We do understand a few important aspects of population growth, which should get more media attention than they do. The human population was roughly 5.7 billion in 1995. About 149 million children were born that year, and about 56 million people died, which means the population grew by 93 million. The growth rate of the human population has been roughly the same for many decades. Anything growing in proportion to its size is growing exponentially .
Any population growing exponentially has a fixed doubling time .
And indeed, the world population did double in size in the 37 years between 1950 and 1987. Unless we slow the growth rate the population will double again in 40 years and when I'm in my seventies the world will have 11 billion people. In order to stop population growth we need to achieve replacement reproduction , which occurs when the average couple has just two children who survive to have children of their own. Unfortunately, achieving replacement reproduction doesn't immediately yield Zero Population Growth, because of population momentum . This next bit isn't so obvious.