<p>Extreme heat causes by far the most fatalities of any type of hazardous weather (<a href="https://www.weather.gov/hazstat">https://www.weather.gov/hazstat</a>). As the globe continues to warm owing to the additional heat trapped by increasing concentrations of greenhouse gases in the atmosphere, areas prone to excessive heat have expanded, some verging on uninhabitable. We present a new metric – the accumulated dangerous heat index – to identify regions in Northern Hemisphere summers where the combination of temperature and humidity causes hazardous heat conditions. We assess changes in the magnitude and duration of dangerous heat, attribute those changes to shifts in temperature and/or humidity, and investigate possible links with amplified warming over high-latitude land. These results provide further motivation to reduce emissions of heat-trapping gases, and they will assist decisionmakers and planners in preparing for a future with an increased threat of fatal heat.</p>