A critical part of adapting to the higher temperatures that climate change brings will be the deployment of existing technologies to new sectors and regions. This paper examines the evolution of the temperature-mortality relationship over the course of the entire 20th century in the United States both for its own interest but also to identify potentially useful adaptations that may be useful in the coming decades. There are three primary findings. First, the mortality impact of days with a mean temperature exceeding 80° F has declined by about 70%. Almost the entire decline occurred after 1960. There are about 14,000 fewer fatalities annually than if the pre-1960 impacts of high temperature on mortality still prevailed. Second, the diffusion of residential air conditioning can explain essentially the entire decline in hot day related fatalities. Third, using Dubin-McFadden's discrete-continuous model, we estimate that the present value of US consumer surplus from the introduction of residential air conditioning (AC) in 1960 ranges from $83 to $186 billion ($2012) with a 5% discount rate. The monetized value of the mortality reductions on high temperature days due to AC accounts for a substantial fraction of these welfare gains.