From wood to coal and from coal to oil and thence to gas, the energy density of these fuels increased with each step. We were getting more power from burning comparatively less fuel. On the one hand, the fuels themselves were getting considerably cleaner: wood combustion emits up to 110 kilograms of carbon dioxide per gigajoule of energy, compared with about 60 kilograms of CO2 for each gigajoule provided by natural gas. However, those improvements could not offset the fact that there were more and more of us consuming the energy.[4]
