Half a billion years of good weather: Gaia or good luck?

Abstract

Earth’s climate has been unusually stable over the Phanerozoic (Greek: visible animal life) eon, the most recent 550 million years of Earth’s history during which organisms have been large and hard enough to form plentiful fossils. For the previous 4000 million years, most of the planet’s lifetime, living things were microscopic, and the climate was much more varied. Oxygen isotope ratios in ancient seashells suggest that, in the Phanerozoic, globally averaged temperatures have generally varied by less than 10 °C from their present value of 15 °C (Veizer et al. 2000). The record of ice-rafted debris preserved in ancient seafloor sediments tells a similar story of a climate that, while showing interesting variations, has rarely differed greatly from that of the present day. This contrasts with the previous 4 billion years of Earth history which is characterized by occasional snowball-Earth episodes (Hoffman et al. 1998), when temperatures were suppressed at least 20 °C, and which also shows evidence of periods when our world was 40 °C warmer than today (Schwartzman 1999). The consistent Phanerozoic temperatures are particularly surprising in the light of the 5% increase in solar luminosity over that time, the 15-fold reduction in atmospheric carbon dioxide concentrations (Berner 1997) and the inevitable variations in Earth-albedo resulting from continental growth and the evolution of land-plants, among other factors. It should be emphasized that there is no problem with understanding why there was significant climatic variability prior to the Phanerozoic. Astronomical, geological and biological processes were constantly altering the solar luminosity, the Earth’s albedo and the greenhouse gas concentrations in our atmosphere and these changes could, in principle, have caused temperature fluctuations even higher than those seen. The real problem is in understanding why temperature fluctuations dropped so dramatically during the Phanerozoic. A widely accepted explanation for long-term climate stability is that chemical-weathering of continental rocks by weakly carbonated rainfall gradually scrubs the greenhouse gas carbon dioxide from the atmosphere and, since this chemical reaction is temperature dependent, the result is a negative-feedback control on climate (Walker et al. 1981). However, while this process can certainly keep the Earth habitable (i.e. it can maintain temperatures allowing the existence of liquid water) it does not seem capable of providing the very fine control on temperature that is characteristic of the Phanerozoic. The coincidence of the emergence of multicelled organisms and of climate stability supports James Lovelock’s Gaia hypothesis that the Earth’s biosphere evolves to maintain optimum conditions for life. Indeed one of the best known Gaia-based models (DaisyWorld, introduced by Watson and Lovelock in 1983) shows how a simple biosphere can control global temperature and enhance climate stability. However, anthropic selection offers another possibility: perhaps climate stability is necessary for complex life and so complex life only evolved because Earth, by chance, provided a stable environment. The probability of such good fortune may seem small but, given the probable number of planets in the universe (>10), it is almost bound to happen on a few lucky worlds. This planetary-level form of anthropic selection, in which we find ourselves living on an unusual planet simply because the conditions necessary for the emergence of intelligent life are rare (Ward and Brownlee 2000), should be distinguished from the more commonly discussed cosmological-level anthropic selection which postulates the existence of multiple universes only some of which have laws of physics which are life-friendly (Barrow and Tipler 1986). Anthropic ideas remain controversial, largely because it is notoriously difficult to provide clear evidence in their favour. It will always be difficult to disentangle feedback from good luck when looking at geological or biological surface processes, but astronomical factors do not have this problem. For example, there is no mechanism whereby the eccentricity of the Earth’s orbit could vary in response to climate change on the Earth’s surface. If our world turns out to have an unusually circular orbit when compared to terrestrial planets around most stars, this is likely to result from anthropic selection. The Moon’s properties too, may provide evidence in support of anthropic selection.