1.3 The right temperature for life

The beginning of this section explored: ‘Why does life exist on Earth but not, apparently, on our neighbouring planets?’ Part of the answer is that the Earth’s distance from the Sun, in combination with the greenhouse effect of its atmosphere, maintains a range of temperatures on the planet that are suitable for oceans and life to survive. Mars, with its thin atmosphere and weak greenhouse effect, is too cold for life to flourish, while Venus, with its dense atmosphere and intense greenhouse effect, is too hot. The Earth is just right.

This is sometimes referred to as the Goldilocks thesis, named after the children’s story Goldilocks and the Three Bears, where Goldilocks finds the temperature for Baby Bear’s porridge to be ‘just right’. The good news (there sometimes is some) is that theoretical calculations suggest that we will not be able to pump enough carbon dioxide into the atmosphere to cause a runaway greenhouse effect like that on Venus. This is backed up by evidence from the Earth’s history: the oceans and life have survived much higher levels of greenhouse gases than now, although life has often suffered setbacks, including several episodes of mass extinction when a large proportion of all species have become extinct in a comparatively short geological time.