California heat wave: Climate change is fueling warm nights this week - San Francisco Chronicle

California heat wave: Climate change is fueling warm nights this week
Overnight lows are expected to be in the 70s in much of the Central Valley this week. Climate change is making these temperatures five times more likely, according to attribution research from Climate Central, a nonprofit climate research and communication organization. Baron/Lynx

The first heat wave of 2024 is hitting inland California this week, and a spell of exceptionally warm nights is predicted. Temperatures may fail to drop below 70 degrees in parts of the Central Valley, Wine Country and the interior East Bay both Wednesday and Thursday morning, 10 to 15 degrees above normal for early June.

Bakersfield, Fresno and Visalia could all match or set new record warm low temperature records Wednesday.

These warm nights, a fingerprint of climate change, are dangerous to human health, especially for people without air conditioning, because it doesn’t allow bodies to cool down.

Advertisement

Article continues below this ad

“This is an important issue because the health statistics indicate that the ‘hot night’ heat waves result in greater human health impacts, including mortality,” said Nick Bond, emeritus Washington state climatologist.

Nighttime temperatures are especially sensitive to climate change due to the greenhouse effect. In fact, the excessively warm overnight lows in the Central Valley this week were made at least five times more likely by human-caused climate change, according to Climate Central, a nonprofit climate research and communication organization.

Nights are warming about twice as fast as daytime temperatures in the U.S., according to Andrew Pershing, vice president for science at Climate Central. Since 1970, summer nights have warmed 3.4 and 6.4 degrees in San Francisco and Fresno, respectively. Nighttime temperatures in Reno have climbed by an alarming 17.4 degrees in the same time.

“During the day you have a lot of variability in terms of temperature, and a lot of that comes from solar radiation, but at night you remove that away and the climate gets quiet,” Pershing said. “What you're left with is the gases in the atmosphere that have warmed up during the day and now they're radiating that heat back at you.”

Advertisement

Article continues below this ad

The greenhouse effect is caused by atmospheric gases, such as carbon dioxide, that absorb heat and radiate it earthward. This effect occurs naturally, and without it the planet would be uninhabitable at night, but burning fossil fuels adds excess heat-absorbing gases to the atmosphere and results in extra warmth.

Another naturally occurring greenhouse gas is water vapor, which varies regionally on a day-to-day basis to a higher degree than greenhouse gas concentrations.

The amount of water vapor in the atmosphere over California this week is expected to be 50% to 100% more than normal for early June due to lingering moisture from an atmospheric river that hit the Pacific Northwest. A warmer atmosphere can hold more water vapor, a double whammy for nighttime temperatures.

“Clouds, high humidity and increased carbon dioxide and other greenhouse gas concentrations serve to minimize this (nighttime cooling) effect,” Bond said.

Advertisement

Article continues below this ad

Previous studies categorized California heat waves into two distinct types and concluded that humid nighttime heat waves are projected to intensify more than dry daytime heat waves with human-caused climate change.

Reach Anthony Edwards: anthony.edwards@sfchronicle.com