California will start ranking heatwaves by how extreme they are

Extreme heat is the deadliest weather-related disaster in the US

Everyone knows that some hurricanes are more powerful than others — a Category 5 storm poses a lot more danger than a Category 1 storm, hitting land with far more powerful winds.

But no such rating system exists for heatwaves, another climate hazard.

At least not yet.

A bill signed by California Governor Gavin Newsom this month will create a new ranking system for heatwaves — helping to inform the public on just how dangerous extreme temperatures could get, especially in coming decades when devastating heatwaves are likely to become more common.

The timing is fortuitous, as California just experienced its worst heatwave of the year — breaking daily, monthly and even some all-time records, with daily highs well above 100 degrees Fahrenheit (38 degrees Celsius) in most of the state.

“California has been battling record breaking extreme heat all week,” California Assemblywoman Luz Rivas, who sponsored the bill in the state legislature, said in a statement from the governor’s office.

“Unfortunately, each summer we are experiencing extreme heat weather events that are hotter and more devastating than the last.”

The legislation requires the state’s Environmental Protection Agency to develop a ranking system for extreme heat by 2025.

The ranking will be based on forecasts of how long and how intense the heat will get, potential dangers to human health and local factors like the “urban heat island” effect, where cities get much hotter than the surrounding area because of all the extra heat absorbed by surfaces like asphalt and concrete.

Once complete, the ranking system will have recommendations or triggers for public policies that try and mitigate the danger of extreme heat and ways of measuring the damage inflicted by a severe heatwave.

California has previously considered naming heatwaves, much like meteorologists do with hurricanes, as a way to call attention to the danger of specific extreme heat events.

The National Weather Service has three tiers of warnings for heat danger, ranging from “heat advisory” to “excessive heat warning”. While these alerts have some general criteria behind them, they’re a little more flexible and relative to standard temperatures in a region.

The bill was just one of many climate-related laws passed by the state legislature and signed by Governor Newsom this year — including $54bn in climate spending, restrictions on new oil and gas wells in residential neighbourhoods and extending the life of Diablo Canyon, the state’s only remaining nuclear power plant.

The early September heatwave in California brought brutal temperatures to most of the state for days on end. Areas in the deserts around Los Angeles regularly saw daily highs rise over 110F (43C), as did cities up and down the Central Valley.

Sacramento, the state capital, broke its all-time heat record last Tuesday, with a high of 116F (47C).

This kind of heat can be extremely dangerous to human health, especially for people with underlying health conditions and the elderly. In addition, people without access to air conditioning, like homeless people, face incredible danger from extended heat events.

Heat illness can range from sunburns and heat exhaustion to potentially fatal heat strokes, where the body is unable to cool itself down. Extreme heat kills more people than any other disaster in the US, according to federal data.

Dangerous heatwaves are likely to get a lot more common in the coming decades as the climate crisis grows.

A UN climate science panel has warned that if the plant reaches 2C of warming above 19th-century temperatures, heatwaves that once occurred every 10 years would happen every other year and get 2.6C hotter.

Already, the planet has warmed 1.1-1.2C above 19th-century temperatures and is forecast to reach 2.7C of warming by the end of the century, according to the Climate Action Tracker, an independent analysis of global climate policy.