Every once in a while a photograph of a migrant's tragic death (usually that of a child) catches the public's imagination.
The image of 3-year-old Alan Kurdi, fleeing from the Syrian civil war, dead facedown in the surf on a Turkish beach in 2015, triggered a wave of sympathy that ended with Germany opening its borders to 900,000 refugees that year – and Hungary building a border fence to keep them out.
Here we go again. A picture of 23-month-old Valeria Martinez, tucked into her father Oscar's T-shirt, both dead, facedown on the banks of the Rio Grande, has unleashed a similar wave of sympathy in the United States, although it certainly hasn't reached the White House. And once again most of the migrants are claiming to be refugees.
Borders will start slamming shut in countries, mostly in the temperate zone of the planet, where the climate is still tolerable and there is still enough food to eat. And don't believe the myth that you cannot really shut a border.
In fact, few of the migrants fit the legal definition of refugees in either case. The Arabs and Afghans trying to get into Europe had fled genuine wars, but they were already in Turkey, which is quite safe. They just wanted to move on to somewhere with better job opportunities and a higher standard of living. That's understandable, but it doesn't give you right of asylum as a refugee.
The same applies to the migrants crossing the Mediterranean from Africa to Europe, even though thousands are drowning in the attempt. They are fleeing poverty, or dictatorial regimes, or even climate change, but they are not fleeing war.
Neither do they have a "well-founded fear" of persecution on account of race, religion, nationality, political opinion, or membership in a particular social group. That is the language of the United Nations Convention Relating to the Status of Refugees of 1951, so they don't qualify as refugees. You may feel sorry for them, but there is no legal duty to let them in.
The Refugee Convention was incorporated into US law in the Refugee Act of 1980, so few of the people now seeking entry at the Mexican border qualify either. This matters, because although 20 years ago 98 per cent of the people crossing the border were young Mexican men seeking work, more than half are now entire families from El Salvador, Guatemala and Honduras – and most of them claim to be refugees.
They are not, and that (not Donald Trump) is why US courts are rejecting at least three-quarters of the applications for refugee status. You may wish that the law took a more generous and humanitarian view, but it does not. And if you think things are bad now, they will be 10 times worse in 20 years' time.
Global heating is starting to bite. We're still on the learner slopes, but the droughts and the floods, and the crop failures they cause, are multiplying, especially in the tropics and the sub-tropics where temperatures are already high.
In the worst-hit areas, which include the "northern triangle" of Central America, family farms are failing, some people are going hungry, and the number of people on the move is starting to soar. This is precisely what unpublished, in-house government studies were predicting 20 years ago in countries such as the US and the United Kingdom. Now it's here.
As the number of migrants goes up, the willingness of host populations to receive them will inevitably go down.
Five per cent new population in a decade will feel disruptive to some people, especially if there are big cultural differences between the old population and the immigrants, but most people will accept and adapt to it. Ten per cent in a decade is definitely pushing it, even though it's only 1 per cent a year. And 20 per cent new population in a decade would generate a huge political backlash in almost any country on Earth.
That's human nature. You may deplore it, but it's not going to change. And behind uncomfortable considerations of what the politics will permit lies the even starker reality that they can't all come. Twenty years from now far more people will be desperately wanting to move than the destination countries could possibly accommodate.
So the borders will start slamming shut in the countries, mostly in the temperate zone of the planet, where the climate is still tolerable and there is still enough food to eat. And don't believe the myth that you cannot really shut a border.
You can do so quite easily if you are willing to kill the people who try to cross it illegally, and the governments of the destination countries will probably end up doing just that. Their military and their civil servants, if not their politicians, were already having grim internal debates about it 15 years ago.
Sorry to spoil your day.
* Gwynne Dyer is the author of Growing Pains: The Future of Democracy (and Work)