Excessive heat: Why is it so hot this summer?

July 12, 2023:

In the next few days, temperatures in parts of the American Southwest could approach 120 degrees Fahrenheit, potentially breaching existing heat records and endangering human lives.

Summer is always warm, but this is already among the hottest ones ever. In early July, several days set global (albeit unofficial) temperature records. Last month was the warmest June on planet Earth since record-keeping began in 1850, according to the nonprofit group Berkeley Earth. And it’s not just the air that’s baking; the oceans are exceptionally hot, too.

Climate Reanalyzer/Climate Change Institute/University of Maine

Why is all this heat happening now? Global weather cycles as well as long-term trends are converging just right to send temperatures soaring. The worst part: As sweltering as this summer has been, it may be one of the cooler years of the rest of our lives as the planet continues warming.

1. “Heat domes” have settled over parts of the world

Heat waves — an extended period of extreme temperatures — are caused by a buildup of high pressure in the atmosphere. This pressure compresses and heats up the air below it. As the air descends, it pushes out cooler, fast-moving air currents and squeezes away clouds, giving the sun an unobstructed line of sight to the ground.

A map of the United States with a heatwave weather symbol covering most of the map.

The ground — soil, sand, concrete, and asphalt — then bakes in the sunlight, and in the long days and short nights of summer, heat energy quickly accumulates and temperatures rise. Additionally, waves in atmospheric air currents known as the jet stream can hold this high-pressure lid in place for extended periods.

This pattern is sometimes called a “heat dome” because the heat is trapped under a dome of pressure. Parts of the US, including Florida, California, and Arizona, are now baking under these heat domes.

A power station in Glendale, Arizona, on July 11 during a severe heat wave that’s gripping the American Southwest.

2. We’re in a big El Niño year

There are also broader meteorological effects at work. Earlier this summer, the US Climate Prediction Center announced that it detected conditions of El Niño, a far-reaching climate pattern caused by warm water spreading along the equator in the Pacific Ocean.

This El Niño could layer on additional warming, as abundant heat in the Pacific is transferred to the atmosphere; in fact, it may have already contributed to early-season heat waves in Asia.

3. Climate change is making heat waves more extreme

Perhaps the simplest explanation for all of this heat is climate change. “Hotter extremes are one of the most obvious consequences of rising global temperatures,” John Nielsen-Gammon, a climatologist at Texas A&M University, told Vox earlier this month.

Our cars, factories, and power plants have warmed the world by about 1.2 degrees Celsius (roughly 2 degrees Fahrenheit) since the dawn of the Industrial Revolution. We’re on track to blow past 1.5 degrees Celsius in a matter of years without a dramatic reduction of fossil fuel emissions and the proliferation of technologies to pull carbon dioxide out of the air, according to the world’s top climate scientists.

Charts from NOAA show that heat wave length and intensity have increased every decade since the 1960s.

While that increase might sound modest, it makes weather extremes much more likely. You can see this in the graphs above — each measure of heat waves is trending in the wrong direction. So for scientists, record-setting temperatures are not surprising; they’re what we should expect.

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