Many countries around the world are experiencing a demographic transition that is occurring due to increased longevity combined with low fertility rates. However, demographic momentum means that the global population is expected to continue to increase until at least the middle of the century, likely peaking near 10 billion. Creative Commons Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales; http://creativecommons.org/licenses/by-nc-sa/2.0/uk/