More than 45 million people in US face dangerous heat

Relentless heatwave expected to expand into the Midwest and Northeast

People watch the sunset from a peak at Papago Park in Phoenix, Arizona. AP

A heatwave that has broiled the south-western US is forecast to reach the Midwest and Northeast with more than 45 million people under heat advisories.

More than 12 states were issued heat advisories by the National Weather Service, stretching from Southern California to the Midwest state of Montana and back down to Florida.

Phoenix, Arizona's capital, has faced a brutal onslaught of extreme heat. The southwestern city has experienced 25 consecutive days of temperatures exceeding 43°C, far surpassing a streak of 18 consecutive days that was set decades ago. Overnight temperatures are bringing little solace, too, with temperature lows not dropping below 32°C for more than two weeks.

The metropolitan Phoenix area has reported 18 heat-related deaths this year. Another 69 suspected heat deaths are being investigated, the Maricopa County Department of Public Health said.

Extreme heat is the number-one weather-related killer in the country, according to the National Oceanic and Atmospheric Administration.

Heat index values are also expected to exceed or be near 32°C in Kansas City, Missouri; Minneapolis, Minnesota; and along the I-95 motorway corridor.

“The heat in the Southwest, Southern/Central Plains and southern Florida is also expected to persist over the coming days with many places likely to rival high and low temperature records,” the NWS said in its Tuesday bulletin.

“Excessive Heat Warnings are in effect across the Desert Southwest and Heat Advisories are in effect over portions of the Great Plains and southern Florida.”

This month has been the hottest that the planet has experienced.

Partly responsible for the blistering temperatures is a natural phenomenon called El Nino, which creates periods of warm water as east-to-west trade winds weaken. As a result, areas experience hotter and dryer conditions then normal.

A study released on Tuesday pinned much of the responsibility on climate change, arguing that the record-breaking heatwaves oppressing North America and Europe would not have occurred without it.

“European and North American temperatures would have been virtually impossible without the effects of climate change,” said Izidine Pinto of the Royal Netherlands Meteorological Institute.

Heatwaves are expected to occur every two to five years if average global temperatures increase by 2°C above pre-industrial levels, the study's researched reported.