The doom-wanking fetish of certain Canadians about the United States is such a weird one.
So, let's pick this one apart.
I've been to Arizona during July. Yeah it's hot, but things pretty much operate normally, just like they do in Winnipeg during an extreme cold snap. Maybe not 100%, but 90%+ easily.
So, the hypothetical future situation where Arizona is a few degrees warmer on average likely means the heat will be more intense for longer during the hottest months. That still leaves 8-9 months of reasonably
pleasant weather, even in the warmer parts of the state.
The hottest day ever recorded in Phoenix was 50C on June 26, 1990. The masses didn't die on that day, people didn't flee the city. Life went on, people still flocked there after that day. A couple of degrees up from that record effectively changes nothing about the place. Sure, some fools probably got heatstroke like fools get frostbite in Canada during cold snaps, but that's expected.
It would be akin to an alternate world where Winnipeg gets a little colder in the winter. The city would continue to function well, because the whole infrastructure of the city is designed for those extremes. Whether the coldest temperature is -35C or -40C is mostly irrelevant past a certain point, because human engineering mitigates the downsides effectively. Things slow down and measures are taken. Life goes on and much of the year is not that extreme.
Second, the population of the whole state of Arizona is ~7.5 million, many who live outside the hottest part of the state. Even if the worst happens, there are many states around with less extreme climates in that country that will easily absorb so-called 'refugees' in a worst-case scenario, because in terms of the United States' population of 330 million+ people, 7.5 million is small.
Third, Canada's population growth is mostly driven by domestic politics and demographics, not the bogeyman of potential illegal immigration. Want to slow our contribution to climate change, if one is so scared of its implications? Stop flying people from the opposite side of the globe to cold, GHG-intensive Canada by the million per year.
Some just can't get their hands out of their pants, though.