What’s Causing This Record-Breaking Heat?

Temperatures are set to skyrocket to potentially record-setting levels across the U.S. Southwest as yet another heat dome entrenches itself over the southern tier of the country and Mexico. This is just one of the many punishing, deadly heat waves that have baked locations around the world, from China to Algeria, this spring and summer.

The trend toward more frequent, longer-lasting and more intense heat waves is a hallmark of the climate emergency that has resulted from humans burning fossil fuels and releasing planet-warming greenhouse gases. These heat extremes—along with exceptionally warm ocean waters—contributed to the first week of July becoming the hottest week on record globally, based on preliminary data, according to the World Meteorological Organization (WMO). That milestone came just after the hottest June on record. With an El Niño event in place that is set to further boost global temperatures, experts expect that more monthly—and potentially even yearly—heat records could be topped.

The latest heat wave is courtesy of an intensifying high-pressure system over the Southwest which is affecting a broad swath of the southern third of the country. Such systems feature sinking air, which gets compressed and heats up. The accompanying clear skies also allow plenty of sunlight to stream to Earth’s surface, further raising temperatures.

Though the Southwest is synonymous with summer heat, this event will be notable for both its magnitude and its longevity. Phoenix, Ariz., has already seen 11 consecutive days with high temperatures above 110 degrees Fahrenheit, which is about five degrees F above average for the area at this time of year. That’s the fourth longest period of such days in the city’s history, and if it continues, it could set a record-long streak.

Texas is also continuing to swelter under high temperatures, as well as high humidity caused by moist air flowing in from the Gulf of Mexico. Places in the state and in Mexico have already seen weeks of brutal heat and humidity, “breaking daily record high temperatures over and over again for days on end,” said Daniel Swain, a climate scientist at the University of California, Los Angeles, and the National Center for Atmospheric Research, during one of his regular “virtual climate and weather office hours,” hosted on YouTube.

Western Canada has also experienced multiple bouts of prolonged heat this summer, contributing to the worst wildfire season the country has ever seen. On July 8 a temperature of 100 degrees F was recorded in one town in the Northwest Territories—the farthest north of 65 degrees latitude where that has ever happened in the Western Hemisphere, the Washington Post reported.

Extreme heat—particularly if it is prolonged—is extremely dangerous and potentially deadly. Heat is the number-one weather-related killer in the U.S., causing more human deaths than hurricanes, tornadoes and floods combined. It is especially dangerous for young children, the elderly and those with health conditions such as asthma and heart disease. Heat is also a major concern for people who work outdoors and unhoused people. A new study in Nature Medicine found that more than 60,000 people died from heat-related causes during blistering heat waves in Europe last summer. That toll suggests the heat adaptation plans that cities and countries have put in place such as early-warning systems are insufficient to protect vulnerable populations. And the findings come as areas around Europe, particularly Italy, Spain and Portugal, face brutal heat again this summer. North Africa, too, has seen astounding heat. On July 6 the temperature in Adrar, Algeria, didn’t drop below 103 degrees F, even at night, the Associated Press reported. High nighttime temperatures add to the danger of heat waves because the body doesn’t get a chance to cool off and recover.

These heat extremes on land have combined with record-hot ocean temperatures around the world to send overall global temperatures to record highs. In his YouTube discussion, Swain noted that the first week of July “appears to be hotter than any week that we’ve observed since we’ve been recording temperatures.” Those temperature estimates are not taken solely from observations. As Swain explained, “we don’t have thermometers covering every square inch of the Earth,” so scientific groups use computer models to interpolate between data points, generating what is called a reanalysis. Different groups use slightly different methods, which can yield slightly different estimates for the hottest days, weeks and months—particularly on daily scales, where data are much noisier.

With El Niño in place and likely to strengthen, there is a good chance for many more heat records to be broken. “We are in uncharted territory, and we can expect more records to fall as El Niño develops further and these impacts will extend into 2024,” said Christopher Hewitt, director of climate services for the WMO, in a recent statement. “This is worrying news for the planet.”

But individual records are ultimately less important than the long-term warming trend they are emblematic of. That trend shows temperatures increasing everywhere over time. An average summer today, for example, would have been considered a hot summer several decades ago. Likewise, a hot summer today will be an average one a few decades from now.