Astronomers and meteorologists now generally agree in the opinion that the so-called solar constant—the extraterrestrial solar radiation integrated over all wave lengths—is subjected only to very small variations, as far as the short time interval of not quite a century, covered by our observations, is concerned.Not so if we consider time intervals of billions of years. On the basis of what we now know about the sources of energy, which are keeping the sun hot, and ofthe physical constitution of the sun. F. Hoyle has made a theoretical computation of what will happen to the sun, as regards its output of energy and its dimensions. In 10 million years we get, according to Hoyle, an increase in the solar constant of about 0·2 per cent, but after about 6 billion years the solar constant will be about 3 times its present value.Hoyle draws the conclusion from his theory that the organic life on the earth will not be able to persist more than about 2–3 billion years more, whileafter that period the temperature will be higher than now by 50°C or more, an evaluation evidently founded on the idea that the present albedo of the earth remains unchanged.The present author has started from the idea, at first put forward by Sir George Simpson, that the system earth-atmosphere is functioning as a kind of large-scale thermostatic device, where changes in direct radiation from the sun are largely compensated by an increase in the albedo of the system, resulting from an increase in cloudiness. Applying to this idea his own results as regards the albedo of the turbid atmosphere, and also recent results on the albedo of dense cloud sheets, the author suggests that the critical intensity of solar radiation will occur first after 6 billion years, which gives a lifetime of organic life, which is almost the double of that derived by Hoyle.The discussion suggests that before the critical temperature is reached, the earth can be expected to go through a period of increasing cloudiness and turbidity, ending in a state where the atmosphere remains permanently cloudy.One must finally regret to have to state that there are possible causes of a total destruction of organic life on the earth which are, at present, of much greater actuality than those considered here.