E. Linacre and B. Geerts
The intensity of the Sun’s radiation has increased since the creation of the solar system, about 4.7 billion years ago. Initially it was 70% of what it is now, and during the Carboniferous, about 300 million years ago, when the first dinosaurs appeared and tropical vegetation grew abundantly, it was about 2.5% less than today. It has been shown (1) that the solar 'constant', as a percentage of its current value of 1373 W m-2, can be described as
[1 + 0.4(1 - t/4.7)]-1
where t is the time in billions of years since the creation of the solar system. So in 4.7 billion years from now, the Sun would be about 67% 'hotter', in terms of its outgoing radiation. During the evolution of the Earth climate conditions developed suitable for flora and fauna to flourish, and these conditions were sustained notwithstanding the increase in solar radiation. This is a key building block of the Gaia hypothesis (Section 1.2) (2).
Possible temporary variations lasting on the order of 10 million years (every 300 million years or so) may account for glacial periods on Earth: the Pleistocene is a glacial period (containing several Ice Ages), the previous ones were 300 and 700 million years ago.
The 'solar constant' also varies on the shorter time scales, from days to years, on the order of 1-5 W m-2 , for instance due to the development or decay of sunspots.
(1) Clough, D.O. 1981. Solar interior structure and luminosity variations. Solar Physics, 74, 21-34.
(2) Kirchner, J.W. 1991. The Gaia hypothesis: Are they testable? Are they useful? In Schneider, S. (ed.) Scientists on Gaia. Cambridge, MA: MIT Press.