One of the clearest findings of climate science is that global warming amplifies the intensity, duration and frequency of extreme heat events. These events occur on multiple time scales, from a single day or week, to months or entire seasons. When heat extremes last for a period of several consecutive days, the event is called a heat wave. Globally, the number and duration of heat waves as well as the frequency of hot days and nights has increased since 1950 in response to rising temperatures.

Top lines

  • The link between climate change and extreme heat is firmly established.
  • Climate change has already increased the intensity, duration and frequency of extreme heat events and will continue to amplify these events in the future.
  • Recent extreme heat events expose the dangers and costs of living in a warming world.

What scientists know well, and what's left to learn, about extreme weather and climate change. Heat waves are well understood and documented. Climate Central

Century increase in the number of monthly heat records, given as the ratio of the observed number of extremes at  17 global weather stations to that expected in a stationary climate. Coumou and Rahmstorf, 2012

The level of humidity in the air has also increased, making it more difficult for the human body to cool down during heat events. These impacts are being felt worldwide, with the area experiencing extreme summer temperatures growing well over ten times in the past 30 years. In the US, new record-breaking high temperatures have outnumbered new record lows by a ratio of 2:1. By mid-century, people in the US can expect a four to sixfold increase in the number of days exceeding 35°C (95°F).

The impacts of these trends can be devastating. Nine out of the ten most deadly heat waves have occurred since 2000, causing 128,885 fatalities globally. In the US, exposure to extreme heat is already the primary cause of weather-related mortality. As climate change drives more frequent and longer-lasting heat waves, associated illnesses and deaths multiply, especially in metropolitan areas and communities at higher latitudes, which are not adapted to such extreme temperatures.

Climate change amplified extreme heat events

How climate change increases the probability of extreme heat events. IPCC

For example, from 1951-1980 the occurrence of extremely hot summertime temperatures impacted much less than 1 percent of the globe. Now, thanks to global warming, the rare extremes from 1951-1980 impact 10 percent of the globe. Through probabilistic analysis, scientists can determine with a high degree of confidence whether climate change is at fault for specific extreme heat events. This analysis also shows how even a small-seeming change in average global temperature—0.72°C (1.3°F)since 1951—can lead to dramatic regional changes in extreme events.

In addition to these global scale trends, scientists are also investigating the role that changes in the frequency, persistence and duration of regional circulation patterns may have on extreme temperature trends and have detected an influence.

Climate trends

In its most recent report, the UN Intergovernmental Panel on Climate Change (IPCC) expressed confidence that “globally the length and frequency of warm spells, including heat waves, has increased since the middle of the 20th century,” with data supporting these trends particularly strongly in Europe, Asia and Australia.

The US National Climate Assessment finds that, “Prolonged periods of high temperatures and the persistence of high nighttime temperatures have increased in many locations (especially in urban areas) over the past half century.”
In addition, a January 2015 study found that during the period 1973-2012, many urban areas across the globe experienced significant increases in the number of heat waves, with the largest number of heat waves occurring in the most recent decade studied, 2003-2012. Another study found that seventy-five percent of moderate heat events—which are those historically occurring once every three years—are due to climate change. In Europe, global warming has doubled the continent’s risk of heat waves.

Health impacts

Exposure to extreme heat is already a significant public health problem and the primary cause of weather-related mortality in the US. As temperatures continue to increase due to climate change, heat stress is expected to worsen.

The limits to which humans can work at a range of temperatures and humidity. Guardian

In very dry conditions, humans can be physically active outdoors in temperatures of up to 40°C, but in humid conditions, the cutoff drops below 30°C because it is harder for the body to cool itself through sweating and evaporation. When the combination of heat and humidity becomes too high, continued exposure to heat leads to heat illness. Heat cramps are the first step, followed by exhaustion and eventually heat stroke.

Using the Wet Bulb Globe Temperature index—which measures the combined effect of heat and humidity in the Sun—scientists have determined that a sustained wet-bulb temperature exceeding 35°C (95°F) is likely to be fatal even to fit and healthy people. At this temperature our bodies switch from shedding heat to the environment, to gaining heat from it. Thus a wet-bulb temperature of 35°C is the threshold beyond which the body is no longer able to adequately cool itself.

study analyzing the impacts of extreme heat in New York City found that increasing numbers of low- and middle-income households will be exposed to dangerous heat extremes as a consequence of global warming. Another study analyzing heat stress in Rhode Island found that state residents of all ages experience more medical distress when the temperature rises over 75°F. Warmer temperatures are also expected to reduce labor capacity considerably and decrease air exchange between indoors and outdoors, leading to an increase in indoor air pollution, a problem already responsible for 4.3 million deaths annually (primarily from burning coal and biomass indoors).

Recent events

Extreme and record-breaking heat waves are signals of climate change, because records are more likely to be broken when climate change runs in the same direction as natural variation.

Studies have shown that global warming increased the odds of the 2003 European heat wave, the 2010 Russian heat wave, and the Australia’s record breaking summer of 2013. A study from the World Weather Attribution group also found a connection in real time between climate change and Europe’s July 2015 heat wave. The impacts of these events are devastating. Nine out of the ten most deadly heat waves have occurred since 2000, causing 128,885 fatalities globally.


According to the US National Climate Assessment, “Climate models project that the same summertime temperatures that ranked among the hottest 5% in 1950-1979 will occur at least 70% of the time by 2035-2064 in the U.S. if global emissions of heat-trapping gases continue to grow...By the end of this century, what have previously been once-in-20-year extreme heat days (1-day events) are projected to occur every two or three years over most of the nation.” 

This has serious implications for human health, and even viability. One study finds that warming of roughly 7°C would create small zones where humans would likely be unable to survive. The same study finds that warming of 11–12°C would expand these zones to encompass most of today’s human population.

2015 extreme heat events

  • India: May 24 - 30, 2015—2,500 people died in India’s second-deadliest and the world’s fifth-deadliest heat wave on record. On May 23, India experienced a temperature of 42.2°C (108°F) in the northeast town of Bhubneshwar with a dew point of 29.3°C (84.7°F), creating conditions for a heat index of 62°C (143.6°F). Peak wet-bulb temperatures measured in the heatwave were around 30-31°C (86°F).

  • Pakistan: June 11 - 24, 2015—1,150 died in Pakistan’s deadliest heat wave, joining India just weeks after its record heat wave in the world’s top ten deadliest heat waves. Temperatures topped out with a high temperature of 44.8°C (112.6°F) and a heat index of 121°F on June 20.

  • Western and Central Europe: June 28 - July 7, 2015—Across parts of Europe, temperatures reached up to 7°C (12-13°F) above average. The UK had its hottest July maximum temperature on record, as did the Netherlands; several locations across France recorded their hottest temperature ever, and Paris recorded its second hottest. July was the hottest month on record in Spain, with Madrid recording its hottest ever June and July temperatures. Germany set a new national record for the hottest temperature ever recorded on July 5 at 40.3°C (104.5°F). In Switzerland, Geneva observed its hottest day in history on July 7 at nearly 40°C (103.5°F).

  • Iran: July 31, 2015—Iran city registered the second-highest heat index on record of 74°C (165°F). The registered temperature was 115°F, but with a dew point temperature of 90, the air felt so hot the heat index was off the charts.

  • Middle East: mid-July - early August, 2015—In Turkey, a heatwave in Istanbul led to 100 people drowning as they attempted to beat the heat. In Israel, nighttime temperatures reached a record high and as did electricity consumption. In Egypt, temperatures of 46°C (114°F) and high humidity led to the death of 60 people and hospitalization for 581 others. In Iraq, citizens protested amid extreme heat as power outages made it difficult to cope with temperatures of 50°C (122°F).

  • Japan: July 27 - August 5, 2015—Tokyo experienced an unprecedented eight consecutive days of extreme temperatures above 35°C (or 95°F), leading to 55 heat-related deaths and 11,000 hospitalizations. The previous record was just four consecutive days. From August 3 to August 8, Japan’s weekly death toll from heatstroke hit a record high, with 32 victims.

  • Texas: August 6, 2015—Electric demand in Texas reached an all-time peak, breaking the previous record set four years ago and pushing prices to an 18-month high as air conditioners were relied upon to survive a heat wave with temperatures nearing 100°F.

  • Central and Eastern Europe: August 7 - 14, 2015—Berlin and over 100 other locations in Germany set or tied all-time highs with temperatures over 38°C (100°F); in Poland, Warsaw registered its hottest August temperature on record, and officials cut electricity supplies for industry, the first time it’s had to do this in more than two decades.

Worst extreme heat events on record

  • Russia: July 28 - August 8, 2010—While second to Europe’s 2003 heat wave in terms of fatalities, Russia’s 2010 event was the worst in recorded history in terms spatial extent and temperature anomalies when compared to the long-term average. Temperatures in the central region of the country, including Moscow, were around 10°C (18°F) above what they should have been for the time of year. An estimated 55,000 people died from respiratory illnesses and heat stress.

  • Europe: June - August 2003—Centered in France, the 2003 heat wave in Europe was the most lethal heat wave in world history, killing more than 80,000 people, including 14,800 in France alone. It was also the most intense European heat wave in terms of temperatures in at least 500 years. Extreme maximum temperatures of 35 to 40°C (95 to 104°F) were repeatedly recorded in July and to a larger extent in August in most of the southern, and central countries from Germany to Turkey.

  • Chicago: July 13 - 17, 1995—The Chicago heat wave of 1995 was the worst heat wave on record in the United States, leading to 733 heat-related deaths over a period of five days. The temperature hit 106°F, and the heat index climbed above 120°F.


Relevant Scientific Quotes

U.S. Fifth National Climate Assessment

“Prolonged periods of high temperatures and the persistence of high nighttime temperatures have increased in many locations (especially in urban areas) over the past half century. High nighttime temperatures have widespread impacts because people, livestock, and wildlife get no respite from the heat. In some regions, prolonged periods of high temperatures associated with droughts contribute to conditions that lead to larger wildfires and longer fire seasons” (Chapter 1, p. 9).

“As expected in a warming climate, recent trends show that extreme heat is becoming more common, while extreme cold is becoming less common. Evidence indicates that the human influence on climate has already roughly doubled the probability of extreme heat events...The incidence of record-breaking high temperatures is projected to rise" (Chapter 1, p. 9).”

“The number of extremely hot days is projected to continue to increase over much of the United States, especially by late century. Summer temperatures are projected to continue rising, and a reduction of soil moisture, which exacerbates heat waves, is projected for much of the western and central U.S. in summer. Climate models project that the same summertime temperatures that ranked among the hottest 5% in 1950-1979 will occur at least 70% of the time by 2035-2064 in the U.S. if global emissions of heat-trapping gases continue to grow (as in the A2 scenario). By the end of this century, what have previously been once-in-20-year extreme heat days (1-day events) are projected to occur every two or three years over most of the nation. In other words, what now seems like an extremely hot day will become commonplace” (Chapter 2, p. 39).

IPCC Fifth Assessment Report

“Since about 1950 it is very likely that the numbers of cold days and nights have decreased and the numbers of warm days and nights have increased overall on the global scale, that is, for land areas with sufficient data. It is likely that such changes have also occurred across most of North America, Europe, Asia and Australia” (WGI, Chapter 2, p. 212).

“Warm days and nights have increased and cold days and nights have decreased for most regions of the globe" (WGI, Chapter 2, p. 213).”

“Increasing numbers of studies are finding that the probability of occurrence of events associated with extremely high temperatures has increased substantially due to the large-scale warming since the mid-20th century” (WGI, Chapter 10, p. 916).

“It is very likely that, on average, there will be more record high than record cold temperatures in a warmer average climate. For example...the current ratio of 2 to 1 for record daily high maxima to low minima over the USA becomes approximately 20 to 1 by the mid-21st century and 50 to 1 by late century in their model simulation of the SRES A1B scenario. However, even at the end of the century daily record low minima continue to be broken, if in a small number...[C]old extremes will continue to occur in a warmer climate, even though their frequency will decline” (WGI, Chapter 12, p. 1066).

A key climate change risk is "declining work productivity, increasing morbidity (e.g., dehydration, heat stroke, and heat exhaustion), and mortality from exposure to heat waves. Particularly at risk are agricultural and construction workers as well as children, homeless people, the elderly, and women who have to walk long hours to collect water (high confidence)” (WGII, Chapter 13, p. 811).