tl;dr:

Conclusions

This study shows that the frequency of and population exposure to extreme heat index conditions in the US will increase substantially by mid-21st century under a range of emissions and population change scenarios. By late century, depending on the scenario, these changes amount to a 4- to 20-fold increase in person-days per year of high heat index conditions from 107 million historically to as high as 2 billion. The current extreme heat alert system used by the National Weather Service relies on specific heat index thresholds. This work illuminates how, across much of the country, those seldom-crossed thresholds become frequently surpassed over the course of this century, putting millions of people at risk.

Economic development, technological advances, and improved communication efforts have reduced heat-related mortality in the US in recent decades (Davis et al 2003). Given the future frequency and extent of dangerous heat events, however, additional efforts to help people cope with extreme heat, particularly in places unaccustomed to such heat historically, will likely become necessary. With late century extreme heat index conditions and exposure under RCP8.5 being roughly double of that under RCP4.5, reductions in global greenhouse gas emissions are a complementary strategy for managing the future impacts of extreme heat in the US.