In “Countdown: Our Last, Best Hope for a Future on Earth,” author Alan Weisman explored what it would take to bring the world’s population down to a sustainable level. The book is a sequel of sorts to Weisman’s bestselling “The World Without Us.”

The idea of “The World Without Us,” said Weisman, was to see how nature could recover from the effects of climate change without humans around to get in the way. But his hope in writing the book was to inspire the discovery of a way to add humans back into the equation.

“Then I discovered that every four to four and a half days we were adding a million people to the planet and this is clearly not a sustainable figure,” said Weisman.

He traveled to 21 different countries in search of the answers to four questions:

How many people can we fit on the planet? How much nature do we need to ensure our own survival? Can people of various cultures be convinced that it's in their best interest to curb population growth? And how do we design an economy that isn’t based on constant growth?

Using oxygen use and food production as parameters, Weisman determined that a sustainable world population would be around 2 billion people—five billion fewer than the world’s current population.

The world’s population explosion is in part due to medical advances and in part to an increased ability to produce food brought on by the use of nitrogen fertilizers and genetically modified organisms. Weisman believes these methods of food production are dangerous and wants the world to return to a more natural way of producing food.

“Before chemistry took over the food supply, there were about 2 billion people on Earth. And that’s pretty much the same figure, between 1.5 and 2 billion or so, that could be consuming [oxygen] and live at a level that we’d like to live, at least a European level of existence,” said Weisman. “If everyone had [no more than] two children, and some people had fewer … over the next two generations or so we would start bringing ourselves down to a natural population decline that would give us time for our economy to adjust.”

In the countries he visited, Wiesman discovered ways societies were encouraging lower birth rates in more positive ways than China’s one child policy. In Mexico, for example, a popular soap opera demonstrated the economic advantages to having smaller families.

No matter the means of encouragement, however, the best method of contraception is girl’s education, said Weisman. When girls have the opportunity to go to school, they wait longer to have children and have better means of supporting their family once they do.

Despite what can be read as a rather depressing premise, Weisman hopes his book maintains a positive outlook.

“Nature does not let any organism exist for very long beyond its limits before there is a population crash,” he said. “We have the ability to handle ours gracefully rather than let nature do it to us, and that’s what I hope we do.”

Weisman is speaking at the St. Louis County Library tonight at 7:00 p.m. For more information, visit the SLCC website.

St. Louis on the Air provides discussion about issues and concerns facing the St. Louis area. The show is produced by Mary Edwards and Alex Heuer and hosted by veteran journalist Don Marsh.