Solar output and temperature

Our Sun is an average star, and like most other stars increases its output of energy, or luminosity, as it grows older. This behaviour is due to a kind of internal greenhouse effect. Stars generate heat by fusing hydrogen atoms in their hot centres, to make helium. This gas behaves in a star rather like carbon dioxide in our planet’s atmosphere: it acts as a blanket to impede the escape of heat, so raising the temperature of the star’s interior. This greatly increases the star’s fusion reaction and so its heat output. In the 4.6 billion years of our Sun’s existence, it will have increased its heat production by 25 per cent, as the graph above shows. The dotted line (1) shows the effect this should have had on Earth’s surface temperature. The solid lower plane (2) shows the actual history of Earth’s surface temperature, so far as it is known. The difference between the two is considerable, and is an important part of the evidence for Gaia. It suggests that some temperature regulating effect due to life’s involvement is at work - probably the level of the biologically controlled greenhouse gas, carbon dioxide. As shown on the graph, this has steadily fallen over the period of life on Earth, compensating for the increased solar heat reaching the planet. (see variation 19)