And: E/G is the energy intensity of the GDP F/E is the carbon footprint of energy If we want to get F, total carbon to zero, we have to get one of these four factors to zero. Assuming that at some stage this century we must drive F (total carbon rate) to zero, this implies at least one of these factors must also be zero. But which ones? Setting P = 0 is the human extinction plan and I think we can rule that one out. Nonetheless humanity is now entering an entirely new demographic state we have never been in before; many developed nations are now at zero or even negative population growth. This has long term implications well worth exploring in a future post.