Global warming may cause more frequent and more intensive heatwaves resulting in an increase in heat-related deaths. A heat wave is a prolonged period of excessively hot weather, and this may be accompanied by high humidity.

There appears to be no universal definition of a heat wave and the term is relative to normal weather in an area. For instance, In the Netherlands a heat wave is defined as period of at least 5 consecutive days in which the maximum exceeds 25 °C (77 °F), provided that on at least 3 days in this period the maximum temperature exceeds 30 °C (86 °F). In the United States, a heat wave is usually defined as a period of 3 or more consecutive days above 90 °F (32.2 °C).

Temperatures that people from a hotter climate consider normal can be termed a heat wave in a cooler area if they are outside the normal pattern for that area. Severe heat waves have caused catastrophic crop failures, thousands of deaths from heat stroke, and widespread power blackouts due to increased use of air conditioners.

Severe heat waves can lead to deaths from heat stroke. Older people, very young children, and those who are sick or overweight are at a higher risk for heat-related death. Heat waves are the most lethal type of weather phenomenon, overall. Between 1992 and 2001, deaths from heat waves in the United States numbered 2,190, compared with 880 deaths from floods and 150 from hurricanes.

If a heat wave occurs during drought conditions which dries out vegetation, it can contribute to wildfires. During the disastrous 2003 heat wave in Europe, fires raged through Portugal, destroying over 3010 km² (740,000 acres) of forest and 440 km² (108,000 acres) of agricultural land and causing an estimated 1 billion pounds worth of damage. This heat wave killed tens of thousands. Much of the heat was concentrated in France, where nearly 20,000 people died.

If a heat wave is predicted or happening in your area the American Red Cross has some useful tips on what to do.