Seattle’s nights are hotter than ever. Climate change means more to come

It’s probably been hard to sleep lately if you’re one of hundreds of thousands of Seattleites without air conditioning.

This week saw three of the hottest nights on record, and these high temperatures can put additional stress on the human body, especially in neighborhoods dominated by concrete and asphalt. 

In the Puget Sound region, average monthly lows are rising in all summer months, with August continuing to see the highest lows. And, while minimum temperatures are up year-round, the summer months — July to September — have seen the sharpest increases.

Scientists who study the climate track low temperatures to better understand how greenhouse gas pollution contributes to warming. Rising nighttime temperatures — higher lows — demonstrate both climate change and its impacts.

This August’s lows have reached a record high, according to a Seattle Times analysis of data provided by the National Oceanic and Atmospheric Administration. From Monday to Wednesday, Seattle’s minimum temperatures were the highest in 130 years. So far this August, the average minimum of 61.2 degrees is the highest the city has felt at night. 

Heat has embroiled much of the world this summer, as scientists recorded the hottest June and July on Earth since record-keeping began. Record-high temperatures, fueled by climate change, are becoming increasingly common.

 In the Puget Sound region, as elsewhere, climate change may be more obviously manifesting through rising low temperatures, rather than highs.

When people release greenhouse gases like carbon dioxide and methane into the atmosphere by driving combustion engine cars or turning on a gas stove, the cooling rate is reduced. 

Earth’s surface cools at night, releasing heat back into the air. But greenhouse gases in the atmosphere trap some of the heat in. That has a bigger effect on nighttime temperatures than daytime temperatures, said Dennis Hartmann, an atmospheric science professor at the University of Washington.

In July, August and September, the Puget Sound region’s average low temperatures have risen 3 degrees over the century, peaking in 2022.

Seattle has gotten even warmer. Over the past 75 years, the average lows have risen 3.7 degrees, peaking last year at 2.6 degrees higher than the rest of the region. Augusts in Seattle are now 4.2 degrees higher than 75 years ago.

In the Seattle area, most people are accustomed to onshore flows or wind moving west to east from the Pacific Ocean, also known as nature’s air conditioning. Typically those flows keep overnight lows in the 50s and provide some reprieve from hot summer days, said Harrison Rademacher, a meteorologist for the National Weather Service in Seattle.

While the deadly 2021 heat dome broke heat records, people are at greater risk of heat-related illnesses if temperatures don’t fall at night, said Kristie Ebi, a professor in the University of Washington Center for Health and the Global Environment. 

“Our core body temperature needs to be within a relatively narrow range to protect ourselves and our organs,” Ebi said. 

“The heart, lungs or kidneys don’t really like to be hot,” Ebi continued.

If people are out in the heat during the day and it stays hot at night, they don’t get the respite they need.

While everyone is susceptible to health complications from prolonged heat exposure, babies, older adults, pregnant people, and the immunocompromised are most vulnerable. 

A growing density of concrete, asphalt and other heat-absorbing surfaces has left more than half of Seattle’s population residing in “urban heat islands” where daytime temperatures are over 8 degrees higher than they would naturally be. The trapped heat raises nighttime temperatures 2 to 5 degrees above surrounding areas.

Like most other U.S. cities, Seattle’s historically marginalized neighborhoods have fewer trees and less green space. Residents are more likely to be exposed to air pollution and heat, and to be burdened by preexisting health conditions.

For example, on a hot summer day, communities along the Duwamish might experience temperatures 10 or more degrees higher than neighborhoods up north like Ballard, a King County study revealed.  

Communities can ease the impacts of heat through distributing trees and green space more equitably, Ebi said. In the short term, as heat wave warning systems are rolled out, public officials should task trusted voices with warning and educating their community about the risks of heat, the professor said.

More information on how to stay healthy in the heat is available at the Department of Health’s website.