Across the globe, warming temperatures are increasing the risk of hot weather and decreasing the risk of colder weather. Hot summer days are getting hotter, as well as more frequent, sometimes stretching into multiday heat waves.
During the past decade, daily record high temperatures have occurred twice as often as record lows  across the continental United States, up from a near 1:1 ratio in the 1950s. By midcentury, if greenhouse gas emissions are not significantly curtailed, scientists expect 20 record highs for every record low. The ratio could be 50:1 by the end of the century. By the 2050s, many of the Mid-Atlantic States including urban parts of Maryland and Delaware could see a doubling of days per year above 95 degrees F. In parts of the South, the frequency of days above 95 degrees F could triple, to over 75 days per year. While climate change will not mean the end of cold weather — scientists still expect record lows to occur in 2100 — the odds have clearly shifted (see map).
Extreme heat can also increase the risk of other types of disasters. When heat occurs in conjunction with a lack of rain, drought  can occur. This, in turn, can encourage more extreme heat, as the sun’s energy acts to heat the air and land surface, rather than to evaporate water. Hot, dry conditions also increase the risk of wildfires, like the ones in 2013 in Colorado that were fueled by record heat and an ongoing drought. 
Extreme heat is the most deadly natural disaster in the U.S., killing on average more people (about 600 per year ) than hurricanes, lightning, tornadoes, earthquakes, and floods combined. The Billion Dollar Weather Disasters database compiled by the National Oceanic and Atmospheric Administration lists heat waves as four of the top 10 deadliest U.S. disasters since 1980 . Two heat waves in 1980 and 1988 were particularly deadly, contributing to 10,000 and 7,500 deaths respectively. These two events account for the vast majority of extreme weather-related deaths in the database, with Hurricane Katrina (1,833 deaths) as the only non-heat wave event that caused more than 500 fatalities.
High humidity and elevated nighttime temperatures (i.e., when nighttime low temperatures remain relatively warm) appear to be key ingredients in causing heat-related illness and mortality. Heat stress occurs in humans when the body is unable to cool itself effectively . Normally, the body can cool itself through sweating, but when humidity is high, sweat will not evaporate as quickly, potentially leading to heat stroke. When there’s no break from the heat at night, it can cause discomfort and lead to health problems, especially for the poor and elderly.
High temperatures at night can be particularly damaging to agriculture. Some crops require cool night temperatures , and heat stress for livestock rises when animals are unable to cool off at night . Heat-stressed cattle can experience declines in milk production, slower growth, and reduced conception rate s.
Higher summer temperatures will increase electricity use, especially during heat waves. An increase in cooling demand is already apparent  over the past 20 years. Although warmer winters will reduce the need for heating, modeling suggests that total U.S. energy use will increase in a warmer future. In addition, as rivers and lakes warm, their capacity for absorbing waste heat from power plants declines. This can reduce the thermal efficiency of power production, make it difficult for power plants to comply with environmental regulations regarding their cooling water, and can make it harder to get permits for new facilities.
Communities can bolster their resilience and reduce the impacts of extreme heat by: