In this project, a ‘heat wave’ model is proposed for producing two-dimensional mortality improvement scales with solid statistical justifications. In the ‘heat wave’ model, short and long-term mortality improvements are treated respectively as ‘heat waves’ that taper off over time and ‘background improvements’ that always exist. Using the model, one can derive two-dimensional mortality improvement scales with minimal expert judgment. Moreover, with likelihood-based inference methods, the uncertainty surrounding the best estimate of mortality improvement rates can be quantified.