The most recent generation of climate models exhibits an alarming increase in high climate sensitivity models compared to previous generations. Because the calculation of equilibrium climate sensitivity (ECS) requires simulations of a thousand years or more, most studies estimate ECS using shorter model integrations. However, the most widely used method for estimating ECS from shorter simulations underestimates ECS. Previous studies attributed this underestimate to the time-dependence of climate feedbacks. Here we demonstrate that it actually arises from an underestimate of the radiative forcing. We present a modified method that corrects for this underestimate and is shown to better agree with ECS calculated from “long run”, millennium-scale simulations. This method reveals that the actual number of “too hot” models is roughly double that previously diagnosed, with one out of every three CMIP6 climate models having an ECS greater than 5K - the “very likely” upper bound on ECS.