Is the Heat Wave Caused By Climate Change? This Is What Scientists Say
This article, originally published in November 2021, was republished on July 20, 2022, as Europe and North America suffered through record-setting heat waves. As the world continues to warm, scientists say everyone will experience more of these extreme weather events.
Everyone gathered in Glasgow for the COP26 climate conference is aware that the world is heating up and human activity is responsible—but how much of an individual weather event can be attributed to climate change?
A strange and dangerous one occurred this summer. As a historic heat wave paralyzed the North American Pacific coast on June 28, a group led by researchers in the U.K. and Netherlands called World Weather Attribution gathered virtually to start a task that just a few years earlier many had considered impossible. They would rapidly diagnose climate change’s impact on weather.
Within nine days, they reported that the unprecedented temperatures were “virtually impossible” without climate change. Greenhouse gases made them likelier by at least a factor of 150.
CANADA
BC
Vancouver
Pacific Ocean
Urban areas
Seattle
WA
ID
Portland
OR
CANADA
BC
Vancouver
Pacific Ocean
Urban areas
Seattle
WA
ID
Portland
OR
BC
CANADA
Vancouver
Pacific Ocean
Urban areas
Seattle
WA
ID
Portland
OR
BC
CANADA
Vancouver
Urban areas
Seattle
WA
Portland
ID
OR
WWA is more interested in people and society than meteorology, and strives to understand the effects of climate on weather, and of weather on people. The effort is led by Friederike Otto, senior lecturer at Imperial College London’s Grantham Institute for Climate Change and the Environment. WWA’s co-leader and co-founder, Geert Jan van Oldenborgh, died on Oct. 12 at the age of 59.
Extreme event attribution research is conducted by several groups around the world. Columbia University’s Sabin Center for Climate Change Law maintains a website that has now logged more than 415 studies, and the Bulletin of the American Meteorological Society has published annual reviews of climate and weather for every year since 2011. WWA rose to prominence because the team hit a trifecta: It can carry out an analysis that relies on multiple lines of evidence, with great speed and distribute its findings quickly to the world’s media.
In six years, WWA has rapidly analyzed more than 40 heat waves, floods, droughts, fires and even cold spells around the world. That’s a vast change for the profession, which can take many months to produce analysis. The group publishes most of its studies eventually through the rigorous scientific review process. But for real-time extreme-weather event attribution, it relies instead on a well-reviewed process, which can be simplified to three basic stages.
First, it takes in the extent and intensity of the event, and geographically defines which specific measure it wants to focus on. Second, it compiles the meteorological history of the area and analyzes the probabilities of the heat wave occurring in the known climate. The last step is to simulate the event in computer models, and compare it with a would-be world in which greenhouse gases were not warming the atmosphere.
Climate change makes all heat waves worse, the scientists emphasize. The question that remains is, how much?
The group defines not only the most important geographic area, but also the physical metric it wants to analyze—average daily temperature? Maximum nighttime temperature? In this case, it decided to analyze the maximum daily temperature within the boxed coordinates.
WWA then assembled the climate history of the region, pulling together temperature records that go back to 1950. Analyzing these data, using a standard statistical approach, gives the researchers a sense of how rare or common a heat wave is.
This Year’s Heat Wave Has No Historical Comparison
Max. annual daily temperature
(104°F)
40°C
2021: 39.4°C (103°F)
38°
36°
34°
32°
Each dot represents the hottest day of the year for each year going back to 1950
30°
28°
26°
1
2
5
10
50
100
1,000
10,000 Years
Years until such a temperature is likely to reoccur
Note: Temperatures are an average for the entire study area
Max. annual
daily temp.
This Year’s Heat Wave Has No Historical Comparison
(104°F)
40°C
2021: 39.4°C (103°F)
38°
36°
34°
32°
Each dot represents the hottest day of the year for each year going back to 1950
30°
28°
26°
1
2
5
10
50
100
1,000
10,000
Years until such a temperature is likely to reoccur
Note: Temperatures are an average for the entire study area
This Year’s Heat Wave Has No Historical Comparison
Max. annual
daily temp.
2021: 39.4°C (103°F)
38°C
(100°F)
34°
Each dot represents the hottest day of the year for each year going back to 1950
30°
26°
1
2
5
10
50
100
1,000
10,000
Years until such a temperature is likely to reoccur
Note: Temperatures are an average for the entire study area
The hotter the event (right axis), the rarer it is (bottom axis).
With an understanding of the area’s climate history, the members then simulate the same event in two different virtual worlds, with and without greenhouse-gas pollution. Comparing the model results allows them to conclude two things: how much likelier the heat wave is today than it would have been, and how much higher climate change pushed temperatures.
This kind of work has taken decades to develop. The 2021 Nobel Prize in Physics was awarded in part to Klaus Hasselmann, a German physicist who in the 1970s developed the idea that observations and computer modeling together could lead scientists to identify human “fingerprints” on the climate.
Five decades later, WWA applies a similar conceptual approach to extreme weather. The final phase of their work simulates the heat wave in a group of models that generate many virtual heat waves.
In this case, it found that the North American heat wave in June was extraordinary even for our warmed climate. Greenhouse gases made it at least 150 times more likely, and pushed the peak temperature up by 2°C higher than it might have been.
A heat wave like this one might become much more intense and frequent if the global average increases another 0.8°C—or 2°C total since the 19th century. That’s a threshold that could be crossed by midcentury if emissions continue at their current rate. In that case, climate change would add 3°C to the heat wave’s highest temperature, up from the 2°C it added to the Pacific coast in June. The frequency would increase, too, increasing from once in 1,000 years to once or twice every decade.
More Frequent, More Intense Heat Waves in the Future
Current Climate
1.2˚C of warming
Future Climate
2˚C of warming
Such a heatwave is
likely to reoccur every
1,000 Years
5–10 Years
And temperatures are
likely to be
+2˚C Hotter
+3˚C Hotter
Current Climate
1.2˚C of warming
Future Climate
2˚C of warming
Such a heatwave
is likely to
reoccur every
1,000 Years
5–10 Years
And temperatures
are likely to be
+2˚C Hotter
+3˚C Hotter
Such a heatwave
is likely to
reoccur every
And temperatures
are likely to be
Current
Climate
1.2˚C of
warming
1,000
Years
+2˚C
Hotter
Future
Climate
2˚C of
warming
5–10
Years
+3˚C
Hotter
Heat waves are more straightforward than many disasters WWA and other research groups analyze. That’s because it’s concerned with temperature alone, without much interference from complex atmospheric dynamics. Heat waves have become so straightforward that the next step may be to standardize the analyses so that national weather centers can conduct them routinely.
Such a development would free up WWA to work more on compound disasters, where multiple things go wrong at once. That includes droughts, which are marked by both heat and dryness, or post-drought flooding, which occurs when rainfall finally comes, but overwhelms parched land instead of nourishing it.
Widening the circle of organizations that can conduct rapid analyses would also allow researchers to focus on climate-juiced weather in parts of the world that have been understudied so far—which is most of it.
Ultimately, research like this has at least two major ambitions. One is to add data to the decades-old case that fossil-fuel-burning and other emissions of greenhouse gases must end. Fossil-fuel emissions keep flowing—up 5% over last year according to an early estimate—and so the global temperature rises.
The other ambition is to ensure communities are prepared for the heat that’s coming. More than 1,000 people died in the June heat wave. “Almost all of the deaths are preventable,” Kristie Ebi, founding director of the University of Washington’s Center or Health and the Global Environment and a WWA co-author, said in July. “People don’t need to die in heat waves. We know how to protect people.”