We study how uncertainty about climate change severity affects the relative benefits of early abatement and a portfolio of research and development (R&D) in lowering future abatement costs. Optimal early abatement depends on the curvature of the marginal benefit and marginal abatement cost (MAC) functions and how the uncertain parameter affects marginal benefits. R&D in a particular technology depends on whether uncertainty increases early abatement; whether investment lowers marginal costs for that technology; whether R&D lowers the slope of that technology's MAC function; and the shape of that technology's MAC function. We illustrate, focusing on the role of backstop technologies.