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Driest, wettest, hottest: Sacramento’s troubling trifecta of extremes

California’s capital has witnessed its highest temperature, wettest day and longest dry spell in the past 12 months, all made more probable by climate change

A temperature sign seen during a heat wave in Sacramento on Tuesday. (David Paul Morris/Bloomberg News)

California’s capital experienced its hottest day ever observed Tuesday — hitting an unprecedented 116 degrees and exceeding previous September records by a landslide of seven degrees. This latest peak, amid a historic heat wave torching the state, follows a year of significant extremes in Sacramento: In the past 12 months, before this record hot day, the state capital experienced its wettest day on record last October, an event bookended by a record-long dry stretch that wreaked havoc on agriculture throughout California’s Central Valley.

The most recent heat has fueled dangerous wildfire weather, and there’s continued risk of power shortages. Rolling blackouts have been a constant threat in the Golden State in the past several days, as the state’s grid operator experiences record demand.

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It’s all part of a past year that’s been meteorological hell for the Central Valley — and Sacramento has been the scene of a bingo card of climate-fueled weather hazards. The extreme droughts and deluges are two sides of the same coin — all of these records made more probable by a warming planet.

Tuesday’s heat record

The average early-September high in Sacramento is about 90 degrees — hot but manageable in a climate of low humidity. Highs have warmed about 1.4 degrees since the late 1940s, and despite little change in overall yearly rainfall, Septembers are trending drier.

Before this ongoing heat wave, downtown Sacramento had never logged a temperature higher than 109 degrees during the month of September. Then it hit 113 degrees Monday and a staggering 116 degrees Tuesday.

“Obviously we’re starting to see temperatures like this happening in September and October, but it’s more like what we’d see in July or August,” said Emily Heller, a meteorologist at the National Weather Service in Sacramento.

Beating a monthly record by seven degrees is virtually unheard of — it hasn’t happened there since the 1940s, and back then it was possible only because many records were “younger” and easier to surpass. It’s analogous to running a marathon and winning by 10 minutes or more.

It goes without saying that a hotter world will lend itself to more frequent, intense and long-duration heat extremes. Sacramento hasn’t set a monthly cold-temperature record since 1999 but has logged 10 monthly high-temperature records since then.

The graph below illustrates the intersection of weather and climate quite well. There will always exist an inherent degree of natural variability in weather, but human-caused climate change is skewing things hotter. The blue curve marks the distribution of high-temperature observations from 1950 to 1980, and the red curve represents 1991 to 2021.

Notice the trend — the red curve is just a bit further to the right, which suddenly translates to considerably more days in the 94 degree-plus range. That’s the crux of climate change — it’s nonlinear. In other words, a subtle warming of a degree or two translates to perhaps exponentially more high-end extremes. Sacramento is seeing that firsthand.

Drought and flooding — two sides of the same coin

A favorite false talking point among some who reject climate science is that opposites such as drought and flood cannot both be caused by the same phenomenon. But in fact, drought and flooding — both of which have had major effects in Sacramento over the past year — are intimately linked by a premise called Clausius-Clapeyron.

Here’s what the premise says: For every degree Fahrenheit the air temperature warms, the atmosphere can hold about 4 percent more water. It’s an exponential relationship. When moisture is available, as in a saturated environment, i.e. a rainstorm, precipitation rates and totals tend to be more extreme. In the absence of moisture, the air can more efficiently evaporate what little remains in the ground, depleting moisture from the landscape and reinforcing drought. That allows the air to warm further, which entrenches the cyclical process even more.

It comes as no surprise that the past year has included Sacramento’s longest drought on record and most severe one-day rain total. Between March 20 and Oct. 17, 2021, not a drop of rain fell on Sacramento — a streak of 212 days. The previous longest dry spell had stretched 194 days and ended in November 1880.

California’s drought is also leading to more extreme fire behavior and larger, more destructive fires.

The state’s 2021 dry streak ended with flooding. Moderate rain fell Oct. 21, 22 and 23, but then 5.44 inches came down on the 24th alone. It was Sacramento’s wettest day on record — especially bizarre considering September averages only 0.93 inches. Another 0.37 inches fell the next day.

Some research suggests that slower west-to-east movement of the jet stream is in part contributing to weather patterns that get “stuck,” making it easier for the same conditions — for example, prolonged rainfall — to linger before the weather changes abruptly.

The bottom line

No single weather event is caused by climate change — but the intensity, frequency, duration and coverage of many extremes are boosted by human influence.

On a typical day, most of us won’t notice things are a degree or two warmer than they were 50 years ago. But suddenly that degree or two manifests in more broken records when outlier events are shifted into unprecedented territory.

There is evidence to indicate the fundamental patterns of our atmosphere are shifting, resulting in myriad hazards that are challenging communities. Sacramento is proof. And as precipitation and temperature extremes become even more pronounced and dramatic, the impact on communities and everyday life may continue to grow more quickly than society can adapt to.

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