LONDON (Reuters) -- If the world follows the demographic habits of Europe -- and that's a big if -- by the year 2200 it could be home to a population of less than half its current level, living in housing built for almost three times that number.
With the global population estimated to pass 7 billion on October 31, many of policymakers' short-term worries revolve around providing resources for the additional 2-3 billion people expected to be born in the next half-century.
Numbers of this magnitude inevitably conjure up terrifying visions of shortage and chaos. But in fact improvements in food production and technology have allowed population growth to continue unimpeded and relatively smoothly, and the real potential nightmare is of a rapidly aging population, combined with collapsing birthrates in both rich and poor states.
Many demographers and long-term planners say the challenge for the next century will be less dealing with growing numbers of people and more managing the much larger population of aged and perhaps dependent people while finding new strategies to deliver prosperity, jobs and essential services.
The trend has already contributed to the current global financial crisis by driving up health and social care bills and perhaps also undermining productivity. But while politicians tie themselves in knots over short-term worries, experts say there is not enough discussion of longer-term demographic challenges.
"It's not a world that's going to look anything like any world or population that has existed before," says Jack Goldstone, professor of public policy and a leading demographics expert at Washington's George Mason University.
"We thought that overpopulation was going to force humanity to expand outward to the stars. That doesn't look like the problem at all. And the policy framework isn't set up at all to handle these longer-term issues."
With many of the world's poorer countries still seeing strong growth, the global fertility rate -- the number of children born per couple -- remains around 2.5, more than enough to replace every person currently alive.
But in richer countries, the rate has already nosedived. Russia, Singapore and several other developed countries have introduced policies to boost fertility but with mixed success.
Exact predictions vary, but most projections suggest the global population will peak at around 9 billion around 2070 and then start to fall, perhaps very fast.
In the Western world, that date will see both the children of the "baby boomers" -- many of them childless or with fewer children, if current trends hold -- reaching the end of their lives. In the developing world, the "youth bulge" -- the large cohort of young people currently most striking in the Middle East -- will also be dying off.
"The decline in fertility has gone the furthest certainly in the developed world but it is falling very rapidly in most middle-income countries and even some of the more successful lower-income countries," said Daniel Cotlear, a population expert at the World Bank specializing in Latin America.
"With an aging population, that brings challenges."
By 2030, more than a third of the population in a number of Western states as well as some Asian economies, such as Japan and Korea, will be aged over 65.
Many developing states, most notably China with its one-child policy but also a growing number of other nations, will follow suit -- often without the financial resources to help pay for the cost of medical and nursing care.
"It's the seminal issue of our time," says Michael Hodin, executive director of the New York-based Global Coalition on Aging and a senior fellow at the Council on Foreign Relations.
"The numbers are stunning. The exact projections vary but it doesn't really matter because they are all going in the same direction."
In the developed world, most countries rely on a large number of working taxpayers to pay for the care and pensions of a relatively small group of elderly people. In poorer countries, extended families tend to look after the elderly.
Neither of those models, experts say, is designed to cope with the changed demographics of more old and fewer young.
In the short-term, many rich countries have plugged the demographic gap by importing young people from elsewhere in the world, particularly to provide care for the elderly and perform other manual tasks. That, too, may become unsustainable in the years to come as those sources of labor dry up due to falling fertility rates.
Some remain optimistic.
"This will be a much older world but it will also be a much more educated world," said Sergei Scherbov, research group leader at the Vienna Institute of Demography.
"People are becoming healthier. I personally think we will adjust to these things."
Scherbov and colleagues at the World Population Programme of the International Institute for Applied Systems Analysis have put considerable effort into the near-impossible task of predicting the world's population over the coming two centuries.
At one extreme, if the world's fertility rate were to fall to the same level as that of Shanghai, at around 0.8 per couple, then by the early 22nd century population would be falling so fast that it would be under a billion by 2150.
If it were closer to the European Union average of 1.5 then population would fall below 5 billion around 2140 and 3 billion by 2200. In contrast, maintaining the current rate of 2.5 would see it top 15 billion by 2100.
Those projections (link.reuters.com/het54s) assume global life expectancy continues to rise. Should it not, the population slump would be faster still.
"Unpredictability is huge," he says. "(But) it could be a very odd world."
The biggest question that no one has any clear answer to at present, experts say, is whether it is possible to plan for economic growth that will provide jobs and hope for both older workers and those younger people entering the workforce.
In a worst-case scenario of generational conflict, an elderly and middle-aged cohort might block jobs and lobby ferociously to keep up unsustainable entitlements while an angry youth feel denied opportunities and are forced to pay the ultimate financial bill. Some believe that phenomenon is perhaps already becoming visible in parts of the developed world.
"The real problem about the aging population is an economic growth problem," says George Mason University's Goldstone.
"If we have growth, we can afford the pensions and healthcare for the older generation. But if we don't, everyone is going to suffer."
Editing by Sonya Hepinstall