When an unprecedented heat wave struck the Pacific Northwest last summer, the emergency department at Harborview Medical Center in Seattle was fielding patients at a pace not seen since the early days of the COVID-19 pandemic, says emergency room doctor Jeremy Hess.
“It was the first time I felt like I was part of a care team very actively responding to climate change,” says Hess, who is also a public health researcher at the University of Washington. “Personally, it was kind of sad for me.”
Heat is just one example of how climate change is endangering human lives, according to a major report published on Monday by the UN’s Intergovernmental Panel on Climate Change (IPCC). The sweeping report, of which Hess is a co-author, outlines how climate change is already affecting us and how we can adapt to it, touching on everything from agriculture to urban development.
Already, emissions of carbon dioxide, methane, and other greenhouse gases have warmed the globe 1.1 degrees Celsius (2 degrees Fahrenheit). With 1.5°C (2.7°F) of warming, which will be extremely difficult to avoid at this point, those consequences become more dire, and some adaptation strategies will become less effective.
The direct consequences for human health if we fail to take action will be severe and will also worsen social inequities, the report says. Those who have contributed the least to climate change—low-income individuals and developing nations—will bear the brunt of extreme heat, vector-borne diseases, and poor mental health.
“It’s urgent we increase our investments and strengthen our health systems,” says Kristie Ebi, a report author and expert on global health at the University of Washington. “We’re already seeing people die from climate change, and unless we adapt, more people will die.”
Today, the report notes, a third of the world’s population is exposed to heat stress. Depending on what actions are taken to limit emissions, that could increase to 48 to 76 percent of the population by 2100.
The IPCC report is produced by some of the world’s top scientists, experts in their field who review the most recent research on climate change. The resulting report represents the current state of climate change, and it offers a prognosis for how it will alter living conditions in the future.
The last IPCC report that focused on who’s impacted by climate change and how we should adapt was published in 2014, and since then scientists have gathered much more evidence of how emissions are changing the climate, and can say more confidently that climate change is driving deadly disasters and diseases. The chapter on health cites more than 1,600 sources.
Extreme heat, floods, worsening storms, drought, air pollution from wildfires, proliferation of vector-borne diseases like West Nile and malaria—the report is clear, says Robert McLeman, a report author and environmental scientist at Wilfrid Laurier University in Canada: “The risks to human wellbeing are tremendous.”
For the first time, the report details the impact climate change is having on mental health. Natural disasters and prolonged drought are increasingly being linked to post traumatic stress disorder, anxiety, and depression.
The report also notes that extreme heat has become more intense in cities, substantially increasing the risk of heat death to residents of low-income neighborhoods and especially to individuals without homes. (Read more about how tree cover affects urban heat.)
Why is heat so deadly
Heat waves kill more people every year than any other type of weather. A study published last summer found more than a third of all heat-related deaths can now be directly linked to climate change.
“Humans are tropical animals. We evolved out of a hot place in the world,” says Larry Kenney, a physiologist at Pennsylvania State University who was not involved with the IPCC report. “In general, humans can withstand really high temperatures for a short period of time, provided we can sweat and that sweat can evaporate.”
But sweat can’t evaporate when the humidity is too high—and as the planet warms, more people risk being exposed to dangerous combinations of heat and humidity. In recent experiments, Kenney found evidence that the lethal limit might be significantly lower than previously thought.
He and his colleagues monitored the internal body temperatures of healthy individuals as they walked on a treadmill or sat in a room where heat and humidity could be controlled. Earlier studies had suggested that at an outside temperature of 95ºF and 100 percent humidity—or 115ºF and 50 percent humidity—humans would no longer be able to prevent a lethal rise in their internal temperature.
But Kenney found that the limit was as low as 87.8ºF at 100 percent humidity in young, healthy individuals. In the elderly or in those with conditions like high blood pressure, it might be even lower.
“People will die. That’s the reality of it. A lot of heat waves have been really deadly,” says Mojtaba Sadegh, a climatologist at Boise State University. A 2003 heat wave in Europe, for example, caused more than 70,000 deaths.
Sadegh, who was not involved with the IPCC report, published a study earlier this month showing that worldwide, people with low incomes are 40 percent more exposed to dangerous heat waves than those with high incomes, both because they are more likely to live in hot regions and because they are less likely to have access to air conditioning. That stark inequality is expected to increase as climate change intensifies.
“If we don’t adapt, we will continue to see more and more loss. Worse heat waves are coming our way,” he says.
What to do
“We have been thinking about climate change and how it will impact our communities and how we’ll have to act differently for some time,” says Lauren Jenks, the assistant secretary for environmental public health at Washington’s Department of Public Health.
“One of the things we learned from our  heat dome is how fragile the infrastructure can be,” she says. Refrigerators at restaurants and grocery stores, she notes as an example, gave out they struggled to maintain low temperatures in the punishing heat, forcing vendors to throw out perishable food.
In Seattle, where fewer than half of the city’s homes have air conditioning, Jenks says the city is now looking into ways everyone can have access to home AC or a community cooling center.
While climate change is becoming increasingly deadly, proactively providing more equal access to healthcare and creating more resilient cities can save lives, the IPCC report says.
Early warning systems—think of a detailed weather forecast—are one tool the report says city and state agencies can use to help people plan ahead for extreme weather and seek out resources like a cooling center when they need one.
According to the University of Washington’s Ebi, the need to improve both energy grids and healthcare systems for increased heat is “urgent and immediate… we are not prepared.”
“People are suffering and dying right now from climate change, and we’re not seeing an investment to prepare for an even warmer future,” she says.
Her colleague Jeremy Hess, who saw the impact of the 2021 heat dome from inside an E.R., says first responders need to be better prepared for the back-to-back disasters climate change will bring.
“We’re going to be hit by a heat wave, then a forest fire, then the power is going to go out, and then it’s going to happen again,” he says. “It sounds apocalyptic, but it’s true.”