This paper develops two-period analytical and numerical models to study the question: given a stock of greenhouse gases that poses a risk of future damages of unknown magnitude, and the possibility of learning about damages, how do sunk abatement capital and a nondegradable stock of greenhouse gases affect optimal first-period investment? We show that both affect investment, the former negatively and the latter positively. Additionally, endogenous risk--the risk of damages dependent on the stock of...