Lemoine and Rudik (2017) argue that it is efficient to delay reducing carbon emissions, because there is substantial inertia in the climate system. However, this conclusion rests upon misunderstanding the relevant climate physics: there is no substantial lag between CO2 emissions and warming, which policy could rely upon. Applying a mainstream climate physics model to the economics of Lemoine and Rudik (2017) invalidates the article's implications for climate policy: the cost-effective carbon price that limits warming to a range of targets including 2°C starts high and increases at the interest rate.