In July 1936, the central United States roasted during one of the most notable summers of the Dust Bowl-era. Parched lands, low rainfall and a strong ridge of high pressure over the region led to record-breaking temperatures in the Upper Mississippi River Valley — a handful of which still stand today. La Crosse, Wis., experienced temperatures at or above 90 degrees Fahrenheit for two consecutive weeks. Nationally, about 5,000 deaths were linked to the heat wave.

But if those same conditions happened now, the outcomes would be worse.

“If the same weather patterns of the 1930s-era heat waves occurred today again, they would happen in a much, much warmer climate,” said Erich Fischer, scientist at the Swiss Federal Institute of Technology in Zurich. “The heat wave we would experience will be way, way warmer than they were in the 1930s.”

As global temperatures have increased, extreme heat events in the Northern Hemisphere have occurred with greater frequency and intensity. Deadly, record-crushing heat waves have scorched parts of the United States, Europe and the Arctic in just the past two decades. The World Health Organization reports that more than 160,000 heat-related deaths occurred from 1998 to 2017 globally.

Recent studies show the magnitude of extreme heat events, and their effect on people will escalate in coming decades if greenhouse gas emissions from human activities are not slashed. Climate models show record-breaking heat waves and heat stress will more than double in the northern midlatitudes before the end of the century.

More severe record-shattering heat waves

Extreme heat events have been occurring more in recent decades: the 2003 European heat wave, the Russian heat spell in 2010, Australia’s “angriest summer” in 2018-2019, the Siberian heat anomaly in 2020 and the Pacific Northwest heat blitz in 2021.

“These record-breaking behaviors haven’t received enough attention [in adaptation and planning], and, in many places, we haven’t seen anything close to the most intense heat waves possible,” said Fischer. “We need to plan for events that shatter the records we have seen over recent decades.”

In a study published last week in Nature Climate Change, Fischer and his colleagues ran nearly 100 computer simulations to determine the frequency and intensity of record-breaking heat waves with future projections of Earth’s climate. They defined the intensity of the events by the margin by which they broke previous records.

They found week-long record-breaking heat events were up to seven times more likely to occur from 2021 to 2050. From 2051 to 2080, these events were up to 21 more times likely to occur and could happen every six to 37 years somewhere in the northern midlatitudes. These events would break previous heat records by 6.4 to 7.6 degrees Fahrenheit (3.6 to 4.2 Celsius).

The recent Pacific Northwest heat wave exemplified the team’s findings, even though they began working on this study more than two years ago. The event broke previous temperature records by more than 9 degrees Fahrenheit (5 Celsius). Fischer states that “such an event is completely outside the realm of all our expectations” if only considering past observations and not future climate projections.

The team found that the probability of these large-margin, recording-breaking heat waves is directly related to the speed of warming.

“It suddenly matters that we currently warm at 0.2 degrees Celsius [0.36 Fahrenheit] per decade,” said Fischer, stressing that faster rates of warming make setting records more probable.

Since 1880, Earth’s average global temperature has increased by slightly more than 2 degrees Fahrenheit (1 degree Celsius). Two-thirds of that warming has occurred since 1975, at around 0.35 degrees (0.2 Celsius) per decade.

Slowing the pace of global warming would lessen the chance of record-shattering events. Fischer said that if temperatures could be stabilized, then heat events would not break records by such large margins after a couple of decades.

But that is not to say non-record-breaking heat events would be harmless.

“If a heat wave strikes again, even if it’s not record shattering, people would still be very heavily impacted,” Fischer said. “For some of these impacts, it’s often even worse if the same event occurs over and over again, even if it doesn’t have a high magnitude.”

More danger to our bodies

Heat stress is one of the most serious effects of extreme heat events on the human body. It occurs when temperatures and relative humidity are high enough that the body can no longer get rid of the extra heat and cool itself. Heat stress can lead to heat exhaustion, heat cramps, heat strokes and death.

In a study published in the journal Earth’s Future in April, a team of researchers investigated how heat stress would intensify along with general increases in temperature, relative humidity and population in the Lower 48 U.S. states over the next century.

They found the potential impact of heat stress of short- to medium-duration (one to seven days) is likely to double in the United States by 2060 to 2099, if greenhouse gas emissions continue to be high. The risk tripled, though, in places with heavier populations, such as Central California, the Pacific Northwest and the Great Lakes region.

“Ironically, one of the regions for which we predict the greatest increases is the Pacific Northwest (as much as a tripling in under a high carbon emissions scenario), which was struck by the infamous ‘heat dome’ this summer,” wrote Michael Mann, an author of the study and a climate scientist at Pennsylvania State University, in an email.

Unlike in many other heat stress analyses, the team included a proxy for heat acclimatization, or the ability of our bodies to adapt to changes in temperature.

“If there is a gradual increase in the temperature, then our body can adjust to that,” said Ashok Mishra, an author of the study and a civil engineer at Clemson University. “If it’s less variable, our body will observe that pattern. It’s that sudden increase in the heat stress that will bring more problems.”

Mishra explained that temperatures have not been rising in a linear fashion since the 1960s, instead increasing with more variability. The variability makes it more difficult for our bodies to adapt to the sudden changes in temperature. The team accounted for the fluctuating and rapid changes by including intraseasonal variability, which previous studies have not done.

The team combined these measurements along with cases of heat stress from 1980 to 2019, calculating one-day, three-day and seven-day averages of the heat index for each summer during the 40-year period. The researchers compared the data to projected human-driven climate change models for the near future (2020 to 2059) and the far future (from 2060 to 2099). They used population projections to determine where populations were likely to grow or not.

Mann said, however, that these results are probably more conservative than reality. The state-of-the-art models used do not yet capture the full ranges of processes by which climate change is favoring more persistent extreme heat events, such as jet stream resonance.

“The extreme weather events this summer are a reminder that the impacts of climate change could well be worse than the models currently suggest,” wrote Mann. “These events could well exceed our adaptive capacity, underscoring the urgency of actions to prevent a worsening of the climate crisis, i.e. rapid decarbonization of our economy.”