Mapping where climate change worsened heat waves in the United States last week

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

Nighttime temperatures, as opposed to daytime temperatures, have been boosted the most by climate change. As more people are increasingly exposed to hotter nighttime temperatures, which are potentially more dangerous to the body, last week’s figure stands out.


Low temperature at night

Difference from 1991-2020 average

Phoenix

lived a

record overnight

temperature of

90°F (32°C)

Dodge City, Kan., had its

hottest minimum temperature

never seen in a month

of the year of 83°F (28°C)

Low temperature at night

Difference from 1991-2020 average

Experienced Phoenix

an overnight record

Temperature

90°F (32°C)

Dodge City, Kansas, had

its warmest minimum

temperature never observed

in any month of the year

83°F (28°C)

Low temperature at night

Difference from 1991-2020 average

Experienced Phoenix

an overnight record

temperature of 90°F (32°C)

Dodge City, Kansas, had

its warmest minimum temperature

never seen in a month

of the year of 83°F (28°C)

Low temperature at night

Difference from 1991-2020 average

Experienced Phoenix

an overnight record

temperature of 90°F (32°C)

Dodge City, Kansas, had its hottest

minimum temperature ever observed in

any month of the year 83°F (28°C)

More than 3,000 new daily high temperatures were reached in the lower 48 states that week – with almost twice as many unprecedented hot temperatures reached at night as during the day.

“Climate change affects us somewhere every day. It’s a big part of the world we live 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, climatologists have refined ability to relate how climate change has influenced extreme weather events around the world. The field, known as the science of climate attribution, has traditionally been reserved for notable and societal-damaging events, but Climate Central’s new initiative shows how everyday weather that may not make headlines is also changed.


Where low night temperatures

were more likely due to climate change

Where low night temperatures

were less likely due to climate change

Where low night temperatures

had no detectable climate signal

Where low night temperatures

were more likely due to climate change

Where low night temperatures

were less likely due to climate change

Where low night temperatures

had no detectable climate signal

where night low

temperatures

were more likely due

to climate change

where night low

temperatures

were less likely due

to climate change

where night low

temperatures have not

a detectable climate signal

Where low night temperatures were

more likely due to climate change

Where low night temperatures were

less likely due to climate change

Where low night temperatures

had no detectable climate signal

The new tool – called the Climate Shift Index (CSI) – calculates the extent to which daytime maximum and nighttime minimum temperatures are more likely to occur due to climate change. An index score, or CSI, of 2, for example, means climate change has made the day’s temperature twice as likely.

On June 13 alone, Phoenix, Memphis, Nashville, Atlanta, St. Louis, Tampa and Santa Fe, NM all experienced a hot night temperatures that have been made at least five times more likely due to climate change – earning a CSI of 5.


The climate change index shows where

human-caused warming has caused the temperature to rise in an area After Where less likely to arrive

Low night temperatures

Nashville experienced a low temperature night made at least 5 times more likely by climate change

The climate change index shows where

human-caused warming has caused the temperature to rise in an area After Where less likely to arrive

Low night temperatures

Nashville experienced

a stocking overnight

temperature made at least

5 times more likely

by climate change

The climate change index shows where

human-caused warming has caused the temperature to rise in an area After Where less likely to arrive

Low night temperatures

Nashville experienced a

low temperature at night

at least 5 times more likely

by climate change

In contrast, climate change had little or no influence on daytime temperatures in these cities.

“There is just very strong evidence that our nocturnal climate is being altered. … It seems to be especially in this period of early summer“, said Pershing. “It’s really a big way that people in the United States are experiencing climate change.”

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

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

Friederike Otto, climate attribution science expert and co-lead of the Global Weather Allocation initiative, said a very good feature of the tool is that it shows how different natural variability is across the United States — what is and is not related to climate change. The places with the greatest temperature anomalies in the country may not show the strongest fingerprint of climate change. For example, temperature anomalies were highest in the central United States on June 13, but the climate change footprint was relatively small, as seen in Dodge City.

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

“In the central regions, you see big anomalies,” but the influence of climate change is relatively small, said Otto, who helped develop the tool frame but is not involved in the operation. She said the tool “removes natural variability from the climate change signal… [and] makes climate change visible in a way [temperature] anomalies cannot.


Where climate change has made temperatures more likely

low night

Temperature

Difference from

1991-2020 avg.

Where climate change has made temperatures more likely

low night

Temperature

Difference from

1991-2020 avg.

Where climate change has made temperatures more likely

low night

Temperature

Difference from

1991-2020 avg.

Difference from

1991-2020 avg.

Where climate change has made temperatures more likely

low night

Temperature

Difference from

1991-2020 avg.

Difference from

1991-2020 avg.

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

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


people who have lived

night temperatures

who were …

… were After

likely due to

climate change

… did not have

a detectable

climate signal

… were less

likely due to

climate change

People who lived overnight

low temperatures which…

… were more likely

because of the climate

change

… did not have

a detectable

climate signal

… were less likely

because of the climate

change

People who have experienced overnight low temperatures that…

… were more likely

due to climate change

… had no

detectable

climate signal

… were less likely

due to climate change

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

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

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

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

“I think there’s still a huge underestimation of how much climate change is already influencing our daily lives,” Otto said. “There is still no real appreciation of how much the impacts are costing us today. Every time we go out it’s different because of climate change.

Sarah Kaplan contributed to this report.

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