The former grew steadily between 1975 and 1985, but then slipped back to around its original level of 1.8 quadrillion Btu. The latter began the period at the same level as wood but experienced more substantial growth. However, it also fell back to around 2 quadrillion Btu, with a particularly sharp drop in the late 1990s.
Finally, wind power emerged late in the period but showed a gradual rise to around 0.5 quadrillion Btu, suggesting that it, along with biofuels, will replace wood and hydroelectricity as the main sources of renewable every in the future.