The researchers found that this warming and reduced precipitation contribute to a preferential warming of the hottest days of the year.
"We found that the hottest days of the year, or the 'hot tail,' warm more than the typical summer days warm," Diffenbaugh said. "One might expect that an average warming of four degrees would equate to each day warming by four degrees, but in fact the hottest days warm quite a bit more."
This is due, in large part, to a surface moisture feedback. The surface gets dryer as it gets hotter and the dry soil leads to less moisture in the area and less evaporative cooling. The locations of intensified warming on hottest days of the year matched the locations where surface drying occurred, Diffenbaugh said.
With the projected shift to more severe temperatures, the daily temperatures currently found in the hottest two weeks of the summer instead are found in the coldest two weeks of the summer in the future climate scenario, Diffenbaugh said.
"The hottest temperatures we are used to experiencing will become the normal temperatures of the summer, and the hot periods will be magnified," Diffenbaugh said. "Take Paris: If we look at the temperatures that occurred there during the heat wave in 2003, when 15,000 people died, those temperatures are exceeded a couple dozen times every year in the future projection. That means that severe heat waves, such as those rare events that have occurred in the past couple of years, are likely to become far more common."
The study used the National Weather Service Heat Index in the analysis of the heat stress response to increasing greenhouse gas concentrations. The researchers found that areas most likely to face substantial increases in the dangerous heat index were concentrated largely in coastal areas.