Last updated June 27, 2017

Increased Extreme Heat and Heat Waves

Global warming has amplified the intensity, duration and frequency of extreme heat events. The National Academy of Sciences reports and validates numerous studies as well as two major science assessment reviews that definitively identify the fingerprint of human influence in driving the changes observed to date.

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 the historic 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] The trend in global warming has contributed to the severity and probability of 82 percent of record-hot days globally.[2]

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

Record hot 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] The trend in global warming has contributed to the severity and probability of 82 percent of record-hot days globally over the 1961-2010 period.[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


A Shifting Climate

A small shift in climate leads to a dramatic increase in the frequency of extreme temperatures. In a warming climate, extreme cold temperatures decrease precipitously while extreme hot temperatures increase dramatically. 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]


Changing Atmospheric Circulation Patterns

In addition to shifting the distribution of local temperatures, global warming is altering the pattern of atmospheric circulation (e.g. the jet stream) as well. And some of these global changes are dramatically amplifying local heat extremes. Waves in the jet stream are stalling in place (as opposed to moving eastward), leading to blocking and persistent weather patterns that fuel the intensity and duration of heat waves. A video explainer is here.

 


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 the increasing intensity, duration and frequency of extreme heat events. The National Academy of Sciences reports and validates numerous studies as well as two major science assessment reviews that definitively identify the fingerprint of human influence in driving these changes.

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.

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.

Importantly, there has been a dramatic increase in hot night-time temperatures in the US, reducing the number of critically important relief windows during heat waves.[11]

The increase in hot night-time temperatures in particular is one of the signatures of climate change.  The fingerprint of global warming in hotter nights emerged before the impact of warming on hotter days, and was first reported in 2005.

The widely discussed spike in heat events in the U.S. during the 1930s has also been linked to human influence, but not to greenhouse gas emissions.  Rather, widespread change in land-use practices (i.e. plowing up the prairies) led directly to the loss of topsoil from over 100 million acres which, in turn, created created dust-bowl conditions.  And the subsequent loss of evaporative cooling dramatically amplified heat conditions far beyond what would have occurred otherwise.

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]

A June 2017 study found that, between 1960 and 2009, mean temperatures across India rose 0.5°C. The probability of heat-related mortality events of more than 100 people increased by 146% in the same period. In southern and western India, there were 50% more heat wave events between 1985 and 2009 than there had been during the previous 25 years (1960-1984).


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]


Looking ahead

If greenhouse gas emissions continue to rise throughout the 21st century, the Fifth Assessment Report of the Intergovernmental Panel on Climate Change projects that the global average temperature will rise 4.7°F to 8.6°F (2.6°C to 4.8°C) by 2081-2100 (relative to 1986-2005).

The higher end of this estimate puts temperatures in 2100 close to one of the planet's warmest periods in history known as the Paleocene-Eocene Thermal Maximum (PETM), which occurred about 55-56 million years ago. During the PETM, the global mean temperature appears to have risen by as much as 9-14°F (5-8°C) to an average temperature as high as 73°F. During this time the poles were free of ice caps, and palm trees and crocodiles lived above the Arctic Circle.

9°F (5°C) is the difference between the last Ice Age, when half of North America was covered in a mile-thick ice sheet, and today. Whereas that warming occurred over thousands of years, the Earth has warmed by 1.8°F (1°C) in just over 100 years. The projected rate of temperature change for this century is greater than that of any extended global warming period over the past 65 million years.