杏吧原创

FOR the past million years, the world鈥檚 human population has grown almost
continuously. But demographers have identified three great population surges,
each following technological revolutions that dramatically increased the number
of people that the world could sustain.

The first revolution was the invention of tool making, which occurred
gradually around the world between a million and 100 000 years ago. The second
revolution was the invention of farming. This began at the end of the last ice
age 10 000 years ago and spread from core areas such as the Middle East.
Agriculture helped the world鈥檚 population rise from less than 10 million to
about 150 million at the time of Christ and 350 million a thousand years
ago.

But in the 14th century, the number of people in Asia and Europe dropped as a
result of the Black Death鈥攁n epidemic of bubonic plague carried by fleas
on rats鈥攚hich spread from Asia to Europe. It reduced Europe鈥檚 population
by a third during the 14th century. But recovery followed and by the early 19th
century the third great technological revolution, the industrial revolution, was
under way in Europe. Its progress around the world continues today.

This third revolution has already raised the world鈥檚 population to around 6
billion, six times what it was at the start of the 19th century, three times
what it was in 1930鈥攁 single life span ago鈥攁nd almost twice what it
was in 1960. And the population may reach 10 billion before, as demographers
predict, it stabilises. The key features of this revolution have been advances
in food production that have allowed the world to feed more people and new
methods of controlling diseases that have allowed people to live longer.

Demographers have attempted to describe the progress of the current surge in
the world鈥檚 population with a model called the 鈥�demographic transition鈥�. This
transition classically begins with a rise in life expectancy and consequent fall
in death rates, as the health and wellbeing of a society is improved by better
health care, hygiene and nutrition. For a time, birth rates remain high while
death rates fall, causing a surge in population. But eventually social changes
bring about a decline in the birth rate and the population stabilises again.

A change of pace

Prosperity for stability

The transition described well the progress of the first countries to
industrialise. In Europe the declining death rate (from more than 30 deaths per
thousand people each year to below 10 today) took place gradually, beginning in
the 17th century and continuing at a slower pace to the present. Around the
start of the 20th century, population growth rate in Europe peaked at around 1.5
per cent per year before beginning to fall as people chose to have smaller
families and gradually adopted modern methods of birth control.

The move to smaller families reflected social changes. In poor rural
societies, children were vital as labour in fields and as carers and providers
for their parents in old age. But in richer urban societies, they became an
economic drain, costly to educate, clothe and feed. Now, as child death rates
fall, people are more confident that their children will survive to adulthood,
while state pensions and welfare provisions mean that children are not essential
to a happy old age. People can invest in material goods rather than
children.

The whole demographic transition has taken around three centuries in European
countries. But it is taking place much faster in many developing countries
today, especially in East Asia, where it is being telescoped into a few hectic
decades during which national population growth rates have often exceeded 3 per
cent per year, causing populations to double in less than 25 years and reducing
the mean age of national populations to as low as 15 to 18 years.

This is partly the result of the transfer of ready-developed Western
technologies that have accelerated medical improvements and economic change. But
it may also reflect the adoption of dominant Western attitudes to things such as
family planning, and the improved social status of women.

As the industrialised countries have reverted to more or less stable
populations, 85 per cent of the world鈥檚 population growth in the past 50 years
has been in the developing world. But population growth rates are now declining
fast in many developing countries, especially in the rapidly industrialising
nations of East Asia, where increasingly rich and urban populations seek to
reduce family sizes. Countries such as China and South Korea have halved their
population growth rates in 20 years.

Progress has been slower in southern Asia. In India, for instance, death
rates halved between 1945 and 1970, while birth rates remained almost unchanged,
causing the population to double. India鈥檚 fertility rates鈥攖he number of
births per woman鈥攈ave since begun to fall, to an average of 3.5 children
per woman today, according to the Washington-based Population Reference Bureau.
However, in neighbouring Pakistan the average is 5.6 children, the highest in
Asia.

The great divide

Rich and poor

Countries in Sub-Saharan Africa lag even farther behind, with very high
population growth rates. Here, improved health care and hygiene have raised life
expectancy, but in largely poor rural societies there is little incentive to
reduce family sizes. In many African countries average families still have six
or more children, while in Kenya in recent years there have been four times as
many births as deaths. The country has had the world鈥檚 fastest population growth
rate, exceeding 4 per cent per year during the 1980s.

At the United Nations population conference in Cairo in 1994, 28 countries
reported that their fertility rates had risen since the previous population
conference in 1984. Twenty of those countries were in Africa; others included
Pakistan and the US (where the fertility rate rose from 1.9 to 2.1 largely as a
result of the immigration of Hispanics with a higher birth rate).

Demographers say they no longer see the demographic transition as an
inevitable process. Some pessimists argue that countries stuck in the poverty
trap will be unable to provide the social conditions that encourage people to
reduce their family sizes without an unacceptable degree of compulsion. They
point out that during the 1970s and 1980s much of the progress in stabilising
the world鈥檚 population growth rates occurred in China, where the one child
policy鈥攚hich reduced the annual population growth rate to 1 per cent, the
lowest in the developing world鈥攈as at times been pursued at the expense of
basic human rights.

On the other hand, in the 1990s, some of the poorest, least industrialised
and least urbanised countries have made the biggest strides in reducing their
population growth rates without compulsion. For instance in Bangladesh, one of
the poorest nations on Earth, 40 per cent of fertile women use contraceptives,
which are provided free by the government. There is a worldwide decline in
fertility except in parts of two regions: the Middle East and tropical
Sub-Saharan Africa

Some nations appear to be in the process of overshooting the return to stable
populations. Having completed the demographic transition, most European
countries now have birth rates too low to maintain their existing populations.
The average fertility rate in Europe is now 1.4.

There is a long time lag in the translation of smaller family sizes into
lower rates of national population growth. People born today will only have
children in 20 to 40 years鈥� time. So, if birth rates in a country peak now and
then start to decline, the very large number of females born today will still be
fertile and having babies until around the year 2040. Any continuing increase in
life expectancy will exacerbate the delay.

Slow to show

A lifetime of change

This time lag is evident today in the post-war 鈥渂aby boom鈥� generation in
Western Europe. Populations continue to rise in many countries even though
fertility rates are well below the long-term replacement level. For instance,
Britain鈥檚 fertility rate has been below 2.1 for 30 years, but the actual
population is not expected to start falling for some years yet, while Western
Europe鈥檚 population is not expected to stabilise until after 2010. In the
meantime, the residual population increase will be the outcome of two trends: a
rise in the old population and a decline in the young population. In China, the
time lag is even more marked. If all couples have two children from now on, the
present population of around 1.2 billion will still grow to 1.6 billion by the
late 21st century

We can clearly see the time lag in global demographics too. The fertility
rate peaked in 1950, when each woman had an average of five children. Since
then, it has fallen to just 2.9 children. But it was not until the 1990s that
the net annual increase in the global population actually began to fall.

The net effect of these different national rates of population growth, reflecting
the different places of nations along the demographic transition, has been a
surge in the growth of the world鈥檚 population that now seems to have passed its
peak. The annual addition to the world鈥檚 total population peaked at the start of
the 1990s at around 90 million a year. But since then it has been declining, to
just 80 million a year by 1996. On the basis of national trends, demographers
estimate that the world鈥檚 population will stabilise at around 10 billion in the
second half of the 21st century. The population time bomb, it is said, is being
defused.

Some demographers see no reason why the world鈥檚 population should reach a
steady state since more than 50 countries in Europe, the Caribbean and the Far
East, including China now have birth rates below replacement levels and many
more could follow. The UN鈥檚 鈥渓ow range鈥� estimate of future population trends
(see Figure 1) suggests the world鈥檚 head count could peak at 8 billion in around
2045, before starting to fall, perhaps to below current levels by the end of the
21st century.

The Australian demographer John Caldwell, then president of the International
Union for the Scientific Study of Population, said at the 1994 UN population
conference in Cairo that he believed that this was the most likely scenario.
鈥淭he experience of the past 20 years,鈥� he said, 鈥渕akes it much more likely that
we will end up with a declining population.鈥�

One of the most important consequences of a dramatic worldwide slowdown in
the rate of population growth is a rapidly increasing proportion of elderly
people. In the first half of the 1990s, the world鈥檚 population grew by 1.5 per
cent per year, but the number of elderly people grew by 2.7 per cent per
year.

The average age of a world citizen is now 28 years. Only 9 per cent of the
population is over 60 years, while 32 per cent are under 15. By 2050,
projections put the median age at 38, with more than 17 per cent above 60 years
old鈥攙ery similar to the current position in Western Europe. China鈥檚
plunging birth rate means that the proportion of elderly people is rapidly
increasing. It will have reached Western European levels within 35 years, by
which time it will have a higher proportion of elderly people than any other
society in history. In Europe the surge in the numbers of old people is already
putting pressure on health services and pension funds, while Japan is predicting
slower economic growth because of its ageing population. But the impact will
soon be felt in developing countries, too. Some demographers now warn that a
rapid decline in the growth of the world鈥檚 population could be worse than a
moderate decline, because of its inevitable corollary鈥攁 rapidly increasing
proportion of elderly people.

Malthus, Marx and Co

Radical alternatives

MODERN theory on the relationship between population, economics and the
environment begins with the British economist Thomas Malthus. His An Essay
on the Principle of Population was first published in 1798, a time of
social unrest in the aftermath of the French revolution. This extremely
pessimistic work declared that if unchecked human population would grow
geometrically (1, 2, 4, 8, 16 and so on), while the food supply could only grow
arithmetically (1, 2, 3, 4, 5 . . .). Seeing no means of restraining population
growth, he concluded that it could be halted only by hunger and rising death
rates among the poorer classes. His tract was an assault on socialism, and Karl
Marx decried it as a 鈥渓ibel on the human race鈥�. Marx accepted that there was
overpopulation but blamed it on an economic system that valued capital over
people.

Modern environmentalists have revived many Malthusian ideas. In 1968, the US
ecologist Paul Ehrlich wrote in his polemic, The Population Bomb, that
鈥渉undreds of millions of people鈥� would starve to death because of overpopulation
before 1985. Similarly in 1972, the Club of Rome, an influential group of
industrialists and intellectuals, used computer simulations to argue in The
Limits to Growth that there would be a catastrophic collapse of population
in about 2025 as pollution increased and resources ran out (see 鈥淢ission Earth鈥�,
Inside Science, No. 106).

Social analysts have since pointed out that it is poverty and insecurity that
drive people to have large families. They have argued that social and economic
advances can and do reduce population growth rates, as the demographic
transition shows. Population academics, such as the American agriculturalist
Frances Moore Lappe have gone on to argue that the high population growth rates
in the modern world are a result of the abuse of the human rights of the
poor.

Pessimists such as Ehrlich have countered with the argument that the rapidly
growing populations of many of the world鈥檚 poorest nations are destroying
natural resources such as soils and forests so fast that they will never be able
to create the social and economic advances that would put a brake on population
growth. They envisage a Malthusian crisis overtaking first individual nations
and then the world.

But a radical alternative to the Malthusian vision was offered by the Danish
economist Ester Boserup in her 1965 book, The Conditions of Agricultural
Growth. While Malthus argued that agricultural output ultimately limited
population levels, Boserup says that population growth drives technological
advances that raise agricultural output and create wider economic development.
This idea has been taken up by right-wing economists, such as Julian Simon of
the University of Maryland, who argues that although population growth means
there will be more mouths to feed, it also means there will be 鈥渕ore hands to
work and more brains to think鈥�.

DESPITE the massive growth in the world鈥檚 population over the past two
centuries, the parts of the planet that were most densely populated before the
industrial era began鈥擡urope and eastern and southern Asia鈥攁re
generally still the most densely populated today. The Americas and Australia
remain relatively thinly populated outside large cities such as Los Angeles,
Mexico City and Sydney. Africa too remains comparatively underpopulated, despite
pockets of high density along the West African coast, Egypt and the highlands of
east Africa.

But as the populations of other countries stabilise, Africa鈥檚 is expected to
continue rising rapidly for some decades. Its population growth rate is now the
world鈥檚 highest. In 1996, for the first time in recorded history, it had more
people than Europe. And the UN鈥檚 projections suggest that Nigeria鈥檚 population
will triple to 340 million. Overall, the continent鈥檚 population is projected to
quadruple before it becomes the last region of the world to reach a stable
population.

As the world鈥檚 population has grown, millions of people have migrated from
densely populated areas to seek new lands and opportunities or to escape poverty
or persecution. The greatest migration was from Europe, the Old World, to the
New World of the Americas. At the height of the migration, in the first half of
the 19th century, two-thirds of migrants were from Britain and a quarter from
Germany. In the second half of the century most came from Ireland, Italy, Spain
and Eastern Europe.

Britain also colonised another 鈥渘ew鈥� continent, Australia, treating it
initially as a penal colony. Smaller numbers of people moved temporarily to
govern other European colonies and imperial territories. Some people were forced
to migrate, most notably the 15 million slaves transported from Africa to the
Americas before 1850. European colonialists also transported Indians to East
Africa, Malaya and the islands of the Caribbean and Pacific. Other people
migrated to escape persecution or famine. This category includes Jews fleeing
the Russian pogroms in the 19th century or the Nazis in Germany in the 1930s,
and the mass exodus from Ireland during the potato famine of the mid-19th
century.

During the 19th century there were large movements of people within countries
then being 鈥渙pened up鈥�. Millions of people moved from the eastern seaboard of
North America to the empty lands in the west, and from European Russia east into
Siberia. In the first half of the 20th century the expansion of the Soviet
empire brought millions of Russians into central Asia, Siberia and the Russian
far east, and into the Baltic and central Europe. The dissolution of the
colonial empires of the European powers also triggered new movements. Britain
encouraged Caribbeans and South Asians to come to work in Britain during the
1960s, and many North and West Africans moved to France.

The 1990s have seen further mass migrations as more than a million people
flooded out of Eastern Europe after the collapse of the Soviet empire. During
the 1991 Gulf crisis, 5 million foreign workers in Iraq, Kuwait and Saudi Arabia
returned to their homes in Egypt, Jordan, Yemen and the Indian subcontinent.
Meanwhile, civil wars and persecution forced Rwandans to flee into Zaire and
Tanzania, and Iraqi Kurds to enter Turkey, while a continuing stream of Mexicans
and other central Americans moves, often illegally, to the US. Many migrants,
notably the Chinese in Asia and North America, have achieved dominant places in
their hosts鈥� economies. Even so, developed countries are trying harder to keep
out those whom they regard as 鈥渆conomic鈥� refugees, looking for a better life
rather than fleeing persecution.

MOST developing countries around the world now have policies intended to slow
population growth. The major exceptions include a handful of Catholic nations,
such as Chile and Argentina, whose governments oppose most methods of artificial
birth control鈥攖hough Catholic Italy and Spain have the world鈥檚 lowest
fertility rates at 1.2 and 1.15 children per woman respectively. Many Islamic
nations also remain committed to increasing their populations. Some of the
latter, including Saudi Arabia, Sudan, Iraq and Lebanon, boycotted the UN
population conference in Cairo in 1994 to make their opposition plain.

Even so, worldwide about 55 per cent of women of childbearing age now use
contraception, says the UN Population Fund. Which method they use depends on a
complex mix of government policy and social circumstances. In China and Cuba,
governments pressure women to have intrauterine devices (IUDs) fitted, whereas
most German women take the pill. Elsewhere men take the lead鈥攊n Japan, the
condom is king, while Britain has the highest proportion of men in the world
with a vasectomy. In Turkey, government statistics show that withdrawal is the
preferred method.

In some countries a wide range of methods is freely available. But in East
Asia, this is often not true. In China, which has the highest rate of
contraceptive use in the world, 90 per cent of women use the IUD, and across
Asia as a whole, half of all contraception is by sterilisation.

WITHIN countries, many of the largest migrations have been from country to
town. Over the next 30 years, demographers expect that for the first time most
of the world鈥檚 inhabitants will live in urban areas. People are 鈥減ushed鈥� from
the countryside by the mechanisation of farming and environmental decline, and
鈥減ulled鈥� to cities by jobs in new industries and government investment in
services such as schools, hospitals and roads, which are often concentrated in
cities.

Megacities, cities with more than 10 million people, have come to dominate
life in many developing countries. Mexico City, S茫o Paulo, Shanghai and
Bombay each have around 16 million people鈥攎ore than twice the population
of London. Tokyo, the most built-up region in the world, has 27 million
people.

After a generation of very rapid growth, however, the growth of megacities
has slowed in the past decade. A 1996 UN conference on the world鈥檚 living
places, Habitat II, heard how many services have collapsed, causing gridlock,
air pollution, crime and urban squalor. Many of these cities鈥� inhabitants live
in giant shanty towns. However, only 3 per cent of the world鈥檚 population live
in megacities. In most countries, other smaller cities and urban areas are now
growing fastest.