Published online by Cambridge University Press: 01 May 2009
Geological evidence suggests that Gaia is endothermic: her body temperature has varied, but within limits; there has been no runaway greenhouse like Venus, nor deep freeze like Mars. This paper presents a hypothesis that the Earth's climate has been ameliorated by living organisms: they have served either as heaters or air-conditioners, and their ecological tolerance is the sensor of Gaia's thermostat. At the beginning, 3.8 or 3.5 Ga ago, only anaerobic autotrophs capable of tolerating high temperatures thinned out the atmospheric CO2 through carbon fixation. Fossil organic carbon was utilized by anaerobic heterotrophs to reinforce the effectiveness of the late Archean greenhouse, when solar luminosity was weaker than it is now. With the increasing solar luminosity during early Proterozoic time, new life forms such as cyanobacteria evolved, removing CO2 from the atmosphere and storing it in stromatolitic carbonates. Over-eager cyanobacteria may have consumed too much greenhouse CO2 to cause glaciation. Their decline coincided in timing with the rise of the Ediacaran faunas which had no carbonate skeletons. The change in the mode of carbon-cycling may have started the warming trend after the Proterozoic glaciation. The Cambrian explosion was an event when skeletal eukaryotes usurped the function of prokaryotes in removing greenhouse CO2 through CaCO3 precipitation. With the evolution of land plants, coal-makers took over the ‘air-conditioning’ duty. They over-did it, and Permo-Carboniferous glaciation ensued. After a wholesale turnover of the faunas and floras at the end of the Palaeozoic, more CO2 was released than fixed in early Mesozoic time. The warming trend reached its zenith in the early Cretaceous, when flowering trees and calcareous plankton began to flourish. The decline since then, with a temporary restoration during early Palaeogene time, could be a manifestation of the varying efficiency of extracting and burying carbon dioxide, in the form of inorganic and organic carbon. The relation of atmospheric CO2 and climatic variation is documented by study of air bubbles in ice cores. Yet there is also correlation to astronomical cycles. The latter seem to have triggered changes which are amplified by feedback mechanisms of carbon cycling.