If under a worst case scenario parts of the US become uninhabitable during the summer due to global warming, geopolitics could shift sufficiently that Canada might want want to merge. That could happen, for example, if half or more of its resident population was born in the US. http://www.independent.co.uk/environment/earth-may-be-too-hot-for-humans-by-2300-study-1970969.html
Why would half or more of our population be born in the USA?
Migration from the US to northern climes, under a worst case scenario, due to global warming.
http://www.newscientist.com/blogs/shortsharpscience/2010/05/earth-2300-too-hot-for-humans.html The problem is that we cannot survive if our skin temperature exceeds 35 °C for more than a few hours. Although many people live and even work in temperatures of 45 °C or more, sweating keeps their skin cool as long as it’s not too humid.
Put in technical terms, human survival depends on a wet-bulb temperature of less than 35 °C. This is the temperature recorded by a thermometer covered in a wet cloth and kept well ventilated.
“The wet-bulb limit is basically the point at which one would overheat even if they were naked in the shade, soaking wet and standing in front of a large fan,” Sherwood told USA Today.
At the moment, virtually nowhere on Earth has a wet-bulb temperature of more than 30 °C. But with a global rise of 11 °C, huge areas would have wet-bulb temperatures of more than 35 °C for part of the year. According to the climate model used by the team, these regions would include much of the eastern US, the entire Indian subcontinent, most of Australia and part of China. Admittedly, the US has Alaska and the Rockies as a safety net as well. Frankly, I find your framework more likely: I need to appeal to game-changers to break it. That said, worst case climate scenarios could certainly change geo-political interests.
That Americans might want to mirgrate to Canada when things heat up too much is a given, but whether Canada would let them in or not is quite another, and for those who did end up here, there is nothing to indicate that they would want want to join the nation they just left, given that the problems facing that nation caused them to emigrate in the first place.
By analogy, consider the present relationship between Mexico and the USA. The USA has not opened its doors to Mexican immigration, and despite the Hispanic demographic tremendously increasing and making real changes to the American political map, there is no hint of that demographic wanting to merge the USA and Mexico.
I expect that if the world heated up so much as to make much of the USA uninhabitable, all bets would be off as to whether our civilization, let alone our nations, would survive.
Do the Anglophone provinces ever talk secession?
Well, “union” doesn’t mean many gaps need to be crossed. Daily life on both sides of the former border would continue as before, just with more traffic both ways. The existing state and provincial and local governments would continue as they are now. The only difference is where you’re now sending your federal representatives and taxes.
But as to the “social gap,” look, our societies are practically Siamese twins already. Don’t most Canadians live within 100 miles of the border? Haven’t our cultures been cross-pollinating for decades? Isn’t our English similar? And wasn’t Anglophone Canadian culture mostly founded by Americans, Loyalist refugees from the Revolution followed by thousands of Yankee immigrants over the following decades? We have ties, Canuck. We’re your Mother Country, GB is only your Grandmother.
No, not the way Quebec does.
For example, Quebec has never signed the patration of our Constitution and its Charter of Rights and Freedoms. Attempts have been made to come up with something that is agreeable to Quebec and the rest of the Canadian provinces, but so far such attempts have failed, and have stirred the secessionist pot in Quebec.
In these negotiations, all provinces try to cut the best deal they can for themselves, but aside from Quebec, they do not at all threaten to leave the country.
We came close to an agreement once, but it was scuppered by an aboriginal member of the Manitoba Provincial Parliament on the grounds of First Nations not being sufficiently involved in the process.
Well, what do they want?!
Done. What’s it like in Ottawa?
I agree with Muffin and I’ll add that splits are far more common than merger between nations. Closer cross-border ties are possible, but Muffin already covered that. I’ll continue with Devil’s advocacy anyway.
Set universal health care in place for a couple of decades and maybe the social gaps won’t be as large. Also, assume Quebec splits off first. Even then, we are left with the “Why bother?” problem.
Here’s another path: the Pacific Northwest decides they are sick of the US and they wish to join Canada. If there were laws in place that assured the free movement of goods and labor between the two countries, I’d say that they are free to go. From there, we can proceed to the cataclysmic hot wet death scenario referenced above. Add some meteorites and volcanoes too. Just because.
Mother country? No, you’re our southern colony. 
No, daily life would not continue on as before, for the division of legislative powers in our nations are profoundly different, but even if this were to be dealt with, we would still be faced with daily life in Canada being profoundly different that the USA with respect to very practical things, such as social health care, public safety (the gun thing), the military, and the other items I set out in post #26. Out of these would arise a voting block that would move the USA very far to the left of where it presently is.
And dinosaurs? Can we have dinosaurs?
So, there is no actual down side. 
Self-determination.
They want to make their own laws and thereby pick their own course, rather than have laws made by others and imposed upon them.
What, you mean they want more autonomy on their reservations, or whatever you call them there?
Actually, yes. Alberta has a lot of dinosaurs.
I was referring to Quebec, but the same applies to First Nations, only with a lot more complexities. Bear in mind that for the most part, First Nations were considered to be sovereign from the outset, and treaties were made with them as sovereign nations, whereas Quebec was obtained by conquest.
I think we should have a test project. Let’s trade Idaho (provided you clean out the nutters first) for P.E.I.
I thought it was O Canada? Or is that his brother?
Let’s look for a moment at “on their reservations”. The claims made by first nations extend far beyond this in a couple of ways. The first deals with geography, and the second deals with jurisdiction.
With respect to geography, the basic deal for most treaties was that the people of a first nation would live on a reserve in exchange for benefits provided by the federal government, but these people would still have economic use of their much larger traditional territory and they would retain sovereignty albeit subject to the laws of general applicaiton of both the federal and applicable provincial government, and subject to the federal laws specifically pertaining to them (e.g. the Indian Act). Back in the day, the assumption was that this meant that first nation people would live on the reserve and hunt and fish on their traditional territory. Today first nations are asserting that they have a right to the economic use of their traditional territories that incudes mining and forestry and any other possible use, not just hunting and fishing. Natural resourses such as mining and forestry are provincial jurisdiction, not federal, and first nations are federal jurisdition, not provincial, so there is a tremendous muddle over who has what rights to what resources in what geographical areas based on what athority. Mining and forestry are huge up this way, so it is a very important issue.
With respect to jurisdiction, jurisdictional issues are now extending beyond reserves. For example, there is no income tax for funds earned on reserve by a status Indian, and a non-status person or corporation can not execute against a status Indian’s on-reserve assets. When a reserve moves operations and finances off reserve, it raises the issue of whether or not the protection from taxation and execution continues. Another example is the recent dumping of over 100 tonnes of iron into the Pacific by a first nation, ostensibly under its own sovereign authority, contrary to federal regulation and internation agreements made by the feds.
Here’s an analogy. When you buy a house, you want to purchase clear title, rather than be subject to some god knows what ownership interest by previous owners, or if you can’t get clear title due to some easement or other, you at least want to know exactly what it is you are getting, and what it is that you are not getting. Well with Canada, we have a long way to go in determining who has right to what, so forget about getting clear title.