In Alan Weisman’s latest book, “Countdown,” the author travels to 21 different countries to investigate the current global population crisis—and look for a way to return the planet to balance. This week MATTER featured the book’s 12th chapter, which considers Iran’s explosive expansion. We caught up with Alan Weisman about his findings and what they mean for our future on earth.

What’s the big takeaway?

This book is about how many people could fit on one planet without tipping it over. And one of the big issues, of course, is finding a way to get that number down realistically, humanely, and in a way that would be acceptable to most people.

Early in my research I learned that there were some very successful alternatives, but the most outstanding alternative of all was in Iran. As a journalist, that gave me two opportunities. The first was to demonstrate how a really big country could turn around what had probably been the highest rate of reproduction in human history and then bring it down to below replacement rate.

Many Europeans unfortunately fear that muslims are going to overrun them because they proliferate like bunnies—they worry all of Europe will turn into one big muslim nation. I sought to belie those myths and show that muslims are just as capable as anyone else in doing something really wise when it comes to dealing with too much of a good thing that becomes problematic.

From this chapter, in particular, there are two things I hope people will take away: It’s always such a rich opportunity when you can humanize a population or a country that a lot of people dehumanize because it’s literally our enemy. I hope to show that not only are these real people like you and me, but these are people with a deep, incredible cultural and natural history. Whenever I describe the Iranian program, what I hear from people over and over again is that it’s just so logical. That’s what I wanted people to take away from this chapter and from the book itself.

I wanted people to understand that we have a problem: our population grew enormously large, mostly in just one century—the last one. And we are going to have tremendous environmental problems because of it—unless we do something about that or unless nature does it to us (which will inevitably happen). Lastly, there is something that we can do about this that would not be painful. And it would not involve governments getting into our private lives and telling us what to do. The best way to do this—and the Iran chapter shows this—is to give people the choice to do what they want. If you combine that freedom with educating them, particularly educating the women, then the overwhelming majority will choose to have a sustainable number of kids. And it’s incredibly cheap compared to everything else that we’re trying to do for the environment. To make contraception universally available would cost less per year than the US was spending per month in Iraq and Afghanistan in the past decade. This is totally doable. If you combine it with female education, it’s doable much faster, and you get the great benefit of having educated women who can really help ease the transition towards coming up with a sustainable economy for a shrinking and unstable world.

Why is female education part of the answer to our population crisis?

If you let a girl go to school, she will defer her childbearing till her studies are over, and once her studies are done, she’ll have something interesting to do with her life. Most women want to have children, but aren’t able to exercise those skills she has acquired in school if she has seven kids clinging to her. Wherever I went, whether it was a rich country or a poor country—I found that a girl who gets through high school will have two children or fewer.

What will the future look like if we continue to grow at the current rate?

By the middle of the century we will have a little over nine and half billion people. That’s almost 2.5 billion more than we have right now. And to meet this population growth we’re going to have to produce as much food in the next 50 years as has been consumed in the entire human history. The chances of being able to feed 2.5 billion more people are not very good.

The next 2.5 billion people are going to have much more of a severe impact than the last 2.5 billion. You can see it already. We’ve run out of the easy energy sources, so now we’re going for the dirtiest ones. We have run out places to grow more food. We can’t cut anymore forests. We’re already using every bit of arable land we can and we’re force feeding it with chemistry to grow more. Demanding more and dirtier energy and agriculture itself is one of the biggest contributors to greenhouse gas because of the use of things like nitrogen fertillizer. We’re going to have a warmer world. It’s very clear. We cannot go on like this.

What we can do is really start reducing the number. Because the carbon ratio of every child turns out to be very significant.

Growing at the rate we’re growing, by the end of the century, we’ll be at close to 11 billion people. If we made contraception universally available and could reduce the number of kids being produced by just a half a child around the world, we would only have around 6 billion. If we don’t do anything, that could translate to a half a child more per woman—and we’d be heading to a global population of 16 billion by the end of the century.

And that’s just not gonna happen. We’re going to crash first.