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Population size, rate of change, distribution, age structure and migration are all critical aspects of demography. Population size to a great extent governs demand for natural resources and material flows. Population growth enlarges the challenge of improving living standards and providing essential social services, including housing, transport, sanitation, health, education, jobs and security. It can also make it harder to deal with poverty.

Rapid population growth can lead to political and social conflict between ethnic, religious, social and language groups. Increases in the numbers of people living in towns and cities are particularly important because urbanization means big changes in lifestyle, consumption patterns, infrastructure development and waste flows. Population structure - the relative proportions of children, persons of working age and elderly people within a population - has important repercussions for future population growth as well as for matching the provision of education, healthcare, incomes and pensions, to predicted needs. Finally, internal and international migration, whether voluntary or forced, can sometimes ease and sometimes worsen the pressures that other demographic factors and other forces place on society and the environment.

Because so many of the people who will have children over the next 30 years have already been born, much can already be said about population over that period. All of the scenarios assume continued growth in global population, tailing off at the end of the period as more countries pass through the demographic transition. Nearly all the growth occurs in developing countries, with North America the only developed region with noticeable growth. Slightly lower population levels are foreseen in Policy First and Sustainability First, reflecting the idea that policy actions and behavioural changes speed up the transition to slower growth. In Security First, lack of effective policy as well as much slower economic and social development, combine to slow down the transition. This leads to significantly higher population levels in this outlook, regardless of devastating demographic trends or events such as the HIV/AIDS pandemic in Africa that might be expected to have the contrary effect.

Urbanization increases or remains stable in almost all regions in all the scenarios, with the greatest increase in those regions currently least urbanized - Africa and much of Asia and the Pacific. In all regions, much of the development occurs in large coastal cities, a shift with serious implications for the coastal environment.

'North America, Europe and Japan have significantly larger shares of elderly people in all scenarios.'

Apart from the Antarctic sub-region, which has no permanent resident population, current and future population structure differs markedly from region to region. North America, Europe and Japan have significantly larger shares of elderly people, a pattern that persists and increases in all scenarios. This trend is less marked in Security First, where advances in medical science (and hence in life expectancy) make less headway in all regions. Other areas, particularly Africa, West Asia, Latin America and the Caribbean and South Asia, are dominated by youth. Their share of the population in these regions - but not their absolute population size - gradually decreases over the next 30 years in all scenarios.

In terms of migration patterns, Markets First and especially Security First are likely to have more conflicts and inequality, provoking more and more movements of refugees and economic migrants. Whereas more openness is assumed under Markets First, barriers to migration are expected in Security First. Policy First and Sustainability First also assume more open migration, especially for refugees and displaced communities. At the same time, more equitable sharing of resources for economic development and international assistance reduce the need for migration.