Sometimes it is asked, if birthrates are declining, why does the world's population continue to grow? Today's population growth is due to two factors: 1. higher fertility rates in the 1950s and 60s and 2. people are living longer than ever before. The thing to remember is that declining birthrates will equal a declining population worldwide that will be very rapid and almost impossible to reverse.
A nation's demographic future can be seen in its current birthrate. In Europe, the number of children under 5 has declined by 36% since 1960. Worldwide there are 6 million fewer children, 6 and under, today, than there were in 1990. If present trends continue, the United Nations estimates that by 2050 there will be 248 million fewer children in the world then there are now.
Of the 10 countries with the lowest birthrates, 9 are in Europe. Overall, the European fertility rate is 1.3, well below replacement level of 2.1. No European nation has a replacement-level birthrate. Ireland has a highest birthrate at 2.0.
Italy's fertility rate is 1.2. Spain's is 1.1. That means in the not-too-distant future, absent massive immigration, these countries will lose half of their people in every generation.
While birthrates are also plummeting in developing nations, most still have above-replacement fertility - for the time being.
It's interesting to note that John Lennon of Beatles-fame regarded over-population as a myth, as the video link below shows.
His view is supported by many leading demographers, including Dr Nicholas Eberstadt, who holds the Henry Wendt Chair in Political Economy at the American Enterprise Institute.
You can find his paper on population questions, Too Many People.