What is causing the recent US heat waves?

Much of the western United States will be gripped by a heat wave this week, with temperatures in some parts of California soaring above 110 Fahrenheit (43 Celsius) on Wednesday.

The extreme heat is also raising the risk of wildfires. The Wishon Fire, a 350-acre blaze in the Sequoia National Forest in California, was 35 per cent contained.

Here are some factors causing the heat waves, according to scientists.

Also Read | Adapt to intense heat waves and flooding

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 the University of California at Los Angeles.

"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," said Swain.

Jennifer Francis, senior scientist at the Woodwell Climate Research Center, said with climate change the world is seeing shifting wind patterns and weather systems in ways that makes heat waves more intense, persistent and widespread.

Alex Ruane, researcher at NASA's Goddard Institute for Space Studies, said as the world warms, it takes less of a natural anomaly to cause 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."

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 Francis at the Woodwell Climate Research Center.

Heat domes

Warmer oceans contribute to heat domes which trap heat over large geographical areas.

Scientists say the main cause of heat domes is a strong change in ocean temperatures from west to east, which occurs in the tropical Pacific Ocean 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 US National Oceanic and Atmospheric Administration says on its website.

El nino and la nina

Every few years, the climate patterns 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.