The Drake Equation: The L Term L: The time in years during which the civilisation remains in a communicative phase. Let us be optimistic and assume that we can achieve a stable long-term society here on Earth. How long might it survive? There are two factors outside our control that would suggest that L would not be longer than several hundred million years. There is no way that we can prevent the Sun getting hotter. Even without the additional effect due to the release of greenhouse gasses the Earth will begin to get uncomfortably hot within around 500 million years. It is also likely that before this time the Earth would have suffered a major cometary impact. In contrast to asteroids, whose orbits can be computed so enabling us to predict in advance when an impact might occur, many comets are detected only a relatively short before they enter the inner Solar System, thus providing virtually no time for an attempt to destroy or deflect them. But then, for what percentage of that time might we actively attempt to communicate with other civilisations? One would expect that our civilisation here on Earth would be stable, having a sustainable population, food and power supply. There would need to be cheap energy, and perhaps a need for things to do, as the problems of everyday life have been taken care of. Will such a utopia come to exist? Who can tell. Lets be positive, L could be a million years or more, but it is much more likely to be less - perhaps a value around 10,000 to 100,000 years might be a suitable guess for the average value.