Explainer-What's causing the recent U.S. heat waves?

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By Daniel Trotta

(Reuters) - Much of the United States will experience another heat wave this week, with above-normal temperatures forecast for the Pacific Northwest, Southern Plains and the Lower Mississippi Valley.

The extreme heat is fueling a fast-moving California wildfire, just went of Yosemite National Park. The Oak Fire has destroyed 10 structures and is 10% contained.

The U.S. heat wave followed record heat that killed hundreds if not thousands of people and sparked wildfires in Europe.

Following is an explanation of what is causing the heat waves, according to scientists.

WHAT IS A HEAT WAVE?

A heat wave has no single scientific definition. Depending on the climate of a region, it can be determined by a certain number of days above a specific temperature or percentile of the norm.

ARCTIC WARMING AND JET STREAM MIGRATION

The Arctic is warming three to four times faster than the globe as a whole, meaning there is ever less difference between northern temperatures and those closer to the equator.

That is resulting in swings in the North Atlantic jet stream, which in turn leads to extreme weather events like heat waves and floods, according to Jennifer Francis, senior scientist at the Woodwell Climate Research Center.

HEAT DOMES

Warmer oceans contribute to heat domes https://oceanservice.noaa.gov/facts/heat-dome.html, which trap heat over large geographical areas.

Scientists have found the main cause of heat domes is a strong change in ocean temperatures from west to east in the tropical Pacific Ocean during the preceding winter.

"As prevailing winds move the hot air east, the northern shifts of the jet stream trap the air and move it toward land, where it sinks, resulting in heat waves," the U.S. National Oceanic and Atmospheric Administration says on its website.

EL NINO AND LA NINA

Every few years, the climate patterns https://oceanservice.noaa.gov/facts/ninonina.html known as El Nino and, less frequently, La Nina occur. El Nino brings warm water from the equatorial Pacific Ocean up to the western coast of North America, and La Nina brings colder water.

At present, La Nina is in effect. Because summer temperatures trend lower during La Nina, climate scientists are concerned about what a serious heat wave would look like during the next El Nino, when even hotter summer weather could be expected.

HUMAN-INFLUENCED CLIMATE CHANGE

Climate change caused by the burning of fossil fuels is a global phenomenon that is certainly playing a role in what the United States is experiencing, scientists say.

"Climate change is making extreme and unprecedented heat events both more intense and more common, pretty much universally throughout the world," said Daniel Swain, climate scientist at UCLA.

"Heat waves are probably the most underestimated type of potential disaster because they routinely kill a lot of people. And we just don't hear about it because it doesn't kill them in, to put it bluntly, sufficiently dramatic ways. There aren't bodies on the street."

Francis, of the Woodwell Center, said with climate change the world is seeing changing wind patterns and weather systems "in ways that make these heat waves, like we're seeing right now, more intense, more persistent, and cover areas that just aren't used to having heat waves."

Alex Ruane, researcher at NASA's Goddard Institute for Space Studies, said as the world warms, "it takes less of a natural anomaly to push us into the extreme heat categories. Because we're closer to those thresholds, it's more likely that you'll get more than one heat wave at the same time. We're seeing this in the United States."

(Reporting by Daniel Trotta; Editing by Donna Bryson, Rosalba O'Brien and Lisa Shumaker)

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