How difficult is the transition to a low-carbon technological paradigm? Using the entire universe of patents granted by the USPTO from 1976 to 2020, we evaluate the intellectual distance between clean and dirty technologies. We measure intellectual distance by checking how many hops one has to make in the citation network to get from a clean patent to a dirty one (where one hop means the clean patent directly cites a prior dirty patent). We find that 73% of clean patents are connected in some way to dirty patents, while only 7.5% of clean patents directly cite dirty prior art. The mean distance between a clean and dirty patent is about 3 hops. By comparison, for any two randomly selected patents it is 8.5. In addition, we observe considerable heterogeneity across sectors. Our results suggest that targeted clean R&D may be needed to overcome “intellectual carbon lock-in”, and in sectors where clean and dirty technologies are distant, the transition may be less about diversification and more about Schumpeterian creative destruction.