California's Heat Waves Are Getting Deadlier at

Most of us think about heat waves the way we think about a bad oven: brutal while it's on, but you crack the door at night and it cools down. That's how California heat has always worked. Hot days, cool nights. Sweat through it, sleep it off.

That's not how it works anymore.

Sasha Gershunov, a climate scientist at Scripps Institution of Oceanography and head of the Weather Extremes and Climate Impacts Analytics group (they call themselves WeaKlima, which I love), gave a lecture recently for the Osher UC San Diego Distinguished Lecture Series that I've been turning over in my head ever since. The science itself isn't new — global warming, CO₂, greenhouse effect. But the texture of what he described about California's changing heat is specific and strange enough that I think it deserves a closer look. 🌡️

The blanket metaphor is better than the greenhouse metaphor

Before getting into the extremes, Gershunov does something I find genuinely useful: he rejects the greenhouse analogy. A greenhouse works by blocking airflow. The actual mechanism of CO₂ warming is different — it's about radiation wavelengths, absorption, re-emission. His preferred metaphor is a blanket.

"If you put on a thicker blanket, well, then you're going to be hotter in bed, and the same thing is happening to Earth."

Clean. Accurate. And it also explains why we can't just wait it out tonight — because the blanket doesn't lift. The Earth keeps warming until it radiates enough energy back out to space to reach a new equilibrium. As long as we keep thickening the blanket by burning fossil fuels, we're not at equilibrium. The warming continues.

One wrinkle worth noting: Gershunov mentions that water vapor is "a much stronger greenhouse gas than carbon dioxide." That's technically true in terms of total radiative forcing — water vapor accounts for roughly half of the greenhouse effect. But it's doing a different job than CO₂. Water vapor is a feedback amplifier, not the initial driver. CO₂ is what starts the warming; warmer air holds more moisture; that extra moisture amplifies the warming further. So CO₂ is still the dial you're turning. Water vapor is what makes the speakers blow out when you turn it too loud.

A brief, incomplete history of people who figured this out way earlier than we acted on it

Gershunov walks through the intellectual history here, and the dates are worth knowing. In 1856, Eunice Newton Foote presented experiments at the American Association for the Advancement of Science showing that CO₂ (then called "carbonic acid") had unusual heat-trapping properties. Her paper concluded: "An atmosphere of that gas would give our Earth a high temperature."

Foote was present at the meeting, but her work was read aloud by Joseph Henry rather than presented by her directly. Gershunov says this was because "women were not allowed to present there" — which reflects how her story is commonly told, though historians have noted the precise reason Henry read the paper (prohibition, convention, or some other arrangement) isn't entirely settled in the record. What isn't contested: a woman worked out the basic premise of anthropogenic warming in 1856, and we're still arguing about it in 2026.

Swedish scientist Svante Arrhenius took it further in his 1896 paper "On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground", calculating how much the Earth would warm from a doubling of CO₂ and arriving at a figure of several degrees — a prediction that's held up remarkably well against modern modeling.

Then came Charles David Keeling, who in the late 1950s started measuring atmospheric CO₂ from the summit of Mauna Loa in Hawaii — chosen for its isolation from local pollution sources. What he got was the Keeling Curve: a rising line with a small annual wiggle caused by seasonal photosynthesis (plants in the Northern Hemisphere exhaling CO₂ in winter, inhaling it in summer). Gershunov mentions he actually met Keeling at Scripps. Keeling's son Ralph now runs the program. The measurements have never stopped. The curve has never bent down.

The thing nobody told you about the 2006 heat wave

Here's where the lecture got under my skin. 🧬

California had a catastrophic heat wave in July 2006. Gershunov says it was unprecedented — not mainly because of how hot the days were, but because of how hot the nights stayed. Minimum temperature records broke across the state. And people died.

According to coroner's reports cited in Gershunov's research, 147 people died directly from hyperthermia. But he flags a more meaningful number: the California Department of Health Services estimated excess mortality — deaths from all causes above the expected baseline during that period — at closer to 600 statewide. Excess mortality captures the less visible deaths: the person whose heart gave out because heat was the last straw on top of cardiovascular disease, the diabetic whose body couldn't regulate, the elderly woman who didn't turn on her air conditioning because she was waiting for the cool she knew always came.

That last part stopped me. Some of the people who died had working air conditioners and didn't use them. They were following the pattern they'd learned over a lifetime in California: endure the day, survive the night. The night didn't cooperate.

"Most of these people who died were elderly and living alone," Gershunov said. "Some had functioning air conditioning, but didn't turn it on."

About 30 of the dead were young and healthy. He paused before answering his own question about why they were vulnerable. Farmworkers.

The geographic distribution is also counterintuitive: more deaths occurred inland and in the Central Valley, where it was hottest. But more hospitalizations and ER visits came from the coast, where temperatures were lower in absolute terms but far more unusual relative to what coastal residents are acclimated to — physiologically and in terms of infrastructure. There is very little air conditioning on the California coast. There was even less in 2006.

The new flavor of California heat

The 2006 event is what prompted Gershunov to start building a systematic heat wave record for California. What he found, and what's continued to develop in the nearly two decades since, is that California's heat waves are changing character — not just becoming more frequent, intense, and longer (which they are, in line with global trends), but becoming more humid.

That humidity shift is the compounding problem. California's coast naturally stays cooler and drier than inland regions; coastal heat waves used to be rare. What's arriving now are events where the marine air is warmer and wetter than it used to be, which means two things simultaneously go wrong for the human body: it's hotter than you expect, and you can't sweat your way out of it.

Evaporative cooling — sweating — only works when moisture can evaporate into the surrounding air. When the air is already saturated, the sweat just sits there. You're wet and still dying. Gershunov frames this as "a double whammy," and I think that's exactly right. The mechanism isn't mysterious; it's just new to places that didn't used to experience it.

Minimum temperatures are rising faster than maximum temperatures in California's record. Nighttime heat wave activity is climbing more steeply than daytime. The cool-down that the coast relied on, that elderly residents counted on, that farmworkers hoped for after a shift — it's arriving later, if at all.

What Bakersfield's thermometer tells the whole country

One data visualization Gershunov describes is simple but clarifying. Take a long-term weather station — Bakersfield, going back to the 1880s. Count hot temperature records broken versus cold temperature records broken over time. At the beginning of the record, it's roughly 50/50 — no trend yet, just natural variability. Watch what happens as you move forward through the decades. Hot records climb. Cold records decline. By the last 20 years, cold records are essentially gone. Hot records break constantly.

He shows the same pattern for Austin, Salt Lake City, Colorado Springs, San Diego. The Bakersfield finding isn't regional quirk; it's the fingerprint of a warming world rendered in local thermometer readings. The same climate signal — more warming in the polar regions than the tropics, more over land than ocean — was predicted by climate models in the 1970s, before it was measurable. The fact that the models got the spatial pattern right before the warming was even clearly observed is one of the less-discussed but more compelling aspects of climate science's predictive track record.

The 2006 heat wave killed people who were waiting for something that didn't come. The research Gershunov's group has built since then shows that the conditions which made that event deadly — humid air, stubborn nighttime warmth, a coast unprepared for temperatures it had no cultural or physiological memory of — are no longer flukes. They're the new baseline's ceiling.

The question I can't shake isn't about the science. It's about the farmworker in the Central Valley, and the elderly woman in Fresno who had an air conditioner she didn't turn on, and how much of what killed them was temperature and how much was a decades-long failure to connect what the Keeling Curve was saying to the infrastructure, the policy, and the warning systems that should have been built around it. 🌊

By Mei Zhang