The Climate Crisis Is Changing The Definition Of Extreme Heat

We all know it’s hot — and getting hotter.

According to NOAA and NASA, the 10 warmest years globally since 1850 have all occurred in the past decade. In less than 200 years, when the Industrial Revolution began, the global average temperature has increased by more than two degrees Fahrenheit.

While the planet’s climate has changed dramatically over millions of years, atmospheric carbon dioxide levels are now at their highest in millenia due to pollution from human society. Scientific consensus proves the modern, rapid weather shifts we're now experiencing are a result of how we humans, largely through fossil fuel pollution, are changing the climate.

The science is clear that we’re entering a new era of extreme heat. We’re unlikely to keep global temperatures below the threshold scientists have raised the alarm about for years, and weather extremes will only become more frequent and intense if we don’t dramatically cut emissions in the next seven years or soon after.

So, as the earth heats up, what is “normal” vs “extreme” heat? How is the climate crisis affecting heat here in Southern California?

Normal heat in the Southland

Of course, July through September and even much of October in southern California, it's normal for the weather to heat up. Southern California has deserts, mountains and the ocean, so our temperatures vary widely. From geography to tree cover to local weather patterns, there are a lot of factors that naturally make some parts of Southern California hotter than others. My colleague, science reporter Jacob Margolis, explains in more detail in this story.

'Extreme' vs 'normal' heat

What’s normal versus extreme depends on where you live — after all, what’s considered a hot day in Santa Monica is a far cry from a hot day in Palm Springs.

The climate crisis is shifting average temperatures hotter — not every day, or every season, but it’s driving an average increase in both day and nighttime temperatures.

How the climate crisis is expected to affect heat in California.

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Courtesy of the California Legislative Analyst's Office

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That also means longer and more frequent “heat waves,” during which temperatures reach higher extremes and stay there for longer periods of time. A week-long heat wave in early September 2022 was the hottest and longest in the state’s record for that month, which is already one of the hottest months in California.

“Whether you're on the coast, whether you're in the mountains, whether you're in the deserts, whether you're in between, all those averages are going up,” said Alex Tardy, a meteorologist with the National Weather Service (NWS) San Diego office.

In Southern California, the most rapid and dramatic increases are happening in the deserts and the mountains, already extreme environments, Tardy said.

Doing the math

Since 1901, the National Oceanic and Atmospheric Administration (NOAA) has collected temperature records from weather stations across the country. They use that information, combined with other data and computer models to calculate specific “climate normals” for different regions.

The “normals” add up to a 30-year average temperature, or what’s considered a long-term average temperature. NOAA’s National Weather Service uses those average temperatures to decide whether a heat event is extreme or not.

“Abnormal heat is typically daytime temperatures 10 to as much as 20 degrees Fahrenheit warmer than the long-term average,” said Tardy. “When we talk about a heat wave, what we're talking about is really two or more days when temperatures are much above average.”

NOAA's latest climate normals map showing the normals from 1991-2020 minus 1981-2010 normals. Most of the U.S. shows significant warming, especially in the West, Southwest and East, where soem normals jumped by a whole degree already.

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Courtesy of NOAA

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Since our bodies take time to acclimate to changes in temperature, those averages depend on the time of year. For example, Tardy said that an abnormal threshold in mid-July in L.A would be 90 degrees, whereas it would be 110 degrees in Palm Springs.

“We look at the heat risk, we look at the departure from normal, we look at the ability for someone to be able to acclimate to the heat,” Tardy said. “We don't just look at the fact that it's a hundred degrees and that requires a heat warning.”

Every 10 years, NOAA recalculates the climate normals — most recently in 2020 — and overall, averages are going up.

“Normals and averages — it's always been a moving target, but the target is moving up and up and up and warmer and warmer and that's what we've seen the past 10 years,” said Tardy.

Six of the last 10 years have been the hottest on record in Southern California, Tardy said.

Ranking heat waves like hurricanes

When the NWS sees abnormal temperatures in the forecast, that can spark a heat watch or excessive heat warning or advisory. These warnings are meant to help individuals stay safe, inform governments and businesses, and can trigger some legal protections for outdoor workers.

NWS is currently prototyping an extreme heat risk scale that will more directly communicate how different heat extremes translate to health risk. They've already used it in tweets about the current heat wave:

Tardy said improving early communication about the dangers of heat is especially important as the climate crisis makes heat more extreme and frequent — already, heat kills more people than any other weather event in the U.S.

“Heat is a very silent killer,” Tardy said. “It's not something that we see like a hurricane or a flood or a winter storm. It's something that just kind of catches up to you over time. It's one of those things that a lot of people don't take seriously until it happens to them.”

The idea is to rank heat waves similar to how we rank hurricanes. California lawmakers also passed a bill in 2022 requiring a statewide heat wave ranking system by 2025.

The prototype scale for ranking heat waves amid a changing climate.

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Courtesy of NOAA

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“People can really relate to the magnitude or the potential impact on a five versus a one [for a hurricane],” Tardy said. “So we're trying to do that also with heat.”

NOAA’s current prototype, which you can peruse here, uses a color scale to indicate how dangerous forecasted temperatures may be to health. NWS is currently using it to inform its official heat alerts, but it will likely be a few years before it’s used in widespread public communication.

“The overall magnitude of the event — not just, 'It's 110 degrees' — that's something we're really trying to focus on more,” Tardy said.

The scale forecasts seven days in advance and allows a more nuanced view of heat as it relates to health by incorporating data about:

• How significantly above normal the temperatures are at your location.
• The time of the year. 
• How long the unusual heat will last (will overnight temperatures get low enough to lower heat stress? How much warmer than average will those temperatures be?)

Weather forecasting has gotten a lot better, and it is another tool to save lives amid the climate crisis, Tardy said. He hopes the heat risk scale will better communicate the health dangers of heat and help people better protect themselves and their loved ones amid our hotter normal.

“We have to do a better job ingraining it into the culture — that it's not just hot all the time, it's not just hot because it's summer,” Tardy said. “When we start talking about heat alerts or heat warnings, it's something that should be treated as, ‘Hey, this is very unusual.’ And we can't just treat the day or the activity as normal.”

Staying safe in the heat