Last updated May 23, 2017

Increased Extreme Heat and Heat Waves

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—and are defined by temperatures significantly above average for that period.

The climate has shifted significantly, leading to more heat records in every season. The number of local record-breaking average monthly temperature extremes worldwide is now on average five times larger than expected in a climate with no long-term warming.[1] 85 percent of recent record-hot days globally have been attributed to climate change.[2] 

The more extreme the heat wave, the more likely the event can be attributed to global warming. However, the impact of climate change on "moderate" heat waves (i.e. 1-in-3 year events) is also dramatic, with a 75 percent share of such heat events now attributed to climate change.[3]

Record hot days: record cold days

In a stable climate, the ratio of days that are record hot to days that are record cold is approximately even. However, in our warming climate, record highs have begun to outpace record lows, with the imbalance growing for the past three decades.[1] 85 percent of recent record-hot days globally are attributed to climate change.[2]

The world is not quite at the point where every hot temperature record has a human fingerprint, but it's getting close to that.

Noah Diffenbaugh, Stanford University


The more extreme the heat wave, the more likely the event is due to global warming

A small shift in climate leads to a dramatic increase in the frequency of temperatures at the high end. The very most extreme events are the events most affected by climate change. 

… more than half of the hot extremes worldwide...can be attributed to global warming. Not one of these events is solely the direct result of warming, but warming increases their frequency. And the less common and more extreme the hot extreme...the more this can be attributed to a man-made contribution.

Erich Fischer, ETH Zürich[3]

As the average global temperature rises and the climate shifts, hot temperatures that were extreme under the old climate are closer to the middle of the new temperature range. Under the earth's climate system, events closer to the midpoint of the climate range occur much more frequently than events closer to the extremes, as shown in the graphic on the right. The shifting bell curve also leads to the occurrence of never-before-seen extremes in high temperatures.[4][5][6]

...it is the rarest and the most extreme events - and thereby the ones with typically the highest socio-economic impacts - for which the largest fraction is due to human-induced greenhouse gas emissions.[7]


Global trends

Since 1950, the number and duration of heat waves worldwide has increased due to global warming. 

"It is very likely that there has been an overall decrease in the number of cold days and nights, and an overall increase in the number of warm days and nights, at the global scale...It is likely that these changes have also occurred at the continental scale in North America, Europe, and Australia. There is medium confidence of a warming trend in daily temperature extremes in much of Asia. Confidence in observed trends in daily temperature extremes in Africa and South America generally varies from low to medium depending on the region."[8]

Summertime heat extremes—defined as three standard deviations (3σ) warmer than the climatology of the 1951-1980 base period—now cover about 10 percent of global land area, compared to much less than 1 percent of the earth during the base period.[4]

The number of local record-breaking monthly temperature extremes worldwide is now on average five times larger than expected in a climate with no long-term warming, implying that on average there is an 80 percent chance that a new monthly heat record is due to climate change.

Many urban areas across the globe have witnessed a significant increase in the number of heat waves, with the largest number of heat waves occurring in the most recent decade studied, 2003-2012.[9]

The impact on "moderate" heat waves (i.e. 1-in-3 year events) is also dramatic, with a seventy-five percent share of such heat events now attributed to climate change.[7]

The fingerprint of global warming has been firmly identified in these trends.

An April 2017 study found that anthropogenic global warming had a significant hand in the temperatures seen during the hottest month and on the hottest day on record throughout much of the world from 1931–2016.[2] The study found that climate change made heat records more likely and more severe for about 80 percent of the area of the globe with good observational data.[2]


US trends

In the United States, exposure to extreme heat is already a significant public health problem and a leading cause of weather-related mortality.[10]

Heat waves have generally become more frequent across the US in recent decades, with western regions setting records for numbers of these events in the 2000s. There has also been a dramatic increase in nighttime temperatures in the US, reducing the number of critically important relief windows during heat waves.[11]

A July 2015 study found a significant shift in the concurrence of meteorological droughts and heat waves across the United States.[12]

New record high temperatures now regularly outnumber new record lows by a ratio of 2:1.[1] This trend is one of the clearest signals of climate change that we experience directly.

Model simulations using the A2 emissions scenario found that by mid-century, people in the US can expect a four to six-fold increase in the number of days exceeding 95°F (35°C).[13]


Europe trends

Western Europe has seen significant increases in summer mean daily maximum temperature, daily minimum temperature, annual hottest day temperature and warmest night temperature, and an increase in frequency of summer days and tropical nights.[14] Changes in sea surface temperature and sea ice extent explain 62 percent of the warming signal, with the remaining 38 percent explained by the direct impact of greenhouse gases and anthropogenic aerosols.[14]

An October 2010 attribution study found that human activity more than doubled the likelihood of higher temperatures in Europe during the spring and fall seasons from 1999 to 2008. The study also found that human activity at least quadrupled the probability of higher temperatures during summer.[15]


Asia trends

In China, the observed increase in areas affected by heat waves during the past fire decades has been formally attributed to human-caused global warming.[16] A second attribution study similarly finds that global warming influenced temperature extremes in China from 1958 to 2012, with more extreme warm events and less extreme cold events observed.[17]

An April 2017 study looked more closely at the increasing heat trend in China from 1961 to 2015 and found that singular days of extreme heat are diminishing in favor of compound events.[18]

In the eastern portion of the Middle East, high-temperature extremes during the night have risen at more than twice the rate of daytime extremes and the intensity and frequency of hot days have increased.[19]


Australia trends

Anthropogenic activity has increased the risk of Australian heat waves during late autumn similar to the extreme heat observed in 2014 by up to 23‑fold, compared to climate conditions under no anthropogenic influence.[20]

An attribution study found that anthropogenic climate change very likely increased the likelihood of prolonged heat waves like that experienced in Adelaide in January 2014 by at least 16 percent.[20]

Similarly, climate model simulations for 2014 indicate anthropogenic climate change very likely increased the likelihood of hot and very hot November days in Brisbane by at least 25 percent and 44 percent respectively.[20]