BIOTIC FEEDBACKS DURING THE PHANEROZOIC AND THE FUTURE OF GAIA

A modelling synthesis of recent insights into the feedbacks regulating N, P, C and O cycles during the Phanerozoic (the past ~550 Myr) indicates that life has continued to alter 'master variables' of the surface Earth system and to strengthen the mechanisms regulating them. Regulation includes counteracting forcing and perturbation from above (increasing solar luminosity, asteroid impacts) and below (changing volcanic and tectonic activity). For example, the evolution of vascular plants and their spread across the land surface, beginning ~420 Myr ago, increased the rates of phosphorus and silicate weathering, tending to increase atmospheric O₂ and decrease atmospheric CO₂ and global temperature. Subsequently, these environmental variables have been more tightly regulated. The effects of O₂ on fire frequency and on photosynthetic carbon assimilation control plant-induced phosphorus weathering and hence organic carbon burial, providing a sensitive negative feedback that has counteracted geologic forcing and maintained atmospheric O₂ within a narrow range of ~15-25 vol% for the last 350 Myr. The effects of CO₂ and temperature on photosynthesis and plant-induced silicate weathering have generated strong negative feedback on these variables that counteracted geologic and solar forcing. The capacity of present feedback mechanisms to withstand future forcing is assessed using models. Increasing solar luminosity, any increase in continental area and any decline in sea-floor spreading rate will tend to cause loss of CO₂ from the atmosphere. Biotic feedback involving the amplification of silicate weathering by plants should counteract this and is thus predicted to extend the life span of the current biosphere, until catastrophic warming terminates complex life in ~1 Gyr. The emerging understanding adds new dimensions to the Gaia theory of the Earth as a self-regulating system.