The Local Supply of Innovators and the Geography of Innovation: Evidence from Green and Brown Technologies

We study the causal effect of the local supply of sector-specific innovators on patenting activity across US metropolitan areas, distinguishing between innovators active in carbon-intensive ("brown") and environmentally sustainable ("green") technological fields. Using USPTO patent data from 1990 to 2016, we document a marked shift in the geography of green innovation: while brown patenting has long been concentrated in established hubs, green patents — initially more dispersed — have increasingly converged toward the same locations. We build a theoretical framework in which local patenting activity is driven by the supply of green and brown innovators, investigating how their interaction shapes the innovation process. Empirically, we address endogeneity using a shift-share instrument that combines predetermined local technological specialization with exogenous shocks to foreign innovation across CPC sections. We find that a one-unit increase in the local supply of brown innovators raises patenting activity by approximately 0.8%, an effect that is robust across specifications. Together, these findings suggest that green innovation is becoming increasingly embedded in existing agglomeration ecosystems, with important implications for place-based climate policy.