Extreme heat may not trigger the same visceral fear as a tornado, but according to NOAA’s natural hazard statistics, it causes nearly twice as many fatalities in the United States each year – more than any other weather hazard. As the climate continues to warm, that number could rise dramatically in the U.S. and around the world.

Since the late 1800s, human-caused climate change has warmed the Earth’s average temperature by around 1 degree Celsius (1.8 degrees Fahrenheit). That doesn’t sound like much, but a relatively small warming of the average temperature results in a large jump in extreme heat.

Why does a modest increase in the average temperature lead to extreme heat?

Consider a graph of temperatures plotted on a bell curve.

Most temperatures fall near the middle of the curve, and those temperatures would be considered typical. Any temperature that falls on the edges of the curve is considered extreme.

This animation shows that as the middle of the curve shifts just slightly to the warm side, a much larger chunk of the curve moves into extreme territory. In other words, extremely hot days occur more often.

(Credit: Climate Central)

How often is extreme heat occurring today?

Extreme heat occurred very rarely 50 years ago in the United States.

But as a result of climate change, the bell curve has already shifted by one standard deviation interval – a measure that tells you how spread out the values are – according to a 2016 paper by climate scientist James Hansen. As a result, extreme summer heat now occurs about 7% of the time.

The U.S. still sets some record lows, but it’s been setting far more record highs. In fact, recent record highs have been outpacing record lows at a ratio of two to one. This difference could grow to 20 to 1 by mid-century and 50 to 1 by the end of the century.

Hansen’s paper reports that the warming effect has been even larger for the Mediterranean and Middle East. In that area, the bell curve shift in summer is even more dramatic – nearly 2.5 standard deviations. Consequently, every summer is now warmer than average, and the summer climate now lasts considerably longer.

Is there a link between climate change and recent heat waves?

The American Meteorological Society defines a heat wave as “a period of abnormally and uncomfortably hot and usually humid weather.” There have been a number of notable heat waves during the past two decades.

For instance, the European Heat Wave of 2003 is estimated to have caused an astounding 70,000 deaths. Researchers found that human influence at least doubled the risk of a heat wave of that magnitude. In 2010, another 56,000 people died in a heat wave in Russia. A 2011 study concluded that there is an 80% probability the heat wave would not have occurred without global warming.

An exceptional heat wave – nicknamed “Lucifer” – occurred in southern Europe in 2017. In a study conducted by World Weather Attribution, researchers found Lucifer to be 10 times more likely than it would have been in the early 1900s.

Scientists have also studied a 2018 summer event that spread oppressive heat from Japan to Canada, concluding that the size of the event was unprecedented and not possible without climate change.

And more recently, in the summer of 2019, two intense heat waves in Europe just weeks apart shattered various all-time records. Both were linked to human-caused climate change.

The first heat wave in late June 2019 led to a temperature of 115 degrees F in France, eclipsing the all-time country record by more than three degrees. A World Weather Attribution study found that the core of the heat wave was around seven degrees F warmer than it would have been a century ago and that a heat wave of this magnitude is at least 10 times more likely now.

In late July, another history-making heat wave set new all-time record highs in the UK, Belgium, the Netherlands, Germany, and Luxembourg. The city of Paris reached nearly 109 F, breaking the former all-time record by four degrees F. A World Weather Attribution study found the heat wave was 10 to 100 times more likely to occur now than it would have been before the Industrial Revolution.

This same heat wave then built north into the Arctic, resulting in the biggest melt day in recent Greenland history.

It is important to recognize that heat waves are not just restricted to land. In 2016, a quarter of the planet’s ocean surface experienced either the longest or most intense marine heat wave on record. In a study of two affected regions – the Bering Sea of Alaska and waters off northern Australia – researchers concluded that the event was up to 50 times more likely due to human-caused climate change. The abnormally hot water triggered the worst mass coral-bleaching event on record for the Great Barrier Reef.

Researchers also found the 2017-2018 Tasman Sea marine heat wave to be “virtually impossible” without the influence of human-caused emissions.

What can we expect from heat waves in the future?

Our recent past is merely a paltry precursor to a much hotter future. In a 2019 study, researchers at Princeton University found that as global temperatures increase, heat waves will become more frequent and the time between them will become shorter.
An investigation of future regional heat waves finds that the number of heat-wave days may increase by 4 to 34 days per season for every one degree Celsius (1.8 degrees Fahrenheit) of increased global warming. The researchers estimate some tropical regions could experience up to 120 extra heat-wave days per season if the Earth warms by 5 degrees Celsius, which could happen by 2100.

In a 2016 post, Hansen warned that “the tropics and the Middle East in summer are in danger of becoming practically uninhabitable by the end of the century if business-as-usual fossil fuel emissions continue.”

A study released by the Union of Concerned Scientists in July 2019 finds a dramatic increase in dangerous heat index days because of combined higher temperatures and humidity. The researchers found that by mid-century, in the United States, days with 105-degree heat index temperatures will triple. In the current climate, central Wisconsin averages just a few days per summer with heat index temperatures over 100 degrees. By 2050 this number may jump to 20, and by 2085 there may be more than 40 days each year with heat index temperatures in Wisconsin of 100 degrees plus.

Research released around the same time by Swiss Federal Institute of Technology in Zurich, Switzerland, concluded that in general cities climates will shift 600 miles southward by mid century. By 2050 Minneapolis’ warmest month will increase from 80 F to 90+ F.

Dallas, Texas, currently has just a few days above 105 degrees Fahrenheit per year. By 2050 that number is expected to jump to nearly 30 – and by 2100 it may catapult to 60 or more days, according to the nonprofit Climate Central. You can check how your city’s heat is projected to change in the future here and here.

(Credit: Climate Central)

What are the consequences of extreme heat?

A large increase in extreme heat will come with consequences. Here are a few examples:

  • Greater marine heat waves may wipe out the majority of global coral reefs by mid-century, along the vulnerable creatures that rely on them and the human services the reefs provide.
  • Hotter days evaporate more moisture from the ground, drying out vegetation. The result: losses in agriculture, more intense large wildfires, and a longer fire season.
  • Worker productivity will likely suffer as increases in heat and humidity push the limits of human tolerance, making it difficult for outdoor workers to stay cool.
  • A 2017 study concludes: “It’s the sustained nature of heat waves that impose more devastating impacts than extreme temperatures on a single day. Excessive human morbidity and mortality rates are clearly associated with sustained extreme temperatures.”
  • The 2018 National Climate Assessment estimates that in 49 large U.S. cities alone, changes in extreme temperatures are projected to result in more than 9,000 additional premature deaths by the end of the century in a “business as usual” emissions scenario. That’s a conservative estimate compared to an analysis from the Natural Resources Defense Council, which found that an increase in dangerous hot days could lead summertime deaths across large U.S. urban areas to grow from the 1975-2010 annual average of about 1,360 to 13,860 by the mid-2040s, and to 29,850 by late century.
  • There is the danger that humidity will literally become unbearable by the end of this century for people living in regions like the Persian Gulf and South China. According to Dr Tom Matthews of Loughborough University in the UK, once the wet bulb temperature (temperature of the air factoring in cooling as a result of evaporation) reaches 95 degrees F, the body can no longer cool itself through evaporation of sweat, which can lead to heat stroke and death. Wet-bulb temperatures this high are projected to be more common later this century.

What can we do?

These are frightening numbers, but there’s a lot that can still be done now to minimize the impacts. Society could invest in aggressive steps to reduce carbon dioxide and other greenhouse-gas emissions and to increase our abilities to withstand harsher climate conditions.

The National Climate Assessment estimates half of the projected deaths in the U.S. can be avoided if the global community adopts and adheres to a lower emissions scenario. But because some of the projected heating is already inevitable, we will also have to adapt to a warmer world, for example by planting shade trees and ensuring people have access to cooling centers. The assessment report found that fatalities could be reduced – assuming society commits to sufficient adaptation to protect people’s lives.

Editor’s note: Updated on August 21, 2019.

Meteorologist Jeff Berardelli is a climate and weather contributor for CBS News in New York City.