Climate science at a glance

• The IPCC states that the frequency and duration of heat waves worldwide have increased since 1950.[1]
• Over the past 30 years, the geographic area experiencing extreme summer heat has increased by more than ten-fold.[2]
• Climate change made Russia’s 2010 heat wave five times more likely to occur.[3]
• Record breaking high temperatures have outnumbered lows in the US by two to one.[4]

Unusual heat in the Northwest

June was particularly hot in the northwestern U.S., with records broken in Washington,[5] Oregon,[6] Idaho,[7] Utah,[8] Nevada and Alaska (where fires burned nearly 2 million acres.[9][10][11][12] In the last week of June alone, 465 daily and 49 monthly heat records were broken or tied.[13] The recent Lancet Commission on Health and Climate Change report points out that there is “a well-established relationship between extreme high temperatures and human morbidity and mortality.”

Record-breaking June heat had continued into July, prompting the United Nations to issue heat wave warning guidelines for the first time. Those attending 4th of July picnics in 2015 faced hotter-than-normal conditions in many places across the United States.

Extreme heat strikes elsewhere

Two of the ten deadliest heat waves in world history occurred in 2015,[14] and there was extreme heat on four continents.

• Pakistan’s heat wave claimed the lives of over 1,300 people and caused another 100,000 heatstrokes.[15]
• India’s lethal heat wave killed at least 2,300 people in India people.
• In London, the 2015 Wimbledon tennis tournament had players competing on the hottest July day ever recorded in the UK.[16] In France, the same heat wave was so intense that it threatened power supplies.[17] In Spain, Madrid's 104˚F temperature set a new June record.[18]
• Even in South America, where it was winter, heat records were broken by nearly 2.2˚F.[14]

Increased Extreme Heat and Heat Waves

A study published in Nature in June 2015 found that the weather patterns that cause heat waves like the one in the Northwest have become more common in recent years,[19][20] consistent with a line of research connecting the global warming-induced melting of the Arctic with unusual mid-latitude weather patterns, the result of changes in the jet stream.

One of the strongest 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. The more extreme the heat wave, the more likely the event can be attributed to global warming. Roughly 75 percent of extreme heat events globally are attributed to climate change. The signal of climate change is particularly reflected in record-breaking heat waves. Records are more likely to be broken when natural variation runs in the same direction as climate change—in this instance, towards hotter temperatures. 85 percent of recent record-hot days globally are attributed to climate change..[22]

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.[21]

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[22]

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. 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. The shifting bell curve also leads to the occurrence of never-before-seen extremes in high temperatures.[2][23][24]

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.[25] Due to global warming, the most extreme heat events now impact a global area 10 times greater than in the period 1951-1980.[2] 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.[25]

US trends

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.[4]

New record high temperatures now regularly outnumber new record lows by a ratio of 2:1.[21] This trend is one of the clearest signals of climate change that we experience directly. There has also been a dramatic increase in nighttime temperatures in the US, reducing the number of critically important relief windows during heat waves.[4]

Model simulations using the A2 emissions scenario found that by mid-century, people in the US can expect a four to sixfold increase in the number of days exceeding 95°F (35°C).[26]

Global trends

Since 1950, the number and duration of heat waves worldwide has increased due to global warming. The hottest days and nights have become hotter and more frequent, and the most extreme heat events now impact a global area 10 times greater than in the period 1951-1980.[2]

An April 2017 study found that anthropogenic global warming has had a significant hand in the temperatures seen during the hottest months and on the hottest days on record throughout much of the world from 1931–2016.[22] The study found that climate change made heat records were more likely and more severe for about 80 percent of the area of the globe with good observational data.[22]

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.[27]

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.[28]

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