San José Just Had Its Hottest Week Ever, ‘A Harbinger of Things to Come’

The early October heat wave is now over. The National Weather Service reports that the high temperatures broke major records in two Bay Area cities.

San José had its hottest seven-day heat wave ever recorded, and San Francisco had its second warmest string of above-average heat, said Nicole Sarment, a meteorologist at the weather service’s Bay Area office.

“It was the hottest overall,” she said of San José. “It’s definitely significant and especially significant for the time of year.”

San José, a city of nearly a million people, notched an average temperature of 83.4 degrees from Sep. 30 to Oct. 7. Further north in San Francisco, the city recorded an average temperature of 76.9 degrees, falling short of the record-breaking week of heat back in 1939 when the average temperature was 79.3 degrees.

Sarment said that cities like San José and San Francisco “generally run warmer than areas without dark pavement and tall buildings” during heat waves. However, scientists believe these future heat waves will likely top these records because of the effects of climate change brought on by humans burning fossil fuels.

This summer was the hottest on record for San José, California and the world. The excessively hot summer and the beginning of fall point to climate change in action, said Eugene Cordero, a meteorology and climate science professor at San José State University.

“This unusually warm period of time here in October is a harbinger of things to come,” Cordero said. “This is what the climate models have been predicting, and this is what we’re seeing all around the world.”

While it’s hard to immediately tell for certain how much climate change is amping up any individual heat wave, Cordero said the frequency of heat waves is increasing. Some parts of the state, including the Bay Area, are experiencing four to five times the number of heat waves they did back in the 1960s.

“There’s a very clear pattern that a warmer world comes with longer heat waves, more intense heat waves, and even warmer winters,” he said.

“Many scientists, including myself, feel increasingly frustrated at the lack of public action,” Cordero added. “We’ve fundamentally changed the energy exchange in our atmosphere because of extra carbon dioxide and other greenhouse gasses. The planet is warmer, and these outbreaks of high temperatures are going to become more and more frequent.”

Karen McKinnon studies heat waves at the UCLA Institute of the Environment and Sustainability. This week, she published a study looking at extreme summer heat waves. She found that record heat waves showcase how extreme climate change has already become.

“We certainly expect to see these record-breaking events because everything is warming with climate change,” she said.

While her study doesn’t delve into the Bay Area or Coastal California, where major heat waves often happen in early autumn, she expects “similar findings” if she studied the region. “We need to be quite humble as climate scientists because our system is changing before our eyes,” she said.

McKinnon said one string of hot days in isolation doesn’t alone tell the story of how climate change is intensifying heat waves and continued study over time is needed to truly understand.

While the heat wave is still in the minds of residents of San José and San Francisco, city leaders have a short window of opportunity to link it to climate change for residents, said Gabrielle Wong-Parodi, an assistant professor in Stanford’s Department of Earth System Science who studies how people respond to global environmental change, such as heat waves.

“These types of events are focusing events that draw attention not just of policymakers, but also of individual citizens,” she said. “This draws their attention away from other things where our attention is constantly and helps focus people on the climate change happening here in their own backyard.”