People and countries are pushed “To the Edge” by heat waves all over the world.
Millions of people in the U.S. are once again in dangerous heat. Last weekend, hot air covered Europe, making parts of France and Spain feel like they do in July or August. Even though it rained a lot in the south of China, it was very hot in the north and center of the country. Some parts of India started to get very hot in March, but the beginning of the monsoon rains has helped a little.
It’s too soon to say if climate change is directly to blame for the severe heat waves that hit these four major economies, which also happen to be the top emitters of gases that trap heat, at about the same time just days into summer.
Even though global warming is making extreme heat more common around the world, scientists need to do more research to find out if specific weather events have become more likely or stronger because of human activity. (A group of scientists who looked into India’s terrible heat wave this spring found that climate change had made it 30 times more likely to happen.)
Even so, it seems like heat waves are happening in more and more faraway places at the same time lately. This is because of the jet stream and other air currents that affect weather systems around the world.
Studies have shown that this link connects parts of North America, Europe, and Asia. Scientists are still trying to figure out how these patterns might change as the planet gets warmer, but for now, it looks like these places, where so much of the world’s economic activity takes place, will probably keep getting hit by extreme heat at the same time.
A climate scientist at Northwestern University named Daniel E. Horton said, “To have a heat wave, we need heat and the way the air moves that lets the heat build up.” He said, “We’re getting more heat for sure” because of global warming. But, he said, climate change could also change how this heat is spread around the world by air currents that go around the globe.
Extreme weather in more than one place at the same time isn’t just a meteorological curiosity. Each heat wave can make people sick or kill them, start wildfires, and ruin crops. When they happen at the same time, they can threaten the world’s food supply, which has been in danger this year because Russia invaded Ukraine.
Even though heat waves are caused by things like urbanization and how the land is used, scientists are pretty sure that climate change is making them worse. Some scientists say that soon, the world’s worst heat waves may not have any historical parallels from before people started pumping greenhouse gases into the atmosphere. This would make the question of whether climate change is a main cause irrelevant.
Andrew Dessler, a climate scientist at Texas A&M University, said that the warming of the past few decades has already made it hard for scientists to tell what is a heat wave and what is just the new normal for hot weather.
If, for example, the threshold for a heat wave is just the mercury going over 100 degrees Fahrenheit for several days in a row, it’s “not at all surprising,” Dr. Dessler said, to see them happen more often in more places at the same time. “As time goes on, these temperatures will be felt in more and more parts of the world,” he said. “Eventually, with enough global warming, every land area in the mid-latitude Northern Hemisphere would be above 100 degrees.”
Even when scientists look at how often temperatures go above a certain level compared to a moving average, they still find that the number of heat waves happening at the same time is going up a lot.
One recent study that did this found that the average number of days in the Northern Hemisphere between May and September with at least one big heat wave went from 73 in the 1980s to 152 in the 2010s. But there were seven times as many days with two or more heat waves, going from 20 to about 143. From May to September, that’s almost every day.
The study also found that in the 2010s, these simultaneous heat waves affected a larger area and were stronger, with peak temperatures that were almost one-fifth higher than in the 1980s. The study found that on days when there was at least one big heat wave somewhere in the Northern Hemisphere, there were an average of 3.6 of them happening every day.
Deepti Singh, a climate scientist at Washington State University and one of the study’s authors, was surprised by these “dramatic” increases.
Dr. Singh and her co-authors also looked at where heat waves happened most often at the same time over the last 40 years. One thing stuck out: Between 1979 and 2019, more and more large heat waves hit parts of eastern North America, Europe, and central and eastern Asia at the same time. Dr. Singh said this was “more than what we would expect just from the effect of warming.”
She said that the study didn’t try to predict whether heat waves like this will happen more often as global warming goes on.
Scientists are trying to figure out how the jet stream, which has been changing the weather for billions of people for a long time, might be changing now that the Earth is getting warmer. One reason is that the Arctic is getting warmer quickly, which makes the temperature difference between the north and south of the Northern Hemisphere smaller. No one knows for sure how this might be affecting extreme weather.
But the difference in temperatures is one of the main things that makes winds blow and keeps weather systems moving around the planet. Kai Kornhuber, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory, said that as the temperature differences get smaller, these air currents may be slowing down. This means that extreme events like heat waves and heavy rains are likely to last longer.
Dr. Kornhuber said, “The longer a heat wave lasts, the more it pushes natural and social systems to their limits.”
He said that climate change already means that the world will see more extreme weather and more extreme weather at the same time. “These changes in circulation will add to it,” he said. “They will make extremes even worse and happen more often.”