Jun 30, 2023 - Energy & Environment

Study: California's holiday weekend heat wave has direct ties to climate change

: Seagulls fly in front of the setting sun on Santa Monica Beach on April 15, 2023, in Los Angeles, California.

Seagulls fly in front of the setting sun on Santa Monica Beach in Los Angeles, California, where an excessive heat warning has been issued for this weekend. Photo: Gary Hershorn/Getty Images

As the oppressive hot weather that Texans have endured for weeks spreads across the U.S., Californians were preparing for their first major summer heat wave Friday.

State of play: Extreme heat was expected from central California into the desert Southwest Friday into this weekend, with excessive heat warnings issued by the National Weather Service for Californian cities including Los Angeles and San Diego.

Zoom out: The triple-digit temperatures that are set to hit California and across the Southwest are two to five times as likely now due to human-caused climate change, new research shows.

  • The analysis by Climate Central builds on the nonprofit climate science research and communications group's findings that the odds of the anomalous June heat that's been occurring in Texas and Mexico is at least five times as likely to occur, compared with a world without the human-caused increase in greenhouse gas emissions.
  • The nonprofit draws on a "Climate Shift Index," which is based on methods recently published in a peer-reviewed journal and uses a weather model combined with other sources to place daily high and low temperatures in a climate change context.
A Climate Central image showing a heat map of expected triple-digit temperatures across the Southwest.
Photo: Climate Central

Zoom in: "Daily average temperatures during the event are expected to exceed Climate Shift Index (CSI) levels of 2 across a broad region stretching from California to Colorado," per an emailed statement from Climate Central Thursday.

  • "Places in five states (California, Nevada, Arizona, Colorado, Utah) will reach Level 5 which indicates that human-caused climate change made this excessive heat at least 5 times more likely."

By the numbers: Climate Central data shows that compared to 1970, Fresno experiences 36 more risky heat days per year and fellow Californian cities Chico and Sacramento 18 and 10 more, respectively.

Meanwhile, Las Vegas has seen 39 more risky heat days and Phoenix 32 more.

What we're watching: Daniel Swain, a climate scientist at UCLA, writes in a blog post that while temperatures are expected to surge well above early July averages over the holiday weekend, it's not expected to be an exceptional heat wave — though some new daily temperature records could be set.

  • However, Swain notes because there haven't been any widespread major heat waves in the state over the past 30 to 40 days, people and some crops "are not yet acclimated to the heat.'"
  • This "tends to increase the risk of heat-related illnesses and crop damage beyond what the absolute temperatures might suggest (or which might occur during a heatwave of a similar intensity later in the season)," per Swain.

Go deeper: Intense heat waves occur primarily because of climate change

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