3 Bay Area Cities Had Hottest Summer in History as Climate Change Pushes Temps Up

This summer was the world’s hottest on record. The same goes for California. But what about the Bay Area?

San José, Livermore and Napa had their warmest summers in more than 100 years of record keeping, according to a preliminary analysis by National Weather Service meteorologist Dial Hoang, a potential sign that rising temperatures fueled by human-caused climate change are especially sharp in the Bay Area’s inland regions.

“In San José … there’s been a significant warming trend since 2012,” Hoang said.

At a summer average temperature of 72.8 degrees, San José broke its record average of 72 degrees set in 1996. At 71.1 degrees, Napa topped a 2015 record of 70.8 degrees, and with an average temperature of 74.7 degrees, Livermore shattered a 2006 record of 73.9 degrees.

While these areas tend to have hot summers with plenty of days in the triple digits, Hoang said human-caused climate change contributes to the increasing average temperatures. The scientific consensus is that global fossil fuel pollution must be cut in half within this decade to avoid even hotter heat waves and other climate extremes.

“What we do know is that the extreme weather that’s being caused by climate change is becoming more frequent globally,” he said. “Twenty-three of the 24 warmest years on record have occurred since 2001.”

As Bay Area climate scientists study extreme heat to better understand how it affects the human body, cities grapple with ensuring vulnerable populations without access to air conditioning can withstand rising temperatures.

While the records broken in the Bay Area may seem marginal, climate scientists find that even small jumps in average temperature mean people in cities like San José are experiencing more extra hot days and heat waves, said Eugene Cordero, a professor in meteorology and climate science at San José State University.

Some parts of California, including the Bay Area, are experiencing four to five times the number of heat waves they did in the 1960s, Cordero said.

“That difference in the number of heat waves is attributed to climate change,” he said. “This is the human signal to the change of our climate.”

On average, California has warmed by up to 3 degrees Fahrenheit since the early 1990s, and the rate of warming has accelerated over the past several decades relative to the mid-20th century, according to the National Oceanic and Atmospheric Administration. The most intense heat waves would not be impossible without human-caused global warming.

Stanford climate scientists are trying to determine how much human-caused climate change has influenced extreme weather events like heat waves. The researchers trained AI models to predict daily maximum temperatures based on the regional weather conditions and the global mean temperature.

The researchers first focused on a historic 2023 heat wave in Texas and found that human-caused climate change made the heat wave between 2.12 to 2.56 Fahrenheit warmer than it would have been without climate change. They also found that heat waves in Europe, Russia and India over the past five decades could happen several times per decade if global temperatures reach 2.0 degrees Celsius above pre-industrial levels.

“AI hasn’t solved all the scientific challenges, but this new method is a really exciting advance that I think will get adopted for a lot of different applications,” said study senior author Noah Diffenbaugh, professor of Earth system science in the Stanford Doerr School of Sustainability.

While the Stanford study focuses outside California, scientists believe it holds implications for other parts of the world experiencing extreme heat.

“We expect the frequency of these types of events to increase from super rare in this current climate to occurring several times per decade under this possible future climate with double the amount of global warming,” said study lead author Jared Trok, a doctoral student in Earth system science at the Doerr School.

Knowing how warm a heat wave could be is important to David Romps, a UC Berkeley climate physics professor, because the heat people feel during a heat wave is often hotter than the actual temperature recorded. That’s partly due to humidity, which can often be higher in the Bay Area due to the influence of the Pacific Ocean.

“When the temperature is 105 degrees, if it’s high humidity, it will feel hotter than 105,” he said. “That higher humidity is going to impair your ability to sweat and so actually make your body more stressed.”

As climate change continues to heat the planet, daily temperatures will take longer to cool down, impairing the human ability to recover from heat, he said.

“We’re pushing towards conditions that the planet has not seen for a long time and the human species hasn’t seen in terms of a global average temperature,” he said. “We’re pushing into uncharted territory.”

This new heat paradigm is vital for cities across the Bay Area to consider because extreme heat is the deadliest of all types of extreme weather in the United States. Adults over 65, children, people with disabilities, people with substance abuse disorders, pregnant people, those who lack access to cooling and those who work outside are most at risk. Minority groups, which have historically been redlined, and urban communities are disproportionately exposed to heat. Lower-income and unhoused people are also exposed.

San José Mayor Matt Mahan said city officials adapted to the heat this summer by lowering the threshold at which the city opened cooling centers from 105 to 100 degrees.

“We’re particularly worried about our unhoused community because we have over 4,000 people in San José living outside in tents and vehicles without consistent access to air conditioning and even water,” he said.

Mahan said the city is also investing in its urban tree canopy to lower temperatures and provide shade during hot months. This year, the city planted 2,000 trees.

“We had an unusually hot summer and seem to be having more and more of those,” he said. “Our primary concern is that people can get into a cool space and access water and go about their daily lives, but in a way that keeps them safe.”