I study the dynamics of greenhouse gas emissions following disruptions in crude oil markets. Following a 10% unexpected increase in crude oil prices, CO_2-equivalent atmospheric concentration declines by 3.24% relative to trend after two years. Solar energy does not take-up contrarily to common expectations.

I introduce a new empirical strategy to provide the first estimate of the aggregate elasticity of substitution between polluting and non-polluting energy. Exploiting variation in US states’ energy mixes, I estimate an elasticity of 0.50 — much lower than most calibrations assume. Gross complementarity between energy sources complicates the green transition and significantly raises the economic costs of climate change.