A Comment on the Environment and Directed Technical Change

The major claim in Acemoglu, Aghion, Bursztyn & Hemous (2012) (AABH) is that subsidies for research and development of clean technologies are more important than carbon taxes when dealing with climate change. However, they - unconventionally - assume that a patent only lasts for one period. In this note we introduce long-lived patents into the AABH model. This makes the role of a research subsidy for clean technologies in AABH far less crucial and reestablishes the role of the carbon tax. This is good news as it is far easier to tax emissions than to pick the right technologies to subsidize.