Extreme Heat

Global warming amplifies extreme heat events

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. Extreme heat and heatwaves have significant implications for human health, as well as economic and agricultural productivity.

Intensity, duration and frequency of extreme heat events are increasing

The link between climate change and extreme heat is firmly established. In its most recent report, the UN Intergovernmental Panel on Climate Change 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.

Climate change has already increased the intensity, duration and frequency of extreme heat events and will continue to amplify these events in the future.The U.S. 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 between 1973 – 2012, many urban areas globally experienced significant increases in the number of heat waves, with the largest number of heat waves occurring in the most recent decade studied between 2003 and 2012. Another study found that 75 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.

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 U.S., new record-breaking high temperatures have outnumbered new record lows by a ratio of 2:1. By mid-century, people in the U.S. can expect a four to sixfold increase in the number of days exceeding 35°C (95°F).

Extreme heat events expose the dangers and costs of living in a warming world

Nine out of the 10 most deadly heat waves have occurred since 2000, causing 128,885 fatalities globally. In the U.S., 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.

Extreme heat also poses a significant risk to economic productivity. A report by Risky Business shows that increased heat, especially in the Upper Midwest, Southeast and Southwest, threatens labor productivity, energy systems and agricultural yields.

For information on the public health impacts of visit our health page. For more information on extreme heat including the state of scientific understanding, relevant news and specific impacts, visit Climate Signals.

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