It has taken a long time for the world to see the "sudden" migration of thousands of refugees from Africa and the Middle East as a full-blown humanitarian drama. Although it has been happening for years, until a few weeks ago, it was depicted as an unwelcome assault by intruders. It may be, however, that presenting the present emergency as a "crisis" is misleading in a different way, since it encourages us to view the uproar as a unique episode with specific causes - alarming, but a one-off.
Some have sought to place it in the context of previous commotions - the displaced citizens after World War II, the expulsion of Ugandan and Kenyan Asians, the Vietnamese Boat People, the Kosovans - and concluded that it is simply the same old story but with different people. It has happened before, and the world did not stop turning.
It is clear that the world is heading for a crunch over water.
Even this argument is now beginning to look stretched. Anyone resolved to be optimistic about the pace of migration in modern life, seeing it as an inevitable and enriching part of contemporary life, is growing edgy - because this convulsion may be the start of something truly daunting.
After all, the nations that are generating the majority of today's refugees - Syria, Afghanistan and Eritrea - are relatively small, with a total population of some 60 million. If the sort of troubles afflicting these places were ever to erupt in larger regions, like Egypt or Pakistan, then the present turmoil would seem minuscule. The population of those nations is around 264 million - more than Britain, France and Germany combined.
We may, in other words, need a bigger metaphor. There is one close to hand, since what this flow of people most resembles is demographic climate change - or a human global warming. Humans have been nomadic ever since time began, moving in an endless quest for better weather, friendlier neighbours, nicer food, more fun, or just a quieter life. The modern world, with its quicksilver communications and transport links, has merely made the process a thousand times easier.
In this respect, the migratory patterns of human existence really does resemble the engine room of our climate, in which air is driven between areas of high and low pressure on the push-pull factors of high winds and thermal currents. The chasms of inequality between the haves and the have-nots, bridged as they are today by instant wi-fi imagery and slick advertising, are creating irresistible pressure differences of just this sort.
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In contemplating the future, therefore, we might think of migration not as a sudden local eruption, but as the deeper tidal swell of global life. There may exist a literal as well as a figurative truth here. Even if the global warming apostles are exaggerating, and the desertification of North Africa can be prevented by hi-tech desalination systems, it is clear that the world is heading for a crunch over water. The consequence of a major alteration in that natural resource is truly hard to contemplate.
Recent years have seen a number of books on the topic, with titles such as Water Wars: A Looming Threat, Running out of Water, or High and Dry. International VIPs have banged the drum, too. In 2002, Kofi Annan argued that "fierce national competition over water resources" looked set to drive future conflict. Jean Chretien, ex-Prime Minister of Canada, stated: "The future political impact of water scarcity may be devastating." And the pope issued an encyclical on the subject - a bold departure for the Vatican.
American Actor Matt Damon caused a stir at the 2014 World Economic Forum in Davos by speaking out on "the magnitude of the water crisis", and the poet W H Auden wrote a bumper sticker for the argument using his famous line: "Thousands have lived without love, not one without water."
We have grown used to fretting about oil as a limited resource, yet, while we know that water is finite - the National Geographic noted back in 1993 that "all the water that ever will be is right now," - we act as if there is no tomorrow. Consumption has tripled since 1950, and until we arrive at a smarter way of making more of it (which we might, for we are an ingenious species), all we can do is block rivers - a sure cause of conflict. In 1950, there were 500 major dams in the world; now there are 45,000.
It is obvious what this means: population movement on a bracing new scale. If we glance at any map of the future, it is always the same water-shortage trouble spots that burn out in red: North Africa and the Middle East. But that is not the whole story. The water table in India is falling by one-and-a-half inches a year; China faces an even drier future.
That is why there is a new word in the lexicon of international relations: hydro-diplomacy. We had all better learn it while it is still raining.
Robert Winder is the author of Bloody Foreigners: the Story of Immigration to Britain.
The views expressed in this article are the author's own and do not necessarily reflect Al Jazeera's editorial policy.