The Living Will Live A Fifth of All the Years Ever Lived

I am working through an excellent book, Massimo Livi-Bacci's A Concise History of World Population. The long history of human populations were governed by natural limits that kept growth slow and exercised formidable checks on rapid deviations from the norm. Since the Industrial Revolution, though, the rules have changed dramatically. The world population is significantly larger than any premodern civilizations could have contemplated. The rate of growth today is enormously faster than any previous era.

One of the measures that Livi-Bacci uses to make this change apparent is this proportion:

There are about 6 billion people alive today. There have been about 82 billion people ever born in the world. That means that 7% of all the people who have ever lived are alive today. However, since people today live much longer than people in the past did, people born since 1950 (most of the people alive today) will live about one fifth of all the years that people have ever lived.