Green dotted lines = confidence interval 95%. Information is patchy on innovation spillovers or transfers between technologies, sectors, countries. The pace of depreciation of knowledge generated by innovation investments is also not clearly understood. A key lesson is that conclusions about the scale and even sometimes the direction of the economic impacts of policies that encourage the uptake of low-carbon technologies and practices differ considerably across different models, because of their diverse theoretical underpinnings, assumptions, and the corresponding modelling.