Warm Summer Nights

KEY CONCEPTS

• Summer minimum (nighttime) temperatures have warmed nearly twice as fast as summer maximum (daytime) temperatures.

• Since 1970, average summer nights have warmed across the contiguous U.S. by 2.5°F.

• In 40% of 246 U.S. locations analyzed by Climate Central, average summer nights have warmed by at least 3°F since 1970.

• Climate Central’s new Climate Shift Index (CSI) tool shows that climate change is making warm summer nights even more likely than warm summer days for most of the U.S.

Scorching summer sun gets all the attention. It sends us searching for cool shade and keeps us reapplying sunscreen. And it’s a symbol of our warming summers.

But temperatures are also rising after the sun goes down. In fact, extreme summer nights are warming nearly twice as fast as extreme summer days.

Warm summer nights are a serious health risk and come with expensive cooling costs. And climate change is making warm summer nights even more likely than warm summer days, according to Climate Central’s new Climate Shift Index tool.

Summer extremes: nights warming faster than days
When it comes to summer extremes, nights are warming much faster than days.

Last summer, a record 71% of the contiguous U.S. (by area) was affected by extreme minimum temperatures, compared to 45% exposed to extreme maximum temperatures.

And over the last 30 days (since June 18), the U.S. set 1.5 times more daily records for high minimum (nighttime) temperatures than for high maximum (daytime) temperatures.

These summer snapshots from last year and this year aren’t flukes—they’re consistent with long-term trends.

Since records began in 1895, summer minimum (nighttime) temperatures across the U.S. have warmed at a rate of +1.58°F per century. That’s nearly twice as fast as the warming rate observed for summer maximum (daytime) temperatures over the same period. 

How are these nationwide extremes felt locally and on average?

Warming average summer nights
From 1970 to 2021, average summer low (nighttime) temperatures have warmed across the contiguous U.S. by 2.5°F.

Since 1970, average summer nights have warmed by at least 3°F in 40% of 246 U.S. locations analyzed by Climate Central. Nearly a quarter of the 246 locations warmed by at least 4°F.

Places that experienced the most overnight warming since 1970 were: Reno, Nev. (+17.3°F); Las Vegas, Nev. (+9.5°F); El Paso, Texas (+8.1°F); Salt Lake City, Utah (+7.3°F); and Boise, Idaho (+6.9°F).

Climate change is making warm nights even more likely
Our warming climate is making all heat extremes more likely—but especially at night.

Climate Central’s new Climate Shift Index (CSI) tool provides real-time estimates of how much climate change is affecting daily high and low temperatures across the U.S.  

• The CSI shows that high temperatures today, July 20—right in the middle of summer—were made at least twice as likely due to human-caused climate change for 9% of the continental U.S. 

• By contrast, today’s low temperatures were made at least twice as likely due to climate change for 53% of the continental U.S. (equivalent to 68% of the population).

The stronger influence of climate change on today’s nighttime lows is consistent with trends so far this summer.

• According to CSI, from July 1-15 of this year, 20% of the population (24% of the continental U.S.) experienced daytime highs that were made at least twice as likely due to climate change.

• The numbers jump dramatically when we consider nighttime temperatures. Over that same period, 86% of the U.S. population (78% of the continental U.S.) experienced nighttime lows that were made at least two times more likely due to climate change. 

Observed warming trends and current CSI estimates suggest that, with continued heat-trapping emissions, we can expect summer nights to continue to warm and to occur more often. And that brings risks for people and the planet. 

Risks of nighttime heat
Warm nights are a major health risk. When temperatures don’t drop much overnight, people don’t have a chance to recover from more frequent extremely hot days. This can be especially risky in historically cooler climates, in urban heat islands, for vulnerable populations (including the very young and elderly, individuals with chronic illness, and outdoor workers), and for those who face disproportionate exposure to extreme heat (including neighborhoods with lower-income and higher shares of non-white residents).

More warm nights also mean higher demand for air conditioning. A report last year from Climate Central found that 95% of 246 U.S. locations have experienced a rise in cooling degree days since 1970. This doesn’t just mean higher energy bills. It also means higher future risks because when air conditioning units are powered by fossil fuels, their increased use further contributes to human-caused warming.

POTENTIAL LOCAL STORY ANGLES

How is climate change influencing nighttime temperatures near me?

Glad you asked! Climate Central’s latest tool, the Climate Shift Index (CSI), provides real-time estimates of how much climate change is affecting daily high and low temperatures across the entire U.S. Access the free tool anytime to see just how much climate change has altered the daytime and nighttime temperatures you’re experiencing today—and the temperatures your area is forecasted to experience tomorrow and the day after as well. 

Where can I find local solutions to reduce exposure to overnight heat risk?

The EPA’s Community Actions Database documents measures that communities are taking to mitigate the heat island effect in their area. FEMA has a dedicated page with tools, data, and resources to learn more about emergency management to reduce heat risks. For state-specific emergency management information, search your state on the USA.gov site. The EPA’s compendium of solutions for adapting to urban heat stress include planting trees and installing green or cool rooftops, or cool pavements.

LOCAL EXPERTS 

The SciLine service, 500 Women Scientists or the press offices of local universities may be able to connect you with local scientists who have expertise on extreme heat and climate change. The American Association of State Climatologists is a professional scientific organization composed of all state climatologists.

NATIONAL EXPERTS 

Tarik Benmarhnia, PhD
Associate Professor
University of California San Diego
Contact: tbenmarhnia@ucsd.edu
Related expertise: environmental epidemiology; climate change and health
*Available for interviews in English, Spanish, and French

Rachel Licker, PhD
Principal Climate Scientist
Union of Concerned Scientists
Media contact: asiefert@ucsusa.org
Related expertise: impacts of extreme heat

Jorge E. Gonzalez Cruz, PhD
SUNY Empire Professor of Atmospheric Sciences
University of Albany
Contact: jgonzalez-cruz@albany.edu
Related expertise: urban climate and weather
*Available for interviews in English and Spanish

Luis E. Ortiz, PhD
Assistant Professor
Department of Atmospheric, Oceanic, and Earth Sciences
George Mason University
Contact: lortizur@gmu.edu
Related expertise: urban heatwaves
*Available for interviews in English and Spanish

METHODOLOGY

The trend in summer minimum temperatures (June, July, August) was computed using data from the Applied Climate Information System and applying a mathematical linear regression. The national graphic used contiguous U.S. data from NOAA/NCEI Climate at a Glance. Climate Central's local analyses include 247 stations. However, for data summaries based on linear trends, only 246 stations are included due to large data gaps in Wheeling, W. Va.

The Climate Shift Index for high and low temperature represents the conditions averaged across each NOAA climate division. For each climate division, we counted the number of times the Climate Shift Index was greater than or equal to two over the period July 1-15 for both high and low temperatures. We also calculated the number of people in each climate division based on the 2020 Census. This allows us to compute the percentage of the U.S. population and the percentage of the continental U.S. that encountered Climate Shift Index values at or above a specified level.