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Text 1
The Population Surprise
by Max Singer
Will the world's population keep
increasing as commonly believed? What are the factors
that account for its change? Please read the following
article and make out its viewpoints.
Fifty years from
now the world's population will be declining, with no
end in sight. Unless people's values change greatly,
several centuries from now there could be fewer people
living in the entire world than in the United States
today. The big surprise of the past twenty years is
that in not one country did fertility stop falling when
it reached the replacement rate─2.1 children per woman.
In Italy, for example, the rate has fallen to 1.2. In
Western Europe as a whole and in Japan it is down to
1.5. The evidence now indicates that within fifty years
or so world population will peak at about eight billion
before starting a fairly rapid decline.
Because in the past two
centuries world population has increased from one billion
to nearly six billion, many people still fear that it
will keep "exploding" until there are too many people
for the earth to support. But that is like fearing that
your baby will grow to 1 000 pounds because its weight
doubles three times in its first seven years. World
population was growing by two percent a year in the
1960s; the rate is now down to one percent a year, and
if the patterns of the past century don't change radically,
it will head into negative numbers. This view is coming
to be widely accepted among population experts, even
as the public continues to focus on the threat of uncontrolled
population growth.
As long ago as September
of 1974 Scientific American published a special
issue on population that described what demographers1
had begun calling the "demographic transition" from
traditional high rates of birth and death to the low
ones of modern society. The experts believed that birth
and death rates would be more
or less equal in the future, as they had been in the
past, keeping total population stable after a level
of 10-12 billion people was reached during the transition.
Developments over the past
twenty years show that the experts were right in thinking
that population won't keep going up forever. They were
wrong in thinking that after it stops going up, it will
stay level. The experts' assumption that population
would stabilize because birth rates would stop falling
once they matched the new low death rates has not been
borne out by experience. Evidence from more than fifty
countries demonstrates what should be unsurprising:
in a modern society the death rate doesn't determine
the birth rate. If in the long run birth rates worldwide
do not conveniently match death rates, then population
must either rise or fall, depending on whether birth
or death rates are higher. Which can we expect?
The rapid increase in population
during the past two centuries has been the result of
lower death rates, which have produced an increase in
worldwide life expectancy2 from about thirty
to about sixty-two. (Since the maximum─if we do not
change fundamental human physiology─is about eighty-five,
the world has already gone three fifths as far as it
can in increasing life expectancy.) For a while the
result was a young population with more mothers in each
generation, and fewer deaths than births. But even during
this population explosion the average number of children
born to each woman─the fertility rate—has been falling
in modernizing societies. The prediction that world
population will soon begin to decline is based on almost
universal human behavior. In the United States fertility
has been falling for 200 years (except for the blip
of the Baby Boom3), but partly because of
immigration it has stayed only slightly below replacement
level for twenty-five years.
Obviously, if for many
generations the birth rate averages fewer than 2.1 children
per woman, population must eventually stop growing.
Recently the United Nations Population Division estimated
that 44 percent of the world's people live in countries
where the fertility rate has already fallen below the
replacement rate, and fertility is falling fast almost
everywhere else. In Sweden and Italy fertility has been
below replacement level for so long that the population
has become old enough to have more deaths than births.
Declines in fertility will eventually increase the average
age in the world, and will cause a decline in world
population forty to fifty years from now.
Because in a modern society
the death rate and the fertility rate are largely independent
of each other, world population
need not be stable. World population can be stable only
if fertility rates around the world average out to 2.1
children per woman. But why should they average 2.1,
rather than 2.4, or 1.8, or some other number? If there
is nothing to keep each country exactly at 2.1, then
there is nothing to ensure that the overall average
will be exactly 2.1.
The point is that the number
of children born depends on families' choices about
how many children they want to raise. And when a family
is deciding whether to have another child, it is usually
thinking about things other than the national or the
world population. Who would know or care if world population
were to drop from, say, 5.85 billion to 5.81 billion?
Population change is too slow and remote for people
to feel in their lives—even if the total population
were to double or halve in only a century. Whether world
population is increasing or decreasing doesn't necessarily
affect the decisions that determine whether it will
increase or decrease in the future. As the systems people
would say, there is no feedback loop.
What does affect fertility
is modernity. In almost every country where people have
moved from traditional ways of life to modern ones,
they are choosing to have too few children to replace
themselves. This is true in Western and in Eastern countries,
in Catholic and in secular societies. And it is true
in the richest parts of the richest countries. The only
exceptions seem to be some small religious communities.
We can't be sure what will happen in Muslim countries4,
because few of them have become modern yet, but so far
it looks as if their fertility rates will respond to
modernity as others' have.
Nobody can say whether
world population will ever dwindle to very low numbers;
that depends on what values people hold in the future.
After the approaching peak, as long as people continue
to prefer saving effort and money by having fewer children,
population will continue to decline. (This does not
imply that the decision to have fewer children is selfish;
it may, for example, be motivated by a desire to do
more for each child.)
Some people may have values
significantly different from those of the rest of the
world, and therefore different fertility rates. If such
people live in a particular country or population group,
their values can produce marked changes in the size
of that country or group, even as world population changes
only slowly. For example, the U.S. population, because
of immigration and a fertility rate that is only slightly
below replacement level, is likely to grow from 4.5
percent of the world today to 10 percent of a smaller
world over the next two or three centuries. Much bigger
changes in share are possible for smaller groups if
they can maintain their difference from the average
for a long period of time. (To illustrate: Korea's population
could grow from one percent of the world to 10 percent
in a single lifetime if it were to increase by two percent
a year while the rest of the world population declined
by one percent a year.)
World population won't
stop declining until human values change. But human
values may well change—values,
not biological imperatives, are the unfathomable variable
in population predictions. It is quite possible that
in a century or two or three, when just about the whole
world is at least as modern as Western Europe is today,
people will start to value children more highly than
they do now in modern societies. If they do, and fertility
rates start to climb, fertility is no more likely to
stop climbing at an average rate of 2.1 children per
woman than it was to stop falling at 2.1 on the way
down.
In only the past twenty
years or so world fertility has dropped by 1.5 births
per woman. Such a degree of change, were it to occur
again, would be enough to turn a long-term increase
in world population of one percent a year into a long-term
decrease of one percent a year. Presumably fertility
could someday increase just as quickly as it has declined
in recent decades, although such a rapid change will
be less likely once the world has completed the transition
to modernity. If fertility rises only to 2.8, just 33
percent over the replacement rate, world population
will eventually grow by one percent a year again—doubling
in seventy years and multiplying by twenty in only three
centuries.
The decline in fertility
that began in some countries, including the United States,
in the past century is taking a long time to reduce
world population because when it started, fertility
was very much higher than replacement level. In addition,
because a preference for fewer children is associated
with modern societies, in which high living standards
make time valuable and children financially unproductive
and expensive to care for and educate, the trend toward
lower fertility couldn't spread throughout the world
until economic development had spread. But once the
whole world has become modern, with fertility everywhere
in the neighborhood of replacement level, new social
values might spread worldwide in a few decades. Fashions
in families might keep changing, so that world fertility
bounced above and below replacement rate. If each bounce
took only a few decades or generations, world population
would stay within a reasonable narrow range—although
probably with a long-term trend in one direction or
the other.
The values that influence
decisions about having children seem, however, to change
slowly and to be very widespread. If the average fertility
rate were to take a long time to move from well below
to well above replacement rate and back again, trends
in world population could go a long way before they
reversed themselves. The result would be big swings
in world population─perhaps down to one or two billion
and then up to 20 to 40 billion.
Whether population swings
are short and narrow or long and wide, the average level
of world population after several cycles will probably
have either an upward or a downward trend overall. Just
as averaging across the globe need not result in exactly
2.1 children per woman, averaging across the centuries
need not result in zero growth
rather than a slowly increasing or slowly decreasing
world population. But the long-term trend is less important
than the effects of the peaks and troughs5 . The troughs
could be so low that human beings become fewer than
they were in ancient times. The peaks might cause harm
from some kinds of shortages.
One implication is that
not even very large losses from disease or war can affect
the world population in the long run nearly as much
as changes in human values do. What we have learned
from the dramatic changes of the past few centuries
is that regardless of the size of the world population
at any time, people's personal decisions about how many
children they want can make the world population go
anywhere─to zero or to 100 billion or more.
(1916words) TOP
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课文一
出人意料的人口变化
马克斯·辛格
世界人口会像人们通常认为的那样持续增长吗？造成人口变化的因素是什么？请阅读下面的文章，并弄清其观点。
现在起50年后，世界人口将会减少，而且还看不到终止的迹象。除非人们的价值观产生巨大变化，否则几个世纪后，生活在整个地球上的人口可能会比今天生活在美国的还要少。过去20年最令人吃惊的事情是，没有一个国家当其出生率降到人口置换率水平时——每个妇女生育2.1个子女——就停止下降了。例如，在意大利，该比率已降到1.2。整个西欧和日本降至1.5。现在的证据显示大约50年内世界人口将达到80亿的顶点，然后将开始相当迅速地减少。
因为在过去的两个世纪内，世界人口已从10亿增至将近60亿，许多人害怕人口还会爆炸式增长，直到地球无法负担庞大的人口。但是，这就好像担心因为你的宝宝在最初的7年内体重翻了六倍，他就会长到1千磅。在60年代，世界人口每年只增长2%，现在这一比率已降到每年1%。如果上个世纪的生育模式没有根本性变化的话，这将会导致人口的负增长。尽管公众们继续把目光集中在失控的人口增长带来的危险上，上述观点已开始被人口学专家普遍接受。
早在1974年9月，《美国科学》杂志发行了一份人口研究专号，描述了人口学家开始将之称为“人口过渡期”的情况，即从传统的高出生率和死亡率转到现代社会的低出生率和死亡率。专家们当时相信，将来的出生率和死亡率会和过去一样大体相等，这样，在过渡期内人口达到100至120亿这样一个水平后，整个人口将会保持稳定。
过去20年的发展表明，专家们认为人口不会永远增长这一观点是正确的。但他们认为人口停止增长后就会维持该水平却是错误的。专家们推测出生率一旦同新的低死亡率达到平衡后就会停止衰减，所以人口能够保持稳定，这种推测没有得到经验的证实。来自50多个国家的证据显示了一个并不令人惊讶的结论：在现代社会，死亡率并不能决定出生率。长远地看，如果世界范围内出生率不能与死亡率适当保持平衡，那么人口将肯定或者增长或者减少，这取决于出生率高还是死亡率高。我们预计会看到什么呢？
过去两个世纪内人口快速增长是由于低死亡率造成的，这使得世界范围内的人均寿命从约30岁增至约62岁（因为如果我们不改变人类的基本生理构造的话，最高人均寿命约为85岁，所以世界人均寿命的增幅已达到最大可能性的五分之三）。这在一段时期内造成的结果就是人口的年轻化，每一代中母亲的数量将会更多，死亡人口少于出生人口。但是即使在这段人口爆炸期间，现代社会中平均每位女性所生育的子女数（出生率）仍在下降。基于几乎是人类的普遍行为，我们预测出世界人口将开始衰减。美国200多年来生育率一直在下降（除了二战过后出现的短暂的生育高峰期），但是在某种程度上受移民的影响，在25年里它只略低于人口置换率。
很明显，如果许多代平均出生率一直低于每个妇女生育2.1个子女，人口最终一定会停止增长。最近，联合国人口署预测，世界44%的人口居住在那些出生率已经低于人口置换率的国家中，而在除此之外的几乎世界各地，出生率都在快速下降。在瑞典和意大利，出生率早就长期低于置换率，以致于人口老龄化，死亡人数多于出生人数。出生率的降低使世界人均年龄增长，而且会导致40至50年后世界人口的减少。
在现代社会，因为死亡率和出生率在很大程度上是相互独立的，世界人口并不一定要保持稳定。只有世界范围内的出生率能达到平均每位妇女生育2.1个子女，世界人口才可能稳定。但为什么平均出生率一定是2.1，不是2.4或1.8，或其它什么数字？如果无法使每个国家的平均出生率正好控制在2.1，那么就无法保证总体的平均水平正好为2.1。
问题的关键在于，孩子的出生取决于家庭想要养育多少个孩子。一个家庭在决定他们是否还需要一个孩子时通常所考虑的并不是全国或世界的人口数目。谁知道或是在乎世界人口会不会从50.85亿降到50.81亿这类问题？人口的变化是长期的而且变化微小，即使在仅仅100年间总人口就翻番或减半，人们也很难在生活中感受到。世界人口是否正在增加或减少未必会影响某些决策的产生，使将来的人口增长或减少。正如一些系统论研究人士会说的那样，这里不存在一种反馈环
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真正会影响出生率的是现代性。几乎在生活方式已经从传统向现代转变的所有国家中，人们选择的是少生孩子，以致于无法进行人口置换。在西方和东方的国家，在天主教和世俗社会都是如此。在最富庶国家中最富裕的地区同样如此。唯一的例外好像是一些小的宗教社区。我们不清楚穆斯林国家中会发生什么，因为它们几乎都不是现代化国家，但目前看来，其出生率好像也会像其它国家那样对现代性作出呼应。
没有人能肯定世界人口是否会一直减少到非常低的数目，这取决于人们将来的价值取向。在即将到来的人口高峰过后，只要人们仍继续宁愿少生孩子以节省精力和金钱，那么人口将会继续减少。（这并不意味少生孩子的决定是自私的，也许，比如说，这只是出于希望为每个孩子做更多的事情。）
也许有些人的价值观与世界其它地方的很不一样，所以就会产生不同的出生率。如果这些人生活在特定的某个国家或特定的人口群体中，那么即使世界人口变化很缓慢，他们的价值观也会使该国或群体的人口数目产生显著的变化。例如，美国人口由于受移民影响，也由于其出生率只是略低于置换水平，所以在将来的二、三百年内，有可能会从目前占世界人口的4.5%增长到世界人口缩减后的10%。对于一些较小的人口群体来说，如果他们长时间内与平均出生率保持差距，那么他们所占世界人口的份额可能会产生更大的变化。（举例来说：如果朝鲜的人口每年增长2%，而世界其它各地的人口每年递减1%，那么仅在一个生命周期内，朝鲜就会从世界人口的1%增加到10%。）
要是人们的价值观不转变，世界人口就不会停止减少。但人类的价值观是会变化的——价值观，而不是生物的必需要求，才是人口预测中不可捉摸的变数。也有可能在一百、两百或三百年后，当几乎整个世界的现代化水平都能达到目前的西欧国家时，相比于当今的现代社会，人们会更看重孩子。如果是这样，那么出生率就会攀升。而且在达到每位女性生育2.1个子女这样一个平均水平后就会停止上升的可能性并不比降到2.1后会停止下降的可能性大多少。
仅在过去的20年左右时间里，世界人口出生率每个妇女已降低了1.5个。如果再发生如此程度的变化，那么就足以使以前长时间内1%的年世界人口增长率变成长时间内1%的年递减率。也有可能某一天出生率会像最近几十年急速衰减那样急剧增长，虽然一旦整个世界过渡到现代社会之后，出现这样一种急剧变化的可能性要小得多。如果出生率仅增长到2.8，比人口置换率高33%，那么世界人口最终将会有1%的年增长率，即70年内翻一番，在三个世纪内就能增长20倍。
在过去的一个世纪里，在一些国家中，包括美国，出生率衰减持续了很长一段时间才取得了世界人口减少的效果，因为开始衰减时的出生率比人口置换率要高得多。而且，愿意少生孩子的观念与现代社会相关。现代社会中，高生活水准使得时间变得很宝贵，而生孩子从经济上来说没有效益，养育和教育费用也很昂贵，所以只有当经济在世界范围内得到发展后，低出生率的潮流才会遍及世界。但是一旦整个世界都成为现代化国家，且任何地方的出生率都与置换率大体相当时，新的社会价值观又会在几十年内传遍全球。家庭方式可能会继续变化，所以世界人口的出生率会在置换率附近上下波动。如果每次波动只需要几十年或几代人的时间（虽然在其中任一方向的发展可能是长期性的），世界人口变化会保持在一个合理的、狭窄的范围内。
但是，看起来，影响人们作出生育决定的价值观转变缓慢，却流传很广。如果需要一段漫长时间使平均出生率从远低于置换率升到远高于置换率，然后再使它降下来，那么世界人口的走势会发展得很远，然后才会逆转。其结果就是世界人口的巨大涨落——可能会降到10亿或20亿，然后升到200至400亿。
不管人口的涨落是短期、小范围的或是长期、大范围的，世界人口的平均水平在经过了几个周期之后，总体上会形成一种或上升、或下降的趋势。正如全球范围内妇女生育子女的平均水平不一定要正好达到2.1一样，几个世纪内的人口平均水平也不见得就是零增长，而可能是慢增长或慢衰减。但是与长期的趋势相比，人口波峰与谷底所带来的效应显得更重要。人口达到谷底时，人类数量会比古代还要少。波峰时会由于某种短缺而产生危害。
一个隐含的结论是，即使由于疾病或战争使人口大量减少，它们对世界人口的长远影响也不能与由于人类价值观转变所造成的影响相抗衡。我们从过去几个世纪戏剧性的人口变化中所学到的就是，不管任何时候世界人口是多少，人们愿意生育多少子女的个人抉择，能够使世界人口发生各种变化——到零，或者到1000亿，或者更多。
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Text 2
"Suppose the Whole World..."
by Isaac Asimov
Suppose the whole world became industrialized
and that industry and science worked very carefully
and very well. How many people could such a world support?
Different numbers have
been suggested, but the highest figure I have seen is
20 000 000 000. This is ten times the population an
agricultural world could support, and a thousand times
the population a food-gathering world could support.
Let us suppose 20 000 000
000 is the limit, then. How long would it be before
the world contained 20 000 000 000 people?
That depends on how the
world's population growth rate rises or falls. The growth
rate might slow down or even reverse if there are terrible
wars, famines, or epidemics. (Naturally, everyone hopes
such disasters won't happen.) On the other hand, the
growth rate might rise even further. So far in history,
the growth rate has been going up steadily from 0.0007
percent or less before the coming of agriculture to
2.0 percent now. Yet a 2 percent growth rate is not
the highest possible. There are nations in the world
with a growth rate of 3.5 percent, and with population
increasing at this rate it will double in only twenty
years.
We can't be sure, then,
whether the growth rate will go up or down in the future.
Just for the sake of argument, and to keep things simple,
let's suppose the growth rate will stay exactly what
it is now. If it does, how long will it take the world
to increase its population to 20 000 000 000?
If the present world population
of 3 800 000 000 doubles, that will make it 7 6000 000
000; and if it doubles again, the population will be
15 200 000 000. Since each doubling, at a growth rate
of 2.0 percent, takes thirty-five years, it will take
seventy years altogether to reach the 15 200 000 000
mark. Then, fifteen more years will bring the world
population to 20 000 000 000. At the present growth
rate, in other words, our planet will contain all the
people that an industrialized world may be able to support
by about 2060 A.D.
Some young people who are
alive today may someday have children who will live
to see the world of 2060. It may be a world of 20 000
000 000 people, over five times as many as there are
today. If this is all an industrial world can support,
those people will be living at a starvation level─just
barely keeping alive. Surely, that is not a pleasant
outlook for a time only eighty-five years from now.
But wait, perhaps we aren't
allowing for changes in the way human beings live.
Let's go back to the food-gathering
world. At that time, 20 000 000 would have been the
population limit of the world, yet long before that
figure was reached, the world stopped being just food
gathering. Agriculture was developed, and the population
zoomed right past the 20 000 000 mark. In stead of people
starving, the average standard of living rose.
The population limit in
an agricultural world would have been 2 000 000 000,
but long before that figure was reached, the world stopped
being just agricultural. The Industrial Revolution took
place, and the population zoomed right past the 2 000
000 000 mark. Instead of people starving, again the
standard of living rose.
Well, then, is there any
reason to be worried now? Before the new 20 000 000
000 mark is reached, might we not expect something else
to happen that will make it possible for the population
to zoom right past it with the standard of living still
rising?
Let's see —
At the time that agriculture
was first introduced, the world contained about 1/5
of the people it could hold at most. If agriculture
had not been invented, it might have taken perhaps 250
000 years for the food-gathering world to reach its
limit.
At the time
the Industrial Revolution began, the world contained
about 1/2 of the people it could hold at most. If the
industrialization of the world had not begun, it would
have taken about 250 years for the agricultural world
to reach its limit.
Now the world has, perhaps,
less than a fifth of the people it could hold, if it
is really true that 20 000 000 000 is the industrial
limit. Yet the growth rate has grown so high that there
is only eighty-five years left for that limit to be
reached. In short, every time there is a great change
that makes it possible for the world to hold more people,
there is less time for that change to happen. and there
are far more people to suffer if anything goes wrong.
What's more, each new change
comes after a shorter and shorter time. Mankind remained
in the food-gathering stage for hundreds of thousands
of years before agriculture was introduced. Then mankind
stayed in the agricultural age for 10 000 years before
industrialization began. But the Industrial Age will
have lasted only about 300 years before another great
change seems to have become necessary. The next age
will then perhaps last only fifty years before still
another must come about.
Suppose we decide to hope
for the best, however. Let us suppose that a change
will take place in the next seventy years and that there
will be a new age in which population can continue rising
to a far higher level than we think it can now. This
means that there will be a new and higher limit, but
before that is reached, still another change will take
place, and so on. Let's suppose that this sort of thing
can just keep on going forever.
Is there any way of setting
a limit past which nothing can raise the human population
no matter how many changes take place?
Suppose we try to invent
a real limit: something so huge that no one can imagine
a population rising past it. Suppose we imagine that
there are so many men and women and children in the
world that altogether they weigh as much as the whole
planet does. Surely you can't expect there can be more
people than that.
Let us suppose that the
average human being weighs sixty kilograms. If that's
the case then 100 000 000 000 000 000 000 000 people
would weigh as much as the whole Earth does. That number
of people is 30 000 000 000 000 times as many people
as there are living now.
It may seem to you that
the population can go up a long, long time before it
reaches the point where there are 30 000 000 000 000
times as many people in the world as there are today.
Let's think about that, though. Let us suppose that
the population growth rate stays at 2.0 percent so that
the number of people in the world continues to double
every thirty-five years. How long, then, will it take
for the world's population to weigh as much as the entire
planet?
The answer
is─not quite 1 600 years. This means that, by 3550 A.D.,
the human population would weigh as much as the entire
Earth. Nor is 1 600 years a long time. It is considerably
less time than has passed since the days of Julius Caesar.
Do you suppose that perhaps
in the course of the next 1 600 years, it will be possible
to colonize the Moon and Mars and the other planets
of the solar system? Do you think that we might get
many millions of people onto the other worlds in the
next 1 600 years and lower the population of the Earth
itself?
Even if that were possible,
it wouldn't give us much time. If growth rate stays
at 2.0 percent, then in a little over 2 200 years, say,
4220 A.D., the human population would weigh as much
as the entire solar system, including the sun.
We couldn't escape to the
stars, either. Even if we could reach them, even if
we could reach all of them, population would reach a
limit. If the growth rate stays at 2.0 percent, then
in 4 700 years, by about 6700 A.D., the human population
would weigh as much as the entire known universe.
So you see we can't go
on forever at the rate we are going. The population
rise is going to have to stop somewhere. We just can't
keep that 2.0 percent growth rate for thousands of years.
We just can't, no matter what we do.
Let's try again, and let's
be more reasonable. Suppose we go back to considering
the density of population on Earth.
Right now, the average
density of population on Earth is 25/km2.
If the population of the world doubles, then the average
density of population also doubles, since the area of
the world's surface stays the same. This means at a
population growth rate of 2.0 percent per year the average
density of population in the world will double every
thirty-five years.
In that case, if the growth
stays where it is, how long will it take for the average
density of population to become 18 600/km2?
Such a density is almost 750 times as high as the present
density, but it will be reached, at the present growth
rate, in just about 340 years. Of course, this density
is reached only if human beings are confined to the
land surface of the world. Perhaps human beings will
learn to live on the bottom of the ocean or on great
platforms floating on the sea. There is more than twice
as much ocean surface as there is land surface, and
that would give more room for people.
That wouldn't do much good,
however. At the present growth rate, it would take only
forty-five additional years to fill the ocean surface,
too. In 385 years, the average density of population
would be l8 600/km2 over land and sea both.
That would be by about 2320 A.D. But a density of 18
600/km2 is the average density of population
of the island of Manhattan.
Imagine a world in which
the average density everywhere─over land and sea alike,
everywhere, in Antarctica and Greenland, over the oceans
and along the mountains, over the entire face of the
globe─was equal to that of Manhattan. There would have
to be skyscrapers everywhere. There would be hardly
any open space. There would be no room for wilderness,
or for any plants and animals except those needed by
human beings. Very few people would imagine a world
like that could be comfortable, yet at the present
growth rate we will reach such a world in only 385 years.
But let's not pick Manhattan.
Let's try the Netherlands. It is a pleasant, comfortable
nation, with open land and gardens and farms. It has
a standard of living that is very high, and yet its
average population density is 400/km2. How
long would it take for our population to increase to
the point where the average density of the surface of
the world, sea and land, would be 400/km2.
The answer is 200 years,
by about 2175 A.D.
You see, then, that if
you don't want to go past the average population density
of the Netherlands, we can't keep our present growth
rate going even for hundreds of years, let alone thousands.
In fact, we might still be arguing in an unreasonable
way. Can we really expect to have a worldwide Netherlands
in the next 200 years?
No one really believes
that mankind can spread out over the ocean bottom or
the ocean top in the next 200 years. It is much more
likely that he will stay on land. To be sure, there
may be some people who would be living offshore in special
structures, on the sea or under it. They would make
up only a small fraction of all mankind. Almost everybody
will be living on land.
Then, too, not every place
on land is desirable. It isn't at all likely that there
will be very many people living in Antarctica or in
Greenland or in the Sahara Desert or along the Himalaya
Mountain range over the next 200 years. There may be
some people living there, more people than are living
there now, but they will represent only a small fraction
of the total population.
In fact, most of the Earth's
land surface isn't very suitable for large populations.
At the present moment, most of the Earth's population
is squeezed into that small portion of Earth's land
surface that is not too mountainous, too dry, too hot,
too cold, or too generally uncomfortable. In fact, 2/3
of the world's population is to be found on a little
over 1/13 of the land surface of the planet. About 2500
000 000 people are living on 11 000 000 square kilometers
of land that can best support a high population. The
average density on the 11 000 000 square kilometers
of the best land is 230/km2,while
the average density of the rest of the land surface
is just under 10/km2.
Suppose the population
continues to increase at the present growth rate and
the distribution remains the same. In that case, after
thirty years, the average population density of the
less pleasant parts of the earth will reach the 19/km2
figure, but the density of the 11 000 000 square kilometers
of best land will be 400/km2.
In other words, we will
reach a kind of worldwide Netherlands density figure,
for as far as we can go, in about only thirty years.
But will all the world
be as well-organized and as prosperous as the Netherlands
is now? Some of the reasons that the Netherlands is
as well off as it is now is that it has a stable government,
a highly educated population, and a well-organized industrial
system.
This is not true of all
nations, and they need not expect to be as well off
as the Netherlands when they are as crowded as the Netherlands.
Indeed, if they have an agricultural way of life and
a poorly educated people, who don't have long traditions
of stable government, then a population as dense as
that of the Netherlands now is would only bring misery.
In other words, the world can't keep going at the present
growth rate, even for tens of years, let alone for hundreds
or thousands.
The matter of a population
limit is not a problem for the future, then. We might
as well realize that the world is just about reaching
its population limit now.
(2369
words) TOP
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课文二
“假定这整个世界……”
艾萨克·阿西莫夫
假定这世界都工业化了，工业和科学都发展得非常完备，非常好。这样一个世界能养活多少人呢？
人们说了不同的数字，但我看到的最大数字是200亿。这是农业文明时期能养活人口的10倍，是原始渔猎时期能养活人口的1千倍。
那么就让我们假定200亿是人口的极限。这世界达到两百亿要多久呢？
这要看人口增长率的高低。如果有可怕的战争、饥荒和流行性疾病，增长率就会放缓，甚至倒退。（自然，没有人希望这样的事发生。）反之，增长率就会升得更高。到目前的历史表明，其增长率已从农业文明前的不到0.0007%稳步上升到现在的2%。但2%的增长率看来不可能是最高的。有些国家的增长率已达3.5%，以这样的速度增长其人口只要20年就翻倍。
我们并不能确定未来的增长率是上升还是下降。仅为了论证的缘故，并使之简便，我们假定其增长率维持现有的水准。如果这样，这世界要达到人口200亿需多久呢？
如果现有人口3800000000翻倍,那就是7600000000；再翻倍，人口就是15200000000。这样以2%的速率每次翻倍需35年，达到15200000000大关因而要70年。再有15年多就可达人口200亿了。换句话说，以目前的增长率，我们的星球到大约公元2060年就能达到工业文明时期所能养活的总人口数。
现在活着的年轻人的孩子能看到2060年的世界。那是一个200亿人的世界，是现在的五倍之多。如果这是工业文明时期所能养活的总人数，那么这么多人就会处于饥饿的边缘——仅仅勉强活着而已。从现在到那时只有85年了，这确实看来不妙。
但是等一等，也许我们没有考虑到人们生活方式的变化。
让我们回到原始渔猎时期。那时200万就是一个极限，但早在这个数字达到之前，渔猎时期就结束了。农业得到了发展，人口急遽上升很快过了2000万大关。人们并没有挨饿，相反生活平均水平都提高了。
农业时期的人口极限是20亿，但早在达到这个数字之前，世界就不仅仅是农业社会。工业革命发生了，人口飙升超过了20亿大关。人们也没有挨饿，生活水平反而提高。
哎，那么，有什么理由要担心呢？在新的200亿大关没有达到之前，难道我们不能希望有什么事发生，从而使人口达到了那个数目
，同时生活水平也可能相应提高？
让我们看看——
当农业刚刚起步时，世界人口只有它能承载人口的大约五分之一。如果农业没有发明出来，原始渔猎时期可能要25万年才能达到其人口极限。
等到工业革命开始时，世界人口达到了它能承受的二分之一。如果世界的工业化没有开始，农业文明需250年才能达到其人口极限。
如果两百亿确实是工业社会所能承受的人口极限，现在这世界也许拥有的人口未到它能养活的五分之一。但增长率是如此之高，达到这个极限只有85年了。简言之，每次大的变革都有可能使地球养活更多的人，而发生这样的变化时间则更短，而稍出差错则会有更多的人遭殃。
而且，每次新的变革所发生的时间间隔越来越短。在农业文明萌芽前的好几十万年间，人类一直处于原始渔猎时期。在工业文明开始前人类处于农耕文明则达一万年。但工业时代只能持续约300年就必须有一次大的变革。而下一时期有可能只能维持50年就必须再发生一次变革。
不过，让我们设想一下最好的情况。假定在下一个70年里会发生一次变革，在随之而来的新时代里人口可以继续增长到一个比我们现在认为的更大的极限。这意味着有一新的更高的极限，而这个极限还未达到就又有另一新的变革发生，这样地连续不断。假定就这样的事情会永远持续下去。
那么有没有方法确定一个限度，不管发生多少次变革，一旦超过这个限度，无论如何也不可能养活那么多人口呢？
比如说，我们试想这样一个真正的极限：一个巨大的人口数量，你不可能想象人口会超过这个数字。假定我们想象地球上的男女老幼加在一起的重量有整个星球一样重。你一定不会想到还有比这更多的人了。
我们假定人的平均重量是60公斤。那样的话，10亿亿亿亿人的重量就跟地球一样重。这个数字是现有人口的3亿亿倍。
你似乎会觉得世界人口要达到现有人口的3亿亿倍还遥遥无期。那让就我们想想。假定人口的增长率还维持在2%，这样世界人口每35年就加一倍。要多久世界人口的重量就能达到跟整个地球一样重呢？
答案是——还不到1600年。这意味着到公元3550年世界人口的重量将与地球一样重。1600年并不长，它还远远不到从裘力斯·凯撒时期到现在的时间。
你是不是在想，也许在下个1600年里人类有可能殖民月球、火星和其它太阳系的星球？你认为在下个1600年里我们能将千百万的人移到其它星球从而降低地球的人口负担吗？
即使那是真的，留给我们的时间也不多了。如果增长率维持在2%，那么只2200年多一点，即公元4220年，人口总的重量就与包括太阳在内的整个太阳系一样了。
何况我们也不可能逃到其它星球上去。即使我们能去，即使我们能到达所有的星球，人口也将有一个极限。如果增长率还维持在2%，那么在4700年后，即到公元6700年，整个人口的重量将会与整个已知宇宙一样重。
所以，你看，我们不能永远以这样的速度增长。人口增长总归非得要停下来不可。我们绝不能让2%的增长率保持几千年。无论如何也不能。
让我们再试试，更理智一点。让我们回头考虑一下地球上的人口密度问题。
现在，地球上的人口密度是每平方公里二十五人。如果世界人口翻倍，人口密度也相应翻倍，因为世界的表面积是不变的。这意味着，以每年2%的人口增长率计算，世界人口平均密度也会每35年就翻倍。
在这样的情况下，如果增长率还是那样，要多久就会达到每平方公里18600人的人口密度呢？这样的密度是目前的750倍，但以现在的速度，达到它只要大约340年。当然，这样的密度是以人类居住的陆地面积不变为条件的。也许人类能学会怎样在海底或漂浮于海面的平台上居住。海洋的面积是陆地面积的两倍，会给人提供更多的居住空间。
但这也解决不了问题。以现有速率，也只需35年额外的时间就占满了海平面。385年后，无论在海面还是陆地，平均人口密度都将是每平方公里18600人，这是公元2320年的事。不过每平方公里18600人是现在曼哈顿岛的平均人口密度。
想象一下世界的每一个地方——无论陆地还是海洋，在南极洲和格陵兰岛，海洋上和山脉间，在整个地球表面——平均人口密度都如同曼哈顿一样。无处不是高楼大厦。几乎没有开阔的空间。没有荒野。除满足人类所需外没有给植物和动物留有余地。很少有人会相信，这样的一个世界会令人舒服，但以现有的速率，我们达到这样一个世界只有385年。
不过让我们不要以曼哈顿为例，而以荷兰为例。荷兰是一个令人愉快、舒适的国家，有开阔的陆地、公园和农场。其人口密度每平方公里400人，但生活水平却很高。要多长时间我们的陆地海洋平均人口密度将增长到每平方公里400人呢？
答案是200年，到公元2175年。
那么你就明白了，如果你不想超过荷兰的人口密度，不可能让我们以现有的速率增长几百年，更不用说几千年了。事实上，我们还可以不切实际地争论下去。我们在下一个200年里真的全世界都能像荷兰那样吗？
没有人真的相信人类在200年后会生活在海面或海底。最有可能的还是会生活在陆地上。确实，有些人可能会生活在海上，住在特殊的结构中，在海面或海底里。他们也只不过人类的一小部分。几乎人人会生活在陆地。
而且，并不是陆地的每一块地方都适合人居住。就在下一个200年里，要很多人住在南极洲、格陵兰岛、撒哈拉沙漠或喜马拉雅山脉地带都是不大可能的。也许有人会住在那，比现在住在那里的人要多，但他们也只是总人口的一小部分。
事实上，地球表面的大部分都不适合较大规模的人口居住。目前，大部分人都聚集在不热不冷，不干燥，山不多，总体较舒适的地球陆地的小部分地带。事实上，三分之二的世界人口是生活在陆地面积的十三分之一多点的地方。大约25亿人生活在能承载密集人群的1100万平方公里的土地上。在这1100万最好的土地上，人口密度是每平方公里230人，而其余部分的人口密度还不到每平方公里10人。
假定人口继续以现有速率增长且其分布不变。那么30年后，不甚理想的那部分土地的人口密度会达到每平方公里19人，但那最好的1100万土地的人口密度，将会是每平方公里400人。
换句话说，我们在全球达到荷兰的人口密度，就我们所能，只要大约30年。
但全世界都会像现在的荷兰那样制度健全和繁荣昌盛吗？现在的荷兰为什么那么富足，一些原因是它有一个稳定的政府，受到良好教育的民众和机制健全的工业体系。
并非所有国家都是这样，也不可能希望达到荷兰那样密度的人口时有荷兰那么富足。的确，如果这些国家有的只是农业时期的生活方式和贫乏的教育水平，也没有稳固的政府机制，那么像现在的荷兰这么稠密的人口带来的只会是饥荒。也就是说，世界根本不能以现有的速率增长人口几十年，更不用说几百年了。
那么，人口极限的问题并不是未来的事。我们还应该认识到，世界现在就面临人口达到极限的问题。
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