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But look beyond fossil fuels for the most intriguing trends. One is that the energy intensity of the world economy — the amount of energy it takes to produce one dollar’s worth of income — keeps falling, at a rate of about 2 percent. What this means is that even without any change in the relative shares of fossil-based and fossil-free sources in the world’s energy mix, we could have 2 percent annual economic growth without increasing carbon emissions from energy use. Of course that is not enough to address climate change and we need more economic growth than that. It is nonetheless a stunning number, which refutes the claim by some environmentalists that permanent economic growth is fundamentally incompatible with finite physical resources.