Life span of the biosphere

Abstract

There has been life on Earth for at least 3,500 Myr but the assumption that a comparable future lies ahead may not be justified. Main sequence stars appear to increase their burning rate as they age. Thus the Sun, if a typical star, can be predicted to have increased its output by 30% since the Earth's origin 4,500 Myr ago¹. The maintainance of an equable climate since life began probably required some means of planetary thermo-stasis. The Gaia hypothesis proposed by Lovelock and Margulis² included an unspecified biological means for climate control. Walker et al.³ suggests an abiological automatic thermostasis in which the atmospheric abundance of CO₂, a greenhouse gas, adjusts to resist the warming tendency of the increased solar flux. Here we discuss possible links between the biological and geological control mechanisms. It is clear that whatever the mechanism, atmospheric CO² is now close to its lower limit of partial pressure, so the biosphere may soon, in geological terms, be exposed without protection to the predicted progressive increase of solar luminosity.