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Would you please tell me what the meaning of the bold part is? Does "growth" refers to population? Then, how population growth can be due to population ageing?

Only Sub-Saharan Africa and parts of India and South Asia are expected to see further rapid increases in population in the coming decades, with the growth elsewhere mainly due to rapid population ageing. Source

  • consider that growth can be negative. – Jim Jun 6 '17 at 5:14
  • @jim So, the text means in some countries, such as India, growth is positive and in other countries growth is negative? – user233847 Jun 6 '17 at 5:18
  • It may be referring to population growth caused by a cohort of people reaching adulthood, and having children. Even if the overall fertility rate is going down (eg, having 2.5 children instead of 3.5), the fact that a lot of people are reaching their 20s soon and having children may mean that the overall population will keep going up for a while. Just guessing. – Steve Bennett Jun 6 '17 at 5:32
  • Actually population growth due to rapid population ageing makes no sense. Increased longevity (a lower death rate) may increase population over what might have been expected, but all the people getting old have already been born and aren't going to increase the population by getting older. – Xanne Jun 6 '17 at 6:25
  • True, but lets say ten years ago the average lifespan was 60, and for every person that died a new person was born, you would have a stable population with no growth. If each year the average life span was increased by one year and the birth rate stayed the same you would slowly see an increase in the population. – flamesquirrel Jun 6 '17 at 6:37
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"Rapid population ageing" is a shorthand for an increase in the age distribution of the population. In itself this cannot cause population growth. It is necessary (mathematically) that the crude death rate (CDR) be below the crude birth rate (CBR). The CDR is deaths per one thousand people per year (as it is usually expressed), and CBR is live births per thousand per year.

Consider the extreme example where the CBR is zero. The fact that the living folks age, and on average are older, does not increase the population. In fact you could imagine an indefinite increase in longevity without any increase in population if you had no live births (and no net migration).

In developed countries at the present time, the CDR is below the CBR. Given the age distribution of the population and a CBR below replacement, eventually the population is going to decline. Also, the CDR will increase even though people live longer.

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I believe the term 'population ageing' refers to people living longer. The net result in these countries is that the population continues to grow although at a slower rate.

  • In every context I've seen it; it means the [implicitly increasing] average age of citizens. – Flater Jun 6 '17 at 14:53
  • @Flater Aren't these the same thing? 'People living longer' must be referring to 'people living longer on average' to make any sense here. – Edwin Ashworth Jun 6 '17 at 16:41
  • @EdwinAshworth: They are related of course, but used in different contexts. Similarly, mass and weight are related but do not express the exact same thing. You often see population ageing mentioned in relation to how a shrinking ratio of adult workforce is not capable of sustaining the social welfare of a growing ratio of senior citizens. Which is what we're starting to deal with because of the baby boomers now (who will be reaching retirement age in a few years). Population ageing can also occur because of less children per adult, not just longer lifespans for seniors. – Flater Jun 7 '17 at 6:56
  • @EdwinAshworth: I also missed this in my initial reply: "the average lifespan" and "the average age of a country's citizens" are by no means expressing the same thing. The latter expresses the distribution of citizens among age groups, which is something different. It does not care about the expected lifespan of people, it only expresses the current distribution. Just because they can be caused by the same thing does not mean that they are the same thing. – Flater Jun 7 '17 at 6:58
  • @Flater Note that OP says 'the term population ageing refers to people living longer' (bolding mine). Not 'is strictly defined as'. As a paraphrase, it's fine. – Edwin Ashworth Jun 7 '17 at 7:42
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Earlier in the same paragraph of the source document, the author phrases it more explicitly (the emphasis is mine):

The number of people and where they live is changing rapidly. The world's population has doubled over the past forty years, but the pace of growth is slowing...

The author is almost certainly referring back to the decreasing growth rate. The entire paragraph is phrased a little awkwardly, but the sentence you call out in your question would benefit from a modifier before the word growth, e.g., reduced, slower, decreasing.

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