Overnight low temperature

Difference from 1991-2020 average

Cooler

Warmer

Friday

June 17

Thursday

June 16

Wednesday

June 15

Tuesday

June 14

Monday

June 13

Sunday

June 12

Saturday

June 18

A week of highs: See where climate change made heat worse in America

A new tool can determine in real time the role climate change plays in making daily high temperatures more likely

Last week, 96 percent of people in the contiguous United States experienced nighttime temperatures more likely to occur due to human-caused warming. The findings come from a Washington Post analysis of data provided by the nonprofit Climate Central, which released the world’s first tool to show how climate change is affecting daily temperatures in real time.

Overnight temperatures, as opposed to daytime temperatures, were boosted the most by climate change. While more and more people are increasingly exposed to warmer nighttime temperatures, which are potentially more dangerous to the body, last week’s number stands out.

June 13

Overnight low temperature

Difference from 1991-2020 average

Cooler

Warmer

0

-10ºC

+10ºC

-18ºF

+18ºF

Phoenix

experienced a

record overnight

temperature of

90°F (32°C)

Dodge City, Kan., had its

warmest minimum temperature

ever observed in any month

of the year of 83°F (28°C)

June 13

Overnight low temperature

Difference from 1991-2020 average

Warmer

Cooler

0

-10ºC

+10ºC

-18ºF

+18ºF

Phoenix experienced

a record overnight

temperature

of 90°F (32°C)

Dodge City, Kan., had

its warmest minimum

temperature ever observed

in any month of the year

of 83°F (28°C)

June 13

Overnight low temperature

Difference from 1991-2020 average

Cooler

Warmer

0

-10ºC

+10ºC

-18ºF

+18ºF

Phoenix experienced

a record overnight

temperature of 90°F (32°C)

Dodge City, Kan., had

its warmest minimum temperature

ever observed in any month

of the year of 83°F (28°C)

June 13

Overnight low temperature

Difference from 1991-2020 average

Cooler

Warmer

0

-10ºC

+10ºC

-18ºF

+18ºF

Phoenix experienced

a record overnight

temperature of 90°F (32°C)

Dodge City, Kan., had its warmest

minimum temperature ever observed in

any month of the year of 83°F (28°C)

More than 3,000 new daily high temperatures were reached in the Lower 48 states that week – with nearly twice as many unprecedented warm temperatures reached at night than during daytime.

“Climate change is impacting us every day somewhere. That’s a big part of the world that we’re living in right now,” said Andrew Pershing, director of climate science at Climate Central. “Our goal is really to be able to talk about everyday conditions.”

Over the past decade, climate scientists have sharpened the ability to link how climate change has influenced extreme weather events across the world. The field, known as climate attribution science, has traditionally been reserved for notable, damaging events on society, but Climate Central’s new endeavor shows how everyday weather that may not make news headlines is altered as well.

June 13

Cooler

Warmer

0

-10ºC

+10ºC

-18ºF

+18ºF

Where overnight low temperatures

were more likely due to climate change

Where overnight low temperatures

were less likely due to climate change

Where overnight low temperatures

did not have a detectable climate signal

June 13

Warmer

Cooler

0

-10ºC

+10ºC

-18ºF

+18ºF

Where overnight low temperatures

were more likely due to climate change

Where overnight low temperatures

were less likely due to climate change

Where overnight low temperatures

did not have a detectable climate signal

June 13

Cooler

Warmer

0

-10ºC

+10ºC

-18ºF

+18ºF

Where overnight low

temperatures

were more likely due

to climate change

Where overnight low

temperatures

were less likely due

to climate change

Where overnight low

temperatures did not have

a detectable climate signal

June 13

Cooler

Warmer

0

-10ºC

+10ºC

-18ºF

+18ºF

Where overnight low temperatures were

more likely due to climate change

Where overnight low temperatures were

less likely due to climate change

Where overnight low temperatures

did not have a detectable climate signal

The new tool — called the Climate Shift Index (CSI) — calculates how much more likely daytime high and overnight low temperatures are to occur because of climate change. An index score, or CSI, of 2, for example, means climate change made the day’s temperature twice as probable.

On June 13 alone, Phoenix, Memphis, Nashville, Atlanta, St. Louis, Tampa and Santa Fe, N.M., all experienced warm overnight temperatures that were made at least five times more likely because of climate change — earning a CSI of 5.

The Climate Shift Index shows where

human-caused warming made the temperature in a region more or less likely to occur

Less likely

More likely

-5x

-4

-3

-2

-1.5

1.5

2

4

5x

3

June 13

Overnight low temperatures

Nashville experienced an overnight low temperature made at least 5 times more likely by climate change

Dodge City, Kan.

3x

Santa Fe, N.M.

5x

Memphis

5x

Phoenix

5x

Atlanta

5x

Tampa

5x

The Climate Shift Index shows where

human-caused warming made the temperature in a region more or less likely to occur

Less likely

More likely

-5x

-4

-3

-2

-1.5

1.5

2

4

5x

3

June 13

Overnight low temperatures

Nashville experienced

an overnight low

temperature made at least

5 times more likely

by climate change

Dodge City, Kan.

3x

Santa Fe, N.M.

5x

Memphis

5x

Phoenix

5x

Atlanta

5x

Tampa

5x

The Climate Shift Index shows where

human-caused warming made the temperature in a region more or less likely to occur

Less likely

More likely

-5x

-4

-3

-2

-1.5

1.5

2

4

5x

3

June 13

Overnight low temperatures

Nashville experienced an

overnight low temperature made

at least 5 times more likely

by climate change

Dodge City, Kan.

3x

Phoenix

5x

Santa Fe, N.M.

5x

Memphis

5x

Atlanta

5x

Tampa

5x

In contrast, climate change played little to no influence in the daytime temperatures in those cities.

“There’s just really strong evidence that our nighttime climate is altered. … It seems to be especially in this early summer period,” Pershing said. “That’s just really an important way that people in the United States are experiencing climate changes.”

Warm nighttime temperatures are potentially more dangerous than daytime highs. Typically, temperatures dip at night and allow our bodies to cool down from the daytime heat. If temperatures remain elevated, the prolonged heat increases the risk of heat exhaustion, cramps, strokes and even death.

The Climate Central tool employs well-established methodologies previously used in attribution studies of extreme weather events. The team uses data and computer models to create simulations of a world with and without carbon emissions to determine the effect of climate change on daily temperatures.

Friederike Otto, an expert in climate attribution science and co-lead of the World Weather Attribution initiative, said one really good feature of the tool is that it shows how different the natural variability is across the United States — what is and isn’t climate-change-related. Locations with the largest temperature anomalies in the country may not present the strongest fingerprint of climate change. For instance, temperature anomalies were the highest in the central United States on June 13, yet the climate change fingerprint was relatively small, as seen in Dodge City.

Meanwhile, nighttime temperatures in Atlanta hit 74 degrees on Monday. While the temperature anomaly was lower than in other locations on the same day, climate change made the overnight warmth at least five times more likely to occur.

“In the central regions, you see large anomalies,” but the influence of climate change is relatively small, said Otto, who helped develop the framework for the tool but is not involved in the operation. She said the tool “takes the natural variability out of the climate change signal … [and] makes climate change visible in a way that just [temperature] anomalies can’t.”

Where climate change made temperatures more likely

Sunday, June 12

Overnight low

temperature

Daytime high

temperature

Difference from

1991-2020 avg.

Climate Shift

Index

Monday, June 13

Tuesday, June 14

Wednesday, June 15

Thursday, June 16

Friday, June 17

Saturday, June 18

Where climate change made temperatures more likely

Sunday, June 12

Overnight low

temperature

Daytime high

temperature

Difference from

1991-2020 avg.

Climate Shift

Index

Monday, June 13

Tuesday, June 14

Wednesday, June 15

Thursday, June 16

Friday, June 17

Saturday, June 18

Where climate change made temperatures more likely

Overnight low

temperature

Daytime high

temperature

Sunday, June 12

Difference from

1991-2020 avg.

Difference from

1991-2020 avg.

Climate Shift

Index

Climate Shift

Index

Monday, June 13

Tuesday, June 14

Wednesday, June 15

Thursday, June 16

Friday, June 17

Saturday, June 18

Where climate change made temperatures more likely

Overnight low

temperature

Sunday, June 12

Monday, June 13

Tuesday, June 14

Wednesday, June 15

Thursday, June 16

Friday, June 17

Saturday, June 18

Difference from

1991-2020 avg.

Climate Shift

Index

Daytime high

temperature

Difference from

1991-2020 avg.

Climate Shift

Index

Climate scientist Sonia Seneviratne, who was not involved in the project, said the tool is important for communication and could be useful for daily weather bulletins.

“It helps make climate change more tangible,” said Seneviratne, who coordinated the chapter on weather and climate extremes of the recent report by the United Nations Intergovernmental Panel on Climate Change (IPCC). “While attribution of extreme events is probably of higher interest to the general public, it is also important to show that climate change is affecting everyday weather.”

People who experienced

overnight temperatures

that were ...

... were more

likely due to

climate change

300

million

200

... did not have

a detectable

climate signal

100

... were less

likely due to

climate change

0

June 12

June 18

South

West

120

million

80

40

0

June 12

June 18

June 12

June 18

Midwest

Northeast

120

million

80

40

0

June 12

June 18

June 12

June 18

People who experienced overnight

low temperatures that ...

300

million

... were more likely

due to climate

change

200

... did not have

a detectable

climate signal

100

... were less likely

due to climate

change

0

June 12

June 14

June 16

June 18

South

West

120

million

80

40

0

June 12

June 18

June 12

June 18

Midwest

Northeast

120

million

80

40

0

June 12

June 18

June 12

June 18

People who experienced overnight low temperatures that ...

300

million

... were more likely

due to climate change

200

... did not have a

detectable

climate signal

100

... were less likely

due to climate change

0

June 18

June 12

June 14

June 16

Midwest

Northeast

South

West

120

million

80

40

0

June 12

June 18

June 12

June 18

June 12

June 18

June 12

June 18

Some estimates say more than 1,300 people die each year in the United States because of extreme heat, but numbers are higher elsewhere in the world. Between 1980 and 2017, the world’s 150 most-populated cities experienced a 500 percent increase in exposure to extreme heat, according to a February report from the IPCC.

The problem will become even worse as the planet continues to warm, according to the IPCC. If global average temperatures rise to 2 degrees Celsius (3.6 degrees Fahrenheit) above preindustrial levels, 16 times as many people will be exposed to heat waves each year, with people in low-income countries most affected. Under the worst-case warming scenarios, people in tropical regions of Africa could suffer from year-round deadly heat.

[The science of heat domes and how climate change makes them worse]

The Climate Central team plans to roll out real-time daily weather attribution across the globe later this year.

“I think there is still a huge underestimation of just how much climate change already influences our daily life,” Otto said. “There’s still not really an appreciation of just how much the impacts today are costing us. Every time we go outside, it’s different because of climate change.”

Sarah Kaplan contributed to this report.