That's Not Good

Billions of people may face “unprecedented” and deadly heat within 78 years

At our current rates of global warming, dangerous heat will become a near-daily routine for billions of people.

Thermostat rising to high temperature against sun and melting city backdrop

Some hundreds of millions of people have experienced unprecedented and dangerously scorching heat waves this summer, spanning the globe from China to the Midwest.

But new research suggests these currently rare, dangerous heat waves will become an annual occurrence in at least one major American city and a daily reality for billions of people living in the tropics. This is especially true if we don’t get our act together and curb greenhouse gas emissions to meet the Paris Agreement, which proposes limiting global warming to “well below” 2 degrees Celsius and ideally 1.5. The findings were published Thursday in the journal Communications Earth & Environment.

“These results are significant because they suggest that, in the likeliest scenario, billions of people could be exposed every year to levels of heat stress that are basically unprecedented in the current climate,” Lucas Vargas Zeppetello, lead author of the study, tells Inverse. Vargas Zeppetello studies climate variability as a postdoctoral fellow at Harvard University's Department of Earth and Planetary Sciences.

How they did it — Using data from the National Oceanic Atmospheric Administration, the new research forecasts future scenarios of global temperature rise due to climate change and connects that data to local conditions.

This method allowed the scientists to predict how frequently people living in regions of the world will experience extreme heat exposure under scenarios of a 1.5, 1.8, and 2.3 degrees increase in Celsius by 2050 and 2.1, 3.0, and 4.3 degrees by 2100. The scientists then created maps projecting dangerous heat across the globe under these different scenarios.

The first scenario corresponds to an unlikely future where we act decisively to significantly curb greenhouse gases, and the latter scenario assumes a world where we do very little to curb greenhouse gas emissions. Finally, the middle scenario corresponds to the most likely scenario under our current economic and climate trajectory, in which we do some work to curb greenhouse gas emissions like carbon dioxide — the primary cause of global warming.

According to their projections, we have only a 0.1 percent chance of meeting the Paris Agreement’s initial goal of limiting 1.5 degrees Celsius, which is largely in line with the findings of recent UN climate reports.

“If past trends in technology and human population dynamics are any indication, the Paris goals are going to be very hard to meet,” Vargas Zeppetello says.

But that’s far from the grimmest finding of the study.

A figure from the study. The top image shows the average number of days per year when the dangerous Heat Index threshold was exceeded in the historical record (1979–1998). The bottom images show the increase in dangerous heat under various climate change scenarios, ranging from a less likely low-warming scenario (5th percentile) to a worst-case high-warming scenario (95th percentile). The 50th percentile represents the median or more likely scenario.

Vargas Zeppetello et al

What they found — During the period of 1979 to 1998, the paper notes that temperatures exceeded the dangerous Heat Index — which corresponds to temperatures above 103°F — less than 15 percent of the days each year in places like subtropical Africa, the Indian subcontinent, and the Arabian peninsula. Meanwhile, areas in the Global North above 35˚N/S known as the “midlatitudes” rarely experienced heat events of that nature — less than once annually over a 20-year period.

Similarly, the “extremely dangerous” Heat Index — corresponding to temperatures above 124°F that can quickly lead to heat stroke or death — were very rare anywhere in the world.

Fast-forward to 2050 and under the median or most likely scenario, the researchers predict that people living in the tropics and subtropics will exceed the dangerous Heat Index up to 50 percent of the year. Because these areas are already in very hot climates, even modest temperature increases can lead to devasting heat and human consequences.

Fast-forward again to 2100, and the number gets even worse: People living in this region will encounter dangerous heat “most of the days in each year.” They’ll also likely experience “extremely dangerous” heat for 15 days a year, which will require “massive adaptation measures for a large number of people,” according to the study.

“People all across the globe, but especially the global south, are virtually certain to experience some unprecedented heat with unprecedented regularity over the next few decades,” Vargas Zeppetello says.

But that’s just according to the moderate scenario. According to the worst-case scenario of extreme warming exceeding 4 degrees Celsius by 2100, people will experience extremely dangerous heat for more than one month — and potentially longer — annually in the tropics and subtropics. Vargas Zeppetello calls such a scenario “nightmarish.”

“We've definitely never dealt with these environments before, and recent experiences like the heat wave in India suggest it would be catastrophic,” Vargas Zeppetello adds.

Let’s talk about Chicago —If you’re tempted to think extreme heat is largely a problem for people living in tropical climates, then think again. By 2100, people in the Global North will also suffer from significant annual exposure to dangerous heat — something that has rarely occurred at these latitudes in recent human history.

“Many regions in the midlatitudes will experience dangerous Heat Index values on between 15 and 90 days each year,” write the researchers.

The paper spotlights the city of Chicago, which serves as a good stand-in for the Global North because it’s not in a very hot climate but has a recent history of extremely dangerous temperatures, especially during the heat wave of the summer of 1995. That was the only time between 1979 and 1998 when Chicago exceeded 100°F for four consecutive days – and it happened twice that summer.

The summer of 1995 may very well forebode the future of the city. According to the most likely scenario in the new data, Chicago will face a 16-fold increase in “potentially dangerous” heat waves, including 32 events like the 1995 heat wave over a 20-year period near the end of this century. In other words: These dangerous heat waves will become a yearly nightmare.

“Heat waves like the kind that Chicago experienced in 1995 are projected to become a regular occurrence by the end of the century,” the study concludes.

A figure from the study shows the number of days reaching extremely dangerous heat in the past compared to the future under various warming scenarios.

Vargas Zeppetello et al

Why it matters — While data does not necessarily determine the future, Vargas Zepetello says the worst-case scenarios projected by the paper are “really frightening.” The scenarios forecasted by the paper are unlikely any that we have experienced in recent human history, leading to potentially unprecedented implications for human health, especially for the most vulnerable people like children and the elderly.

“I don't know what happens if billions of people are exposed for months of the year every year to these extremely dangerous damages,” Vargas Zeppetello says.

But we can still act to curb greenhouse gas emissions to avoid the worst-case scenario and achieve a more moderate future envisioned under the Paris Agreement, where people will still experience extreme heat stress for some of the year but it will be far more manageable. A world where we get carbon emissions under control and meet the Paris Agreement goals is “substantially better” than one where we don’t, according to the lead author.

“The goal is two degrees Celsius by the end of the century. It's unlikely, but it's still within the realm of possibility, which suggests that we need to do everything we can to make that happen,” Vargas Zeppetello adds.

Matei Georgescu, director of Arizona State University’s Urban Climate Research Center, states the paper’s findings are in alignment with previous research that continues “the parade of alarm bells that are ringing, indeed screaming, at policymakers to undertake strong measures to curb greenhouse gas emissions.” Georgescu was not involved with the study.

“The things we do today are still really, really important for preventing the worst-case climate scenarios,” Vargas Zeppetello concludes.

Pictured here: the sun rises over the Chicago sunline. The city is projected to experience a 16-fold increase in dangerous heat events by the end of the century.


What’s next — The most important thing we can do to stave off these worst-case scenarios is to shift to emissions policies that curb global warming, but more extreme heat is still coming even if we succeed in limiting temperature increases to 2 degrees Celsius.

So, that means we need to amp up our heat preparedness systems to adapt to and reduce the worst effects of heat to prevent further human suffering.

Technological innovations like air conditioning have helped humans adapt to extreme heat, but they also contribute further to climate change and are unaffordable to people in many parts of the world. Georgescu says the development of renewable energy technologies like solar photovoltaic energy can help people sustainably adapt to dangerous heat and ensure “a future that is not at the razor's edge of climatic consequences from our delayed inaction.”

“I fervently believe that the human race can and will adapt to a changing climate,” Georgescu says.

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