Another extreme heat wave in the West threatens all-time highs

It's become a common concern in recent years — extreme heat waves plaguing the western United States. Now, for the third time since the beginning of June, a life-threatening heat wave is underway. While this event is not expected to be quite as unprecedented as the Pacific Northwest heat wave last week, all-time records will be challenged in major cities like Las Vegas, Fresno and Redding, California.

This latest heat wave comes on the heels of the hottest June on record for the United States. A new study published Wednesday shows the Pacific Northwest heat wave from June 27 to June 29 was made at least 150 times more likely by human-caused climate change and, even in today's heated climate, is a rare 1-in-1,000-year event. The study, by a collaboration of 27 climate scientists, warned that if we keep warming the climate, extreme heat waves like that will happen once every 5 to 10 years by mid-century.

Although the Pacific Northwest will be hot in the coming days, the core of this latest heat dome is centered further south, over the Desert Southwest and Southern California. 

Heat domes are persistent mountains of warm air stacked vertically in the atmosphere, caused by slow-moving or even stalled patterns in the jet stream, called blocks.

That means the hottest temperatures will be found in areas like Las Vegas and Redding. In these two cities the highs are forecast to reach around 117 to 119 degrees Fahrenheit. If the mercury breaches these readings, all-time highs will once again fall.

On both Saturday and Sunday the high temperature is forecast to reach 130 degrees in Death Valley. If this threshold is reached, it will tie what is considered the hottest temperature ever reliably measured on Earth, which was set last year in Death Valley. (The world record of 134 degrees, set in Death Valley in 1913, is hotly contested by climatologists, but still considered official.)

The heat wave will peak on Sunday and then begin to ease early next week. But signs that this overall global blocking pattern will relax anytime soon are few and far between. So it is likely that such heat domes will continue to reappear from place to place throughout the summer.

In each case, human-caused climate change makes these heat waves more intense. Scientists have determined a rule of thumb: With the average long-term global temperature increasing by 2 degrees Fahrenheit since 1900, heat wave intensity should increase by about double that, or 4 degrees Fahrenheit. That's according to World Weather Attribution scientist Professor Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute. 

But he says the Pacific Northwest heat wave defied that logic, with temperatures far exceeding expectations. Until recently, he explained, "It looked as if regions' heat waves were well-behaved, with temperatures gradually rising with global warming, about two times faster than the global mean temperature. And then we got this." 

Oldenborgh mentions one possible explanation for these out-of-bounds heat waves is the possibility that we have crossed a threshold in our climate system in which nonlinear effects are starting to play a role. If so, the probability of these kind events has become much larger. This, he says, needs to be investigated further.

One thing is certain: Heat of the intensity being experienced in the western U.S. right now is a killer. In the recent Pacific Northwest heat wave alone, several hundred people died, and the numbers continue to rise. On average, heat waves kill more people in the U.S. than any other type of extreme weather.

Yearly, about 500,000 deaths are attributed to extreme heat, and one recent study found that more than a third of global heat-related deaths are due to the impact of climate change.

There is unanimous agreement in the climate science community that heat waves will grow more frequent and more intense as we continue to heat the planet. We can't call what we are seeing today "the new normal" because it will continue to get worse. 

But scientists also stress that we know how to solve this challenge and we have the tools to do it: Stop burning fossil fuels and temperatures will stop rising. All that is lacking, they say, is the political willpower to make it happen at the pace needed to avert catastrophic impacts.