Surprise! This summer is extremely hot.
How hot? July 4 was the hottest day on Earth since record-keeping began more than 40 years ago, according to scientists at the University of Maine’s Climate Reanalyzer project. As Americans grilled burgers and set off fireworks, the global average temperature reached 62.9 degrees Fahrenheit (17.2 degrees Celsius). The month of June, meanwhile, was the warmest on record, the European Centre for Medium-Range Weather Forecasts reported Thursday.
The heat index neared 100 degrees this week in New York City. That’s nothing compared to the 120-degree temperatures that baked parts of Texas in late June, smashing dozens of records, straining the power grid, and sending thousands of Texans to the emergency room with heat-related ailments. More than a dozen people in the state have died.
On just one day at the end of June, more than 120 million Americans were under some form of heat advisory, according to the National Weather Service. That’s more than one in every three people.
Regions outside the US have also been blasted by spring and summer heat. In April, temperatures in Spain were already breaking 100 degrees; they’ve since exceeded 110 in some places. The heat in Beijing and other regions of China broke records in June, and warm weather is fueling unprecedented wildfires in Canada.
All this, and we’re only a few weeks into summer.
It’s easy to call this weather unusual. Compared to past averages, the world is unusually hot. There are also a handful of natural phenomena beyond climate change that make this summer particularly toasty — a global weather event called El Niño is gripping the planet, for example, which can fuel heat waves and droughts.
Yet from a scientific perspective, there’s nothing surprising about record-breaking heat, dangerous as it may be. It’s actually exactly in line with what scientists have long predicted in a world warmed by climate change. Fossil fuel emissions heat up the planet, and hotter weather makes heat waves more extreme.
“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.
In the years to come, heat waves like those in the American South and Europe are likely to get worse on the whole, not better. So while this summer might be unbearably hot, it’s likely to be one of the coolest summers for decades to come.
Why is it so hot right now?
At a local scale, heat waves — an extended period of above-average temperatures — are often caused by a buildup of high pressure in the atmosphere. This pressure compresses and heats up the air, writes Vox’s Umair Irfan:
The high-pressure system also pushes out cooler, fast-moving air currents and squeezes clouds away, which gives the sun an unobstructed line of sight to the ground. 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.
This region of pressure acts like a lid on a pot, trapping heat so it can’t dissipate. That’s why heat waves are often referred to as “heat domes” — the heat is trapped under a dome of pressure.
It’s this heat dome effect that underlies scorching temperatures in Texas.
Zooming out, however, there are much broader meteorological patterns causing temperatures in Texas and beyond to soar. Large, fast-flowing currents of air called the subtropical and polar jet streams, for example, appear to be wobbling, which can impact how heat blankets parts of the US, as Vox’s Irfan explains.
There’s also El Niño, a far-reaching climate pattern that causes vast stretches of the Pacific Ocean to warm, affecting weather all over the world, including temperature and precipitation. This El Niño could layer on additional warming; in fact, it may have already contributed to early-season heat waves in Asia this year.
What’s more is that all of these meteorological phenomena occur on a planet that’s getting hotter due to greenhouse gas emissions. The impact of jet streams and El Niño may be hard to predict, but the broad effect of climate change is pretty clear: It will make heat waves more common, longer-lasting, and more extreme.
Sorry, summer is only getting hotter and starting sooner
Our cars and factories and power plants have warmed the world by about 1.2°C (roughly 2°F) since the dawn of the Industrial Revolution. We’re on track to blow past 1.5°C 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.
While that increase might sound modest, it makes weather extremes much more likely. You can see this in the graphs below — each measure of heat waves is trending in the wrong direction.
The frequency of these events, for example, has grown from an average of two heat waves per year in the 1960s to six per year in the 2010s and 2020s, according to the US Environmental Protection Agency (EPA). Meanwhile, a typical heat wave in major US cities lasts about a day longer than it did half a century ago, the EPA says.
As for what to expect for the rest of this summer? Unfortunately, a lot more heat, Nielsen-Gammon said. For July through September, the National Oceanic and Atmospheric Administration predicts above-average temperatures for much of the US. There’s not a single region that NOAA expects will be cooler than average.
Heat waves early in the summer can also drive a feedback loop that leads to more heat later in the summer, at least in the southern Plains, Nielsen-Gammon said. The hotter the air the drier the land, and the drier the land the hotter it gets (in part, because drier land lacks moisture, and evaporating moisture has a cooling effect).
The good news is that meteorologists can, to an extent, forecast extreme weather, and climate models are improving. They show that many places will get hotter and hotter, and so theoretically cities can prepare.
“The main tool we have for predicting summertime temperatures in the United States is climate change,” Nielsen-Gammon said.
Update, July 6, 3:30 pm ET: This story was originally published on July 5 and has been updated with new information about the current heat wave.