In a recent survey of potential adult migrants worldwide, 47 million said they would most like to move to Canada. There are only 37 million people in Canada. The same goes for Australia: 36 million would like to move there; only 25 million do live there. Most of these would-be immigrants are going to be disappointed. In fact, Canada lets in just 300,000 immigrants a year; Australia 200,000.
Other developed countries are significantly less popular destinations, but potential migrants amounting to around half the existing populations want to move to the United States, France, Britain, Germany and Spain. They too are going to be disappointed.
In its most generous year, 2016, Germany let in a million immigrants, mostly Syrian refugees, but 80 million Germans are never going to let in the 42 million foreigners who also want to live there. Indeed, former US presidential candidate Hillary Clinton said bluntly last November that "Europe has done its part, and must send a very clear message: 'we are not going to be able to continue to provide refuge and support'".
Ms Clinton was mainly concerned about how anxiety about mass immigration has fuelled the rise of populism in Western countries. That's hardly surprising, given how Donald Trump's tight focus on the alleged criminal and job-stealing propensities of Latino immigrants won over enough formerly Democratic voters in the Rust Belt states to give him the presidency.
It didn't just work for Mr Trump. It helped the Brexiteers win their anti-European Union referendum in the United Kingdom, it brought the populists to power in Italy, and it underpins Viktor Orban's "soft dictatorship" in Hungary (even though Hungary has never let immigrants in, and they probably don't want to go there anyway).
But the fact is that the levels of immigration are not particularly high in the United States and most European countries at the moment. Net migration to the United Kingdom has been stable since 2010; in both the United States and in Germany (with the exception of 2016, in the latter case), net migration is down by half since 2000. Something more is needed to explain the level of anger in these countries.
It is, of course, unemployment, which is much higher than the published (official) figures in every case, and is particularly high in the post-industrial areas that voted so heavily for Mr Trump in the United States, for Brexit in Britain, and for ultra-nationalist parties in Germany. In the United States, according to Nicholas Eberstadt of the American Enterprise Institute, 17.5% of American men of prime working age (24-55) are not working.
So you can see why Hillary Clinton is concerned, but she seems unaware that the pressure of migration is also going to grow rapidly.
According to the UN's International Labour Organisation, there are currently 277 million migrants in the world (defined as people who have left their home countries in search of work, or to join their families, or to flee conflicts and persecution). How many more are still in their home countries but would like to leave? At least a billion, maybe two.
More than half of Kenyans would move to another country if they could, a 2017 survey by the US-based Pew Research Center discovered. More than a third of Nigerians, Ghanaians and Senegalese are actually planning to emigrate in the next five years, according to the same survey. (Good luck with that!)
Even a third of Chinese millionaires would like to emigrate (half if you include moving to Hong Kong as emigration). And all this is before climate change kicks the numbers into the stratosphere.
The chief impact of global warming on human beings is going to be on the food supply, which will fall as the temperature rises. And the food shortages will not affect everybody equally: The supply will hold up in the temperate zone (the rich countries), but it will plummet in the tropical and sub-tropical countries where 70% of the world's people live. They will be desperate, and they will start to move.
That's when the pressure of migration will really take off, and the rich countries are simply not going to let the climate refugees in. Not only would it stress their food supply too, but the numbers seeking to get in would be so large -- two or three times the resident population -- that it would utterly transform the country's character. So the borders will slam shut.
It's a myth that you cannot close borders. You can, if you're willing to kill people. (Think of the Iron Curtain, which successfully divided all of Europe for 40 years.) And the rich countries will, in the end, be willing to kill people.
Gwynne Dyer's new book is 'Growing Pains: The Future of Democracy (and Work)'.