A major study published recently found that members of the public can forecast economic and political events as accurately as experts, and some of them do better than the pundits and economists. It is a lesson of the modern world that having more data does not mean more accurate forecasting.Both the Schengen border-free zone and the eurozone are central projects of EU unity that are now driving disunity. Yet leaders gathering in Malta now to discuss the migration crisis will make rational decisions only if they pay attention to one area of forecasting that can be accurate - the growth or decline of populations.Demographic forecasts tend to be among the most reliable, partly because we already have a lot of information that will not change.A glance at such projections to the middle of this century points to dramatic variations, changing in one generation the political and economic landscape of the world.We learn the UK is likely to overtake Germany as the most populous country in Europe; the US will boom in numbers, while European countries decline; that many emerging economies have populations ageing more rapidly than they might have expected, constraining their future growth.Above all, it is clear that, according to the UN, half of all the population growth globally in the next 35 years will be in Africa. That increase will be in poorer countries, alongside massive growth in numbers in the war-torn Middle East.The numbers are stark. The increase in Africa's population alone is set to be 1.3 billion by 2050, about two-and-a-half times the entire population of the EU today. Put another way, the number of people in Africa and western Asia is expected to increase by more than 110000 every day for decades.Such figures put into perspective a crisis caused by the arrival of several thousand migrants a day.What we have seen in recent months is a hint of what might happen, mere gusts of wind before the approach of a hurricane.The implications for Europe are immense and clear. First, any approach signalling an open door to migration, as in Germany in recent months, will prove unsustainable. There need to be strict limits on migrant numbers from now on.Second, the Schengen zone can survive only if there is a strengthening of its borders - otherwise one country after another will close its own borders, not for a few days at a time, but permanently.Third, Europeans will need to do much more to promote stability, save failed states and avert huge outflows of people within Africa itself, and the Middle East. That means not shying away from intervention abroad, but learning how to intervene more effectively.We will have to do more of what we have been doing in Somalia - funding the African troops who fight terrorism, putting our ships off the coast, establishing a legitimate UN-backed government, and marshalling diplomatic and budgetary support for it. In turn, this will require small, highly mobile armed forces.Hague is the former UK foreign secretary ..