Global population: 11 billion by 2100 is a bit of a 'good news/bad news' story.

"It's not that we've started breeding like rabbits; we've just stopped dying like flies", wrote Peter Adamson back in 1974 to explain the surging growth in world population. Ironically, the world was just starting to take notice of global population increase at the point it started to decline from its peak rate of growth; it has been declining ever since 1974. However, that won't stop an inevitable growth in global population to between 10 and 12 billion people on the planet by 2100. The good news is, it's because we're all living longer. The bad news relates to what the quality of life will be on a planet with over 50% more people than are living on it at present if we don't start preparing for a more sustainable use of the place. And it might not be so much an issue of feeding the extra mouths, but finding useful activities for all those extra pairs of hands.

So why is the world's population forecast to grow from the current 7.3 bn to over 11 bn by the end of the century? The growth rate may be slowing, but it depends on three factors:

• how many countries have birth rates above replacement level (around 2.2 births per woman)
• the rate of extension of life expectancy
• the survival of children up to reproductive ages

The situation is one with a majority of countries now seeing birth rates below replacement level. It is mainly in sub-Saharan African countries where birth rates continue to exceed replacement level, even though the birth rate is declining from previously higher levels. But in the next few decades their figures are likely to come down in the same way they did for Asian countries in the 1980-90s.

The extension of life expectancy is actually a good news story: we're living longer. Vaccines and good nutrition when young, fewer dangerous jobs and medical intervention and preventative treatment means, according to Sarah Harper (Professor of Gerontology and Director of the Oxford Institute of Population Ageing) speaking at the Hay Literary Festival this year, that half the children born in the next few years in developed countries can anticipate seeing their 100th birthday.

Birth rates are likely to continue to decline to a 'desired family size', says Sarah. Which in most of the world is 2 children, although in sub-Saharan Africa this is more like 3-4, even amongst university-educated women. Whether the global population eventually hits the higher end of the projected range, or the lower forecast, depends largely on whether birth rates in sub-Saharan Africa follow the pattern set by the rest of the world and drop below replacement level, or cultural and social factors lead to it plateauing just above replacement level. The latter would mean continued growth for these countries, and with Nigeria likely to over-take the USA to become the world's third most populated country behind China and India, that represents many millions of additional people.

In past decades we worried about how to feed a global population of 10 billion or more. That appears less of a concern now but has been replaced by socio-political apprehension. The MENA region (Middle East and Northern Africa) has a demographic structure typical of a youthful population. The male component is characterised by many young men in their late teens and twenties, who are post-education but with few chances of formal employment. Sarah asks the question whether this group will be a 'demographic dividend' or a driver of socio-political division and conflict. The armed groups conflating the sectarian problems of the MENA countries are largely drawn from this group. When the same age/gender bulge was present in Hong Kong thirty years ago, the city-region took off economically. Here there were social structures and infrastructure in place to take advantage of the demographic dividend, as well as an enlightened perspective on absorbing women into the workforce on equal terms. The same potential contribution may not be utilized effectively in MENA countries with concerning implications for social and political stability - and, consequently, economic prospects.

It is partly this instability that is driving considerable migration from MENA and sub-Saharan countries to Europe, giving an influx of refugees, asylum seekers and economic migrants; people for whom these labels often overlap. Coincidentally, Sarah points out, by 2017 most EU countries will have more 60-65 yr olds leaving the workforce through retirement than 20-25 yr olds coming in: a classic 'ageing' population feature of Stage 5 Demographic Transition countries. The demand for new, young workers from elsewhere will intensify as time goes on if economic and social workforce recruitment needs are to be sustained, let alone, grown. That last need is particularly relevant to ageing affluent populations, such as the USA and western European countries where 'degenerative affluence' has led to an increase in obesity and medical conditions associated with poor lifestyle choices. Sarah forecasts that while mortality rates will continue to improve, we are likely to be more unhealthy in our final five or so years, and require more intensive medical and social care for a longer period in our declining years. The 'elderly disabled' will be an expanding social demographic that will likely absorb considerable investment - both economic and social.

Read more about the changing population growth rates in this Guardian article: 'Over-populated or under-developed? The real story of population growth.

There are some great illustrative visuals (maps and graphs) in this report by the UN: World Fertility Patterns 2015

This video, below, includes Professor Harper and co-authors discussing their book 'Is the Planet Full?' at Oxford Martin School in 2014 (55 mins) It can be accessed, along with other Oxford Martin School videos and publications on population matters from this site: