The C02 emissions increased even less, indicating a pervasive historical trend towards decarbonization of the global economy at about 1.3 per cent per year. The share of fossil energy sources, taken together, increased (from 20 to 80 per cent) between 1850 and now, as did the emissions of C02 (as an unavoidable by-product of combustion). Consequently, energy-related emissions of C02 increased 21-fold to about 6 billion tons of carbon (6 GtC) in 2000. Nevertheless, their increase has remained at a substantially slower pace than that of energy requirements, indicating a strong historical trend towards decarbonization of societies.