A Guide Growth Rate applies where the growth of a technology is exponential. Clearly this cannot continue for ever, at some point a limit will be reached. For example Moore’s law has been followed for about 5 decades but may be prevented from further growth by the laws of physics. This is evident by a flattening off.

Growth curves will have start-up phase, defined by realisation of importance and basic research, a uniformly exponential growth phase and a ceiling phase.

For solar energy the realisation of importance can reliably pinned down to 1972, with the publishing of “Limits to Growth”. The growth of solar energy has followed the Guide Growth Rate until now, five decades later with no sign of flattening off.

Mathematically sigmoid functions have graphs with a characteristic S-shape. Trying to fit any such function to real live growth would be seriously misleading. The shape of the start-up phase, the steady growth phase and the ceiling phase are each determined a very different set of historical, economic and practical constraints, which will be different for each technology. Clearly there can’t be a general rule for predicting how long the steady growth phase will last.

None of the technologies relevant to climate change seem to be show signs of flattening off apart from nuclear where growth has been negative for the last two decades and, unusually, the cost has been increasing.