original source: https://www.ted.com/talks/bill_gates_innovating_to_zero/transcript
This equation has four factors, a little bit of multiplication. So you’ve got a thing on the left, CO2, that you want to get to zero, and that’s going to be based on the number of people, the services each person is using on average, the energy, on average, for each service, and the CO2 being put out per unit of energy. So let’s look at each one of these, and see how we can get this down to zero. Probably, one of these numbers is going to have to get pretty near to zero.