July 23, 2018, was a day unlike any seen before in Japan. It was the peak of a weekslong heat wave that smashed previous temperature records across the historically temperate nation. The heat started on July 9, on farms and in cities that only days earlier were fighting deadly rains, mudslides, and floods. As the waters receded, temperatures climbed. By July 15, 200 of the 927 weather stations in Japan recorded temperatures of 35 degrees Celsius, about 95 degrees Fahrenheit, or higher. Food and electricity prices hit multiyear highs as the power grid and water resources were pushed to their limits. Tens of thousands of people were hospitalized due to heat exhaustion and heatstroke. On Monday, July 23, the heat wave reached its zenith. The large Tokyo suburb of Kumagaya was the epicenter, and around 3 p.m., the Kumagaya Meteorological Observatory measured a temperature of 41.1 degrees Celsius, or 106 F. It was the hottest temperature ever recorded in Japan, but the record was more than a statistic. It was a tragedy: Over the course of those few weeks, more than a thousand people died from heat-related illnesses.
On July 24, the day after the peak of the heat wave, the Japan Meteorological Agency declared it a natural disaster. A disaster it was. But a natural one? Not so much.
In early 2019, researchers at the Japan Meteorological Agency started looking into the circumstances that had caused the unprecedented, deadly heat wave. They wanted to consider it through a relatively new lens—through the young branch of meteorology called attribution science, which allows researchers to directly measure the impact of climate change on individual extreme weather events. Attribution science, at its most basic, calculates how likely an extreme weather event is in today’s climate-changed world and compares that with how likely a similar event would be in a world without anthropogenic warming. Any difference between those two probabilities can be attributed to climate change.
Attribution science was first conceived in the early 2000s, and since then, researchers have used it as a lens to understand the influence of climate change on everything from droughts to rainfall to coral bleaching. As scientists have long predicted, the vast majority of extreme weather events studied to date have been made more likely because of climate change. But the 2018 Japan heat wave is different. As people who lived in Japan knew at the time, the oppressive temperatures were more than unusual. They were unprecedented. In fact, without climate change, they would have been impossible.
“We would never have experienced such an event without global warming,” says Yukiko Imada of the Japan Meteorological Agency.
On June 7, 2019, Imada, Masahiro Watanabe, and others published an attribution study of the 2018 Japan heat wave in the journal Scientific Online Letters on the Atmosphere. They found that the deadly event of the previous summer “could not have happened without human-induced global warming.”
This heat wave is not the first extreme event found to be only possible because of climate change. But it is the first short-lived event, and the first to have direct impacts on human health. Given that tens of thousands were hospitalized and more than a thousand died due to the heat wave, in a sense, these people are the first provable deaths of climate change.
For Watanabe, the result wasn’t unexpected. It was more of a grim inevitability. “It was not that surprising,” he says of his unprecedented result. An event like this was “naturally expected as global mean temperature continued to rise.” But for both Watanabe and Imada, it holds real historical significance. “It is very sensational for me because human activity has created a new phenomenon. Human activity has created a new phase of the climate,” says Imada.
You couldn’t live through this heat wave without realizing that something was unusual. Ayako Nomizu lives in Tokyo. “When I was growing up in the ’80s, if we had 31 or 32 degrees centigrade, that was hot,” she says. “We would say ‘Oh, my God, it’s gonna be really 32 degrees?’ ” Summers recently, and especially 2018’s, concern her. “Now we are seeing 37, 38 [degrees]. It’s crazy. We didn’t really have this kind of heat before.” Nomizu works for Climate Action 100+, a group that helps investors and companies transition to clean energy, so for her, the connection between climate change and the extreme heat in summers is obvious.
Kazuo Ogawa, a 65-year-old landlord who lives in Tokyo, says he has never experienced anything like the heat wave of 2018. His memories of the experience are visceral. “I was so uncomfortable. I took a shower three times a day, I changed my T-shirt three times a day,” he says.
This kind of heat, as the hospitalization numbers and death toll show, is dangerous. Especially so in Japan, where most people didn’t grow up with air conditioning because it was never needed, and where heat exhaustion was basically unknown until recently. To Ogawa and many Japanese, this is a new problem. “Heat exhaustion is called netsuchusho in Japanese. I never heard of this phrase, this illness, 30 years ago,” Ogawa said.
Heat exhaustion and its more deadly version, heatstroke, are simply the physiological changes that occur when someone has an extremely elevated body temperature. There are a lot of mechanisms humans have evolved to prevent dangerous overheating—sweating and other internal changes like increased heart rate and the transfer of blood from organs to the skin can usually keep the body at a safe temperature—but there is a limit to what the body can handle. If the outside temperature gets too intense or high humidity prevents sweat from evaporating and pulling warmth out of the skin, internal body temperature will start to rise. When this happens, blood vessels dilate in an attempt to get rid of more heat, causing a drop in blood pressure that leads to the first symptoms of heat stress—lightheadedness and nausea. As the body continues to heat up, organs swell, and cell-signaling processes, especially in the brain, are disrupted. At this point people begin to fall unconscious, and if their temperature is not lowered quickly, the damage can be fatal.
The most dangerous conditions are when high temperatures coincide with high humidity (a set of interactions measured by wet bulb temperature), and the danger gets worse the longer these conditions last. Older people and children are most at risk because their bodies aren’t as efficient at self-cooling, and children often won’t realize the seriousness of their symptoms and will continue to play or exercise outside. In places that aren’t accustomed to extreme heat, like Japan, many get caught unprepared and uninformed.
The heat wave of July 2018 was a perfect storm. The rains that ended just as the heat wave began had soaked the landscape. Temperatures stayed high day and night, day after day. Many people in Japan, especially the elderly, don’t have air conditioning, and those who did were hesitant to spend the extra money to run it 24/7. In the first few days of the heat wave, kids continued to play outside, construction workers stayed on job sites, and people looking for exercise went about their normal routines. But as the heat wave wore on into the third, fourth, and fifth days, with high nighttime temperatures offering no reprieve, people began to suffer heatstroke in astonishing numbers.
Tatsuro Maesawa, a 36-year-old bike shop owner in Kumagaya, quickly learned the risks posed by the heat. On a bike ride in the first days of the heat wave, he experienced minor heatstroke, so he switched to riding before sunrise. But some did not adapt so well. A few days after his own bout of heatstroke, a regular customer stepped clumsily into the shop while on a ride, complaining. “He said he had a headache and felt powerless,” Maesawa said. “He had reduced concentration.” Having just experienced these same symptoms only a few days before, Maesawa was concerned and immediately recognized what was going on. “I saw he was feeling dull, and the way he walked was not usual.” He offered the man some water and encouraged him to rest in the shop until his body cooled down. The man was eventually OK, but for Maesawa the experience was a powerfully close-to-home demonstration of life in a changed climate.
As the number of hospitalizations climbed, government organizations started to take action. The Tokyo Fire Department issued heatstroke warnings over radio and television. They advised people to drink more water, keep their air conditioners running, and avoid going outside. This posed its own problems, though. With air conditioners running 24/7 across the country, demand for electricity spiked, and prices hit a five-year high. When price hikes were announced, there was even more incentive for people to suffer through the heat rather than spend the extra money. Not running an air conditioner would seem like an economically smart choice, but this quickly became dangerous. As the number of deaths climbed, one energy supplier, Kyushu Electric Power, tried to reverse course by offering a 10 percent discount to anyone over the age of 75.
The heat wave wore on; public concern, hospitalizations, and temperatures rose; and it became apparent just how unprepared the country was. The drink-water-and-stay-inside warnings continued, but government agencies also started asking residents in major cities to spray down sidewalks and streets with water in an attempt to lower temperatures. This, as one expert explained, has little effect on temperature and in fact hurts rather helps with heatstroke: The evaporating water raises humidity locally. After a 6-year-old boy died from heat exhaustion on a school trip to a park, the education ministry issued warnings asking schools to reconsider going outside with students. The only answer Japan seemed to have was to hide.
Officially, more than 70,000 were hospitalized and 1,032 died due to the heat wave. The emergency medical division of the Tokyo Fire Department keeps records of the number of ambulances dispatched each day. Of the 10 days with the most dispatches in their history, the top seven occurred during the week from July 17 to July 23. On July 23, the last and hottest day of the heat wave, the fire department responded to 3,383 calls. As Takashi Komabashiri, assistant manager of the emergency planning branch, explained, the Fire Department was overwhelmed and unprepared for an event of this magnitude.
“We felt a sense of crisis against these numbers. After all, as the number of cases increases, it takes more time for ambulances to arrive,” says Komabashiri.
For people like Katheryn Gronauer, a 30-year-old executive coach in Tokyo, the sound of sirens is now associated with extreme heat. “I remember I heard, like, six ambulances in one day, and my first thought was ‘Oh, it’s summertime.’ ” Maesawa, the bike shop owner, is concerned about the future of his country and the future of his business. Motioning to a group of kids playing baseball across the street, he said sadly, “I think that outdoor sports will become difficult in the future.”
The summer of 2018 broke records, took lives, and shook the belief of many about what the future holds for Japan’s climate. But as Watanabe said, this was not unexpected. The trend had been heading this way for decades. Kazuo Sakamaki, 76, has lived directly across from the weather observatory in Kumagaya his entire life. He said he’s noticed a dramatic change over the years: “I used to be able to endure [summer] with only a fan in the past, but now I can’t live without an air conditioner.”
When I asked him if he knew why the summers have changed, he responded, confidently, “Oh, that’s the global warming that is now making noise.” And then he said something unexpected: “But I don’t know if this area will be affected due to that.”
As a scientist and journalist who has spent years thinking and writing about climate change, I was confused and saddened by the apparent doublethink I saw in Japan. Researchers have shown that living through a climate-related natural disaster—like a heat wave—can change whether a person believes in or cares about climate change. But one strong hurricane won’t change the mind of a climate denier overnight. Much of the research shows that political affiliations and existing beliefs on climate change are extremely hard to overcome, even in people that have lived through extreme weather events. As the coronavirus pandemic so glaringly shows, political leaders often wield more power to convince than science or personal experience.
With the lack of political or cultural leadership on climate change, many Japanese people struggled to fully blame the heat wave of 2018 on the global crisis that both science and common sense show is happening right now. Sakamaki, for example, said he understood that global warming is affecting the temperature of the planet generally, but he wasn’t sure if it had been responsible for that heat wave specifically. This hesitance to connect local experiences to the global problem has haunted climate change researchers and activists for years. And much to their frustration, the direct link between climate change and a single weather event was, for a long time, an impossible thing to prove scientifically.
As an undergraduate ecology student in the early 2010s, I often heard this line used to describe the tricky relationship between climate change and weather: We know climate change will make weather more extreme, but it is impossible to point to any one event and say, “That! That individual event was affected by climate change.” This temporary reality has melted into common wisdom.
Coverage of Australia’s bushfires in early 2020 perfectly illustrates this. Journalists and experts are well practiced in saying that climate change was likely contributing to make fire conditions worse and that long-term warming is going to affect future fire seasons. But they were hesitant to make a direct link between climate change and the fires. Years of this have permeated people’s brains, and the hesitance of people like Sakamaki might be one outcome.
Attribution science is illuminating this formerly invisible link. As Yukiko Imada of the Japan Meteorological Agency explains, “The purpose of [attribution science] is to ask the question: Whether and to what extent can an event be attributed to climate change?” The ability to answer that question long eluded scientists. But now, using attribution analysis, researchers like Imada can separate the signal of climate change from the noise of daily weather.
To actually conduct an attribution study takes a massive amount of computing power, complicated climate models, and precise weather data. But the theory underlying the process is simple and replicable. What an attribution study does is use two computer simulations of the climate—one with all of the anthropogenic carbon dioxide and one without—and assesses the likelihood of an extreme weather event happening in each model.
So, for example, in a model of today’s climate-changed world, the heat wave that crippled France in July of 2019 has an estimated return period of 100 years (meaning it would be expected to happen about once every 100 years). In a model of the world without climate change, such an event would happen about once every 1,000 years or more. Therefore, while climate change didn’t create the opportunity for that heat wave, it made it at least 10 times more likely.
To assess the heat wave in Japan, the researchers did the same thing. They built side-by-side models of a world with climate change and a world without it. Then they calculated the return period for a heat wave as bad as the one in 2018 in each of the models. Finally, they compared the return period of the heat wave in a world without climate change to the return period of the heat wave in today’s climate-changed world. Any difference between the return periods in the two models would be caused by climate change.
They found a big difference. With climate change as it stands today, a heat wave like the one that happened in 2018 has a 2.1 percent chance of occurring in any given year, or a return period of about 50 years. In years when the weather patterns match the specific conditions that produced the heat wave, there is a 19.9 percent chance. In a world without climate change, the likelihood of a heat wave as bad or worse happening in any given year is 0.00003 percent. That is about once every 3.33 million years, otherwise known as never. The 2018 heat wave was impossible until anthropogenic global warming changed the climate.
Attribution studies tend to include something called the factor of attributable risk, or FAR, to describe how much climate change influenced the risk of an extreme event occurring: FAR = 0 would mean climate change had no influence on the risk of the event occurring, FAR = 0.9 would mean that it was responsible for 90 percent of the risk, etc. The 2018 Japan heat wave had an FAR of 1, meaning climate change was responsible for 100 percent of the risk of it happening.
How did climate change cause such a heat wave? It’s not just the result of the Earth being hotter; it’s how this heat influences weather patterns. What happened was a rare situation called a double-high, where a high-pressure system in the lower atmosphere and a high-pressure system in the upper atmosphere stack on top each other. Warmer sea surface temperatures and a warmer atmosphere made the baseline temperature that the double-high added to significantly higher, which pushed the event from unusually hot to record-breaking and deadly.
Watanabe, one of the researchers, was at a bit of a loss for words when I asked him what the future holds for attribution science. “We have nothing more to say once the FAR is equal to 1, at least using this indicator,” he told me. He also had a warning: “We will keep having FAR = 1 events.”
And the Japanese heat wave wasn’t the first. There had already been four other events with FAR = 1. Two were marine heat waves off the coast of Australia, one was the high average temperature of Asia in 2016, and the last was the total global warmth in 2016. The 2018 Japan heat wave differs from these in that it was a singular and short-lived event (not an average) that occurred where people actually live (i.e., not the ocean), which is how it’s gained its other notorious distinction: the first FAR = 1 event to directly cause human deaths.
When I asked Imada and Watanabe about the impact and importance of their work, they had mixed feelings. On one hand, Imada said that the media were showing interest in the work, and she has done many interviews since the paper came out. But on the other hand, she was uncertain as to what, if any, effect the reporting had on how the public thinks about climate change. “People can only read the results from the TV or newspapers, and I don’t know how they are responding to that,” she said.
After speaking with dozens of people about the heat wave in 2018, I found Imada’s concerns to be true. Nearly everyone knew that the temperature record had been broken, and many assumed it was climate change, but not a single person I spoke with knew about the study or that the heat wave had actually been connected to climate change in a definitive way. In fact, most people were shocked when I told them that the official death toll was more than a thousand. Most thought it had been in the dozens or, at most, the low hundreds. In spite of broad national awareness of the concept of climate change, a deeper understanding of how this global problem is manifesting locally seems to be missing, even in a country that is already feeling the deadly effects.
Asuka Suzuki-Parker, a professor and climate researcher at the Kumagaya campus of Rissho University, showed me around the city that was the epicenter of the heat wave. Kumagaya hosted a few games of the Rugby World Cup in 2019, and Suzuki-Parker explained measures the local government had taken to reduce heatstroke risk after the extremes of 2018’s summer. Roads in the city center had been repaved with lighter-colored asphalt that absorbs less heat than traditional streets. Water misters had been installed at the train station. A shade covering had been built between the train station and the rugby stadium. These changes are adaptations, not solutions, and Suzuki-Parker had a fatalistic outlook: “In the short term, what we can do is adapt because the temperature will increase no matter what we do about it.”
After the lesson of 2018, the Tokyo Fire Department increased its number of reserve ambulances and began to ready them ahead of heat waves. Thanks to huge information campaigns, the Japanese public is much more aware of the risks of heatstroke and methods to avoid it. Thousands of schools across the nation have installed air conditioning for the first time, and I was told by dozens of people that they are using air conditioning more frequently in their homes.
In many small ways, Japan is shifting to accommodate a warming future. But if these changes don’t seem like huge strides in the fight for a greener future, it’s because they aren’t. These are adaptations to a warmer world. They are not actions to reduce greenhouse gas emissions and prevent an even hotter one.
The Japanese government has a less-than-stellar plan to meet the challenges of climate change. There are, of course, some people doing what they can to fight climate change. Groups of young activists are hosting workshops and talks, organizations like Climate Action 100+ are trying to convince businesses to adopt more sustainable practices, and there are individuals, in modest ways, making changes to their personal lives in an attempt to help. But there isn’t a sustained, centralized, government-endorsed move to take the necessary amount of action. More than once I was told that change would happen if the government asked for it, but the government hasn’t asked. In the wake of the Fukushima disaster, there has been a switch away from nuclear energy in Japan. The government created a subsidy program for solar energy but is simultaneously investing in coal power plants. Regardless of where the blame falls, the end result is the same as in so many places: Japan is not prioritizing actions to counter the underlying cause of climate change.
Attribution science is giving us the ability to watch, in real time, the consequences of our actions. The future that the heat wave of 2018 represents is one we knew was coming. It is here, today, and attribution science gives scientists and the world the ability to say so with conviction.
There’s another way in which the new field might prove useful. At the end of our conversation, Watanabe paused to reflect on the work he has done. Attribution science compares the world of today with a world without climate change. In some ways, he’s started to see his work as a signpost in history, reminding us of a world that used to exist, but no longer does. Someday, it’s the other simulation, the world without climate change, that will be the curiosity, he thinks. That computer simulation will be the one that tells people something they never got to experience—an image of what the world once was, but will never be again.
This work was completed with the support of the Pulitzer Center on Crisis Reporting.