Millions of people in California, Nevada, Arizona, New Mexico, and Texas have been under excessive heat warnings this week.

While the region is accustomed to sweltering heat, climate change worsened by human activity has led to more extreme weather and the current heat wave has been historically early.

Las Vegas recorded 44°C on Thursday, marking the earliest date in the year that temperature has ever been reached.

“The past few days have been HOT,” observed the city’s National Weather Service, publishing a lengthy list of locations where daily records have tumbled.

Among them, the notoriously scorching Death Valley desert reached 50°C.

An excessive heat warning is in place until Saturday in Las Vegas, where libraries have been converted into cooling stations for residents to escape the furnace, and some events have been forced to move indoors.

At a Trump rally in Arizona, nearly a dozen people were taken to hospitals with heat exhaustion, fire officials told a local ABC affiliate.

Hiking at the popular Camelback Mountain and Piestewa Peak trails in the state’s capital city Phoenix was prohibited due to the heat.

“With temperatures hitting near 110 degrees, it’s not the day for hiking,” the Phoenix Fire Department posted on Facebook.

Coastal regions have largely been spared.

But in a potentially ominous sign of the summer months to come, a number of small wildfires broke out across California.

The largest, around 241km northwest of Los Angeles, burned more than 1,450 hectares of the agricultural Central Valley before being largely contained by firefighters.

After about 20 years of drought and in a climate that is slowly becoming more arid, California has seen an alarming number of destructive fires in recent years.

Wildfires are a natural – and necessary – part of the region’s life cycle.

But climate change, caused by humanity’s burning of fossil fuels that pump greenhouse gases into the atmosphere, is making them bigger, hotter, and more unpredictable.