200 years ago there were less than one billion humans living on earth. Today, according to UN calculations there are over 7 billion of us.1 Recent estimates suggest that today's population size is roughly equivalent to 6.9% of the total number of people ever born.2 This is the most conspicuous fact about world population growth: for thousands of years, the population grew only slowly but in recent centuries, it has jumped dramatically. Between 1900 and 2000, the increase in world population was three times greater than during the entire previous history of humanity—an increase from 1.5 to 6.1 billion in just 100 years.