A warming climate inevitably results in more warm days. As greenhouse gas concentrations rise, the number of cold days and nights are expected to decrease, while hot days are expected to become warmer and more frequent. Below are projections of the number of hot days (above 100°F or 38°C) that the United States may experience from 2080-2099 under low emission and high emission scenarios.

Fortunately, the number of cold-related deaths is projected to decrease as the climate warms; however, the frequency of heat waves is expected to increase. Heat waves can have dire consequences. During the summer of 2003 sustained temperatures of approximately 40°C killed more than 40,000 people in Europe. Elderly people were the worst affected. Climate models forecast that heat waves of this magnitude are expected to become more common, possibly occurring every other year by 2040.