,” Stephan Harding encapsulates their arguments in “Water Gaia”:

Most profoundly, we concluded that without life’s water-retaining abilities, there would be no plate tectonics. In essence, water must be present to “soften” sea floor basalt as it plunges downwards at plate margins to melt and be recycled in Earth’s depths. Without plate tectonics there would be no granite and hence no continents, and no return of carbon dioxide to the atmosphere and thus no regulation of planetary temperature favorable to the persistence of liquid water and hence of life itself. These massive planetary cycles exist only because the biosphere persistently bonds fleeing hydrogen molecules to those of oxygen, replenishing Earth’s deep wells of water. What “Water Gaia” means for our scientific understanding is that, for the planet as a whole, life itself holds on to the water it needs to live and flourish.

The Gaian mechanism here is precisely the manifest and continuously replenished presence of oceans of water moving between the biosphere and the geosphere. Because the loss of water or its components from Earth to space is almost entirely prevented, according to this thesis, in all the various ways that their “Water Gaia” essay details, planetary water retention is actively and continuously achieved. In this way, the Gaian system ensures the terrestrial presence of sufficient water to carry out the various forms of mineral hydration and lubrication necessary to the keep the dynamics of plate tectonics in motion. Harding and Margulis suggest that these precise considerations yield “an interesting and appropriately circular Gaian dynamic . . . : no life, no water →no water, no plate tectonics → no plate tectonics, no life.”

In sum, Earth’s hydrological cycles are themselves much more than just “water in the aggregate” as it runs its planetary courses. Much more than just solar and thermal energies, forces of gravity, strictly geological dynamics, and meteorological systems are in play. Rather, water’s global movements run around and through the organic pulsations of living systems. Its flows are processed both through biological pumps — for instance, by evapotranspiration in plants — and by passage through its rocky channels, whose biotically-enhanced weathering by water releases nutrients locked up in geological formations from previous eons. The Water Gaia thesis is an especially powerful reminder that no Earthly formation can be taken for granted either as eternal or as independent of far wider environmental and ecological contingences. The protection of what we like to call “natural resources” would seem to be a thing that Gaia has been doing on its own behalf for the three billion years of its operation. Viewing local formations against their planetary horizon, regional aquifers are all tributaries of an Earth system whose replenishment is inextricably tied to Gaia’s own self-maintenance.

Stephan Harding and Lynn Margulis, “Water Gaia: 3.5 Thousand Million Years of Wetness on Planet Earth,” in Eileen Crist and Bruce H. Rinker, eds., Gaia in Turmoil: Climate Change, Biodepletion, and Earth Ethics in an Age of Crisis (Cambridge, MA: MIT Press, 2010), 41–59.

Aditya Chopra and Charles H. Lineweaver, “The Case for a Gaian Bottleneck: The Biology of Habitability,” Astrobiology16:1 (2016): 7–22.

Stephan Harding, “Gaia and the Water of Life,” in Bruce Clarke and Sébastien Dutreuil, Writing Gaia: The Scientific Correspondence of James Lovelock and Lynn Margulis (Cambridge: Cambridge University Press, 2022).