Inside one of Vancouver's 25 air-conditioned cooling centers on Wednesday, visitors quietly read books or worked on laptops as the death toll in Canada's British Columbia province rose into the hundreds from a record-smashing heatwave.

"We've had heatwaves before, but not to this extent," said Lou, who provided only her first name. "I'm shocked by how many deaths there have been."

"I have no air conditioning, only a fan at home -- I came here just to work where it's cool."

Canada's westernmost province has been scorched for days by record-smashing heat that reached 49.5 degrees Celsius (121 degrees Fahrenheit) in Lytton, three hours northeast of Vancouver, on Tuesday, surpassing its own previous all-time Canadian record a day earlier.

The heat has killed over 500 people in Canada and the neighbouring US, with the toll rising "by the hour," while meteorologists warned of more extreme temperatures to come.

Among those being mourned was the mother-in-law of infectious diseases expert Tara Moriarty, who said the otherwise healthy senior was afraid to seek respite from the heat because she was only half-vaccinated for Covid-19.

"It's quite devastating," Moriarty said on Twitter. "My partner's healthy mom died of heatstroke in British Columbia (Sunday) night. 

"Heatstroke can kill very fast. If you have family, neighbours, friends afraid to seek cooler places (because of) Covid, check on them every couple of (hours) when it's really hot."

A couple cools off in New York.

The British Columbia Coroners Service reported 486 "sudden deaths" between Friday and Wednesday, compared with 165 normally, while Vancouver police said calls for help overwhelmed emergency phone lines.

"While it is too early to say with certainty how many of these deaths are heat related, it is believed likely that the significant increase in deaths reported is attributable to the extreme weather British Columbia has experienced and continues to impact many parts of our province," the coroner's service said in a statement.

- 'Alarming' numbers -

"We've never experienced anything like this heat in Vancouver," police Sergeant Steve Addison said, "and sadly dozens of people are dying as a result of it."

Vancouver cancelled schools for extreme heat for the first time, while firefighters turned their hoses on anyone in need of cooling.

"It's been superhot in our house, we have really had to rely on friends to give us fans," said Ashley Vaughan, walking with her three children as temperatures began to cool slightly Wednesday. "My kids have been miserable; there was a lot of crying because it was so hot."

Meteorologists said the extreme weather is the result of an extreme heat dome above the Pacific Northwest, a normal summer phenomenon -- but never this hot or early.

"This particular event is completely consistent with the science of climate change: more intensive heat waves, of a longer duration, more extreme heat, earlier in the season," Terri Lang, meteorologist with Environment Canada, told AFP. 

"People in the meteorological community -- weather forecasters and climatologists -- are all holding our breaths looking at the numbers. They're alarming."

Peter Lohuaro, 70, was forced to stop cycling because of dangerously high ground-level ozone, prompting public warnings to stay indoors; but he said the heatwave also helped his joints.

"It's unprecedented -- I've travelled to hot places like Death Valley (in California) and this was hotter," Lohuaro told AFP at a city cooling center. 

"For people who live in apartments without air conditioning or facing south, a lot had to go rent hotel rooms or really suffer."

Heatwave 'on steroids' due to climate change, say experts

The western United States and Canada would likely have experienced a heatwave in the past week even without climate change. But the scale and severity of the record-breaking temperatures were undoubtedly multiplied by the changes to our atmosphere, experts say.

The US states of Washington and Oregon, as well as British Columbia in Canada, boiled this week in all-time high temperatures that have caused dozens of deaths.

"This is one of the most extreme heatwaves that we have seen on Earth, in many years, anywhere, in terms of the deviation from the typical conditions in this particular part of the world," said Daniel Swain, a climate expert at UCLA, noting that temperature records are rarely broken by "more than a degree." 

"In this case, those records were obliterated," he said. "It's really the magnitude and the persistence of this one that is just genuinely shocking."

Canada set an all-time record on Tuesday. In Oregon, temperatures were higher than the maximum recorded in Las Vegas, in the middle of the Nevada desert. And all this in a region with a normally temperate climate at this time of the year. 

The phenomenon causing the scorching heat is called a "heat dome." Hot air is trapped by high pressure fronts, and as it is pushed back to the ground, it heats up even more. 

"It's sort of like a bicycle pump," said Philip Mote, professor of atmospheric science at Oregon State University. "If you compress air into a bike tire, it warms the air."

The condition also prevents clouds from forming, allowing for more radiation from the sun to hit the ground. 

Such conditions are not unheard of: "The pattern was similar to how we always get our heat waves," Karin Bumbaco, a climatologist at the University of Washington, told AFP. "We've seen that pattern before, but it was just much stronger than usual."

- Climate like 'steroids' -

So what is causing the precedent-shattering highs?

"A world without climate change would have still had a heat wave in the Pacific Northwest. It just would have been not quite as record-setting," said Zeke Hausfather, a climate expert at the Breakthrough Institute. 

"Climate is like steroids for the weather," he said "If a baseball player or Olympic athlete is taking steroids, they're still going to perform better some days and worse some days, but on average, their performance is going to go up. And so climate is doing something similar to the weather. That makes it more likely to experience these sort of extremes." 

So-called "attribution studies" will be conducted to determine the exact causes of the event. 

But "I think it's safe to say that there's at least some components of global climate change that contributed to this event," said Bumbaco. 

Temperatures are generally higher in this region, which has warmed by around three degrees Fahrenheit in the past 100 years, so it makes sense for records to be broken little by little. 

However, "it is very possible that climate change increased this heat wave to an even greater degree" than that, said Swain, the UCLA expert.

For example, the drought that has plagued the region for weeks may have bolstered the heat dome because the energy of the sun's rays is no longer being used to evaporate water, so instead it warms the atmosphere more. 

And climate change is already "increasing the severity of drought" in parts of western North American, said Swain. "The answer to the question of whether it would have happened to this extent without climate change is clearly no."

- Adaptation -

It is hard to predict just how often such heat waves will occur again. 

"This particular event was so extreme that it will remain unusual, even in a warming climate," said Swain. "But it has gone from the realm of being essentially impossible to being something that we may well see again."

"The bad news is that even if we could wave a magic wand and get all our emissions to zero tomorrow, the world isn't going to cool back down," said Hausfather.

"We're stuck with the warming... And so we need to be prepared for these sorts of events to be more frequent."

Experts insist on the need to adapt in the medium term: by equipping populations with air conditioners (even if they release harmful emissions in the long term), by rethinking the structure of buildings so that they reflect rather than retain heat, and by planting vegetation. 

But all are unanimous: "In the longer term, obviously, the best way to prevent these things from happening in the first place, or to reduce how much worse they could get, is to reduce global emissions of greenhouse gases," said Mote.