1999 (Christian calendar) was the Year of 6 Billion, the year the human population of Earth was projected to reach 6 billion. According to the United Nations, the 6 billionth person was born on October 12th of that year. The selection of that day was fairly arbitrary. The 6 billionth person may have been born earlier or later than that date, or may not have even been born yet. No one really knows for sure. We aren't taking a head count, and we couldn’t even if we tried. Picking the exact Day of 6 Billion isn't important, though. You pick a day to get people's attention, so you can tell them that the population is still growing and how fast.

So, how fast?

• When my maternal grandmother was born, 82 years ago, it is estimated that there were just under 2 billion people on Earth.

• When my mother was born, 59 years ago, there were 2 billion-plus people on Earth.

• By the time she had her 10th birthday, the population had grown to more than 2.5 billion people, despite the massive casualties of World War II.

• When she gave birth to me, 35 years ago, the population was between 3 and 3.5 billion.

• By the time I celebrated my 10th birthday, there were more than 4 billion of us.

• It is estimated that we reached 5 billion the July I celebrated my 22nd birthday, not quite 13 years ago.

Now, after just 12 more trips around the sun, there are 6 billion of us. I've almost seen our population double in my lifetime. For me, and probably all of you, what's been called the population explosion has been a fact of life for our whole lives. But why? Why does our population keep growing so quickly?

Nothing explains quite like a parable:

You awaken to find yourself a passenger on a train. You don't know how you got there, or where the train is going. You notice out of the corner of your eye that the ground drops off into a deep canyon just beyond the tracks. And the train isn't just riding, either. It's hurtling down the tracks, and the scrub and parched dirt outside seem to be racing past the window. The car you're riding in is rocking back and forth on the tracks madly, threatening to rip itself free and go careening off into the chasm.

It's only then that you notice the deep thrum of the locomotive pulsing through the car, and the sound of metal being strained and twisted. Over it all is a wailing screech, which you quickly recognize as the sound of the brakes being applied to halt the spinning of the train's wheels. Then you realize that everyone else in the car is screaming in desperate fear. They all have both hands stretched to a lever above the window next to their seats, and they're pulling it down and to the rear, their muscles straining, sweat flowing down their faces, veins bulging. Just as you notice that there is a lever above your window, too, a voice over the intercom thrashes its way into your mind.

"Attention all passengers! This is the engineer speaking! We've only been able to slow the train down, and we don't have any chance of stopping it in time at this rate! All of you must pull harder on the brake at your seat or we won't be able to stop before we plummet off the cliff ahead! Pull with everything you've got!"

All the people around you tighten their grips on the brake levers above them and pull hard, down and back, grimacing with the effort. Caught up in the desperation of the moment, you reach over your head to grasp the lever above your window and join them in straining mightily to stop the train. The brakes scream under the increased pressure, and the train seems to slow just a bit, but it's still rocketing down the tracks. You're pulling with all your might, the other passengers seem to be pulling with all their might, and yet the train has barely slowed.

You lunge out of your seat and race to the front of the car, through the door, and on into the next car. It, too, is filled with passengers working mightily on their own levers. Car after car the scene is the same. Finally you reach the locomotive and burst into the control compartment. The engineer whirls to face you.

"What are you doing in here? We need every passenger pulling on the brakes if we're going to have any hope of stopping this train! Get back to your seat!"

"We can't stop the train that way! Everyone but me is pulling and we're still moving! There must be some other way to stop the train!"

"No! The brakes are our only hope! Now get back to your seat and pull!"

The engineer spins back to the instrument panel with fierce determination set in his face. He starts shouting into a microphone: "Passengers! You've got to pull harder!"

Only then do you notice the chamber beyond the control compartment. Through heavy, dark smoke you see figures moving, and a blazing light. You can just make out that the figures are workers. What could they possibly be doing? Why aren't they helping to stop the train? Then the realization hits you: They're feverishly shoveling coal into the combustion chamber of the locomotive.

We're like the people on that locomotive when it comes to dealing with our population growth. Our leaders keep exhorting us to pull harder on our brakes, while other people keep working to add fuel to the fire that drives the engine. And nothing is being done to convince them to stop. For the most part, in fact, they are encouraged to keep shoveling, because hardly anyone seems to realize that their efforts feed the fire that is propelling this locomotive toward the cliff ahead.

Now if I was your typical population activist, I would tell this parable a little differently. I would still describe our population growth as a runaway locomotive on which we are passengers, each with a hand brake above the window at our seat, all being beseeched to pull on our brakes until the train stops. Instead of ending with the discovery of the workers fueling the fire, though, I would have praised the noble work of the people racing to lay track out in front of us, trying to buy us the time to stop the train with our hand brakes. Oddly enough, though, in the real world these are the very same folks I have described as fueling the fire. How can that be?

In both stories, our population growth is a locomotive with considerable momentum built up, pulling us toward catastrophe when we exceed the carrying capacity of the biosphere. In both stories we each have a hand brake, and those of us who choose to pull on this brake can help to slow the train. In both stories the hand brakes are the equivalent of voluntary family planning methods, which do help to brake the train but have not stopped it.

Where the stories diverge, however, is in what comes next. The conventional population activist view is that persuading everyone to use their hand brakes – family planning – is ALL we can do to end population growth. There are lots of things we can do to increase the likelihood that people will use the brakes, but not using the brakes is ultimately what causes our population growth. Convincing people that using them is the right thing to do is all that can stop the locomotive. Put another way, the conventional view is that, if this train is to be stopped, it will be stopped by billions of individuals making voluntary choices to limit the number of children they have. This view looks at population growth atomistically, as the sum total of billions of personal choices.

Conventional population activists are quite aware that the train is only slowing so much, though. They are also quite sure that it’s going to be a while before we can get everyone to pull their own brakes. Even when we do, these activists are convinced that the built-up momentum of the train will carry us for some unknown distance beyond that point. Consequently, they argue that we must not only work as hard as we can to get people to participate in the braking program, we also have to send workers out to lay track in front of the train so that we won’t run out before we can stop ourselves.

What does it mean to lay track? In a broad view, laying track encompasses all the things we do to increase our ability to provide for additional people. For the purposes of my parable, however, laying track means everything we do to increase the human food supply. In other words, we lay track when we increase food production to provide for the inevitable additional human beings.

This atomistic view, however, is a product of our cultural mythology. It ignores biological and ecological reality by treating humans and human population growth as if they aren’t fully subject to the processes of the biosphere. It assumes, in short, that we are exempt from the Law of Life that governs the population growth of all other feeder species, although there is absolutely no evidence to support this assumption. We aren’t exempt from the laws of gravity, or aerodynamics, or thermodynamics, or any of the other fundamental processes of the universe that we have begun to understand. For some reason, though, we insist on behaving as if we are exempt from the laws of population growth.

There is another way to look at our population growth – systemically. Instead of assuming that population growth is solely the result of individual, atomistic choices, the systemic view seeks to identify the fundamental mechanisms within the system which function to fuel our population growth. To understand the population growth of any feeder species, you have to understand the basics of ecology. Most basic is the notion that when the food supply of a population increases, that population will increase in response – more food means more feeders. The reverse is true, too, of course. When the food supply decreases, the feeder population decreases in turn – less food means less feeders.

This is the systemic process we see at work throughout the community of life: a perpetual oscillation of feeder populations rising in number as the food supply increases and declining in number as the food supply decreases – and all without famine except in rare, catastrophic circumstances. We have no problem recognizing this pattern among all the feeder populations in the community of life – except our own. Because we refuse to recognize that we are subject to these same systemic processes, we talk out of both sides of our mouths when it comes to population growth. We push for both voluntary family planning and increased food production, and then wail in fear as our population continues to grow by a billion people every dozen or so years.

Until we understand that increasing food production isn’t buying us time to stop population growth – that it is, in fact, fueling our population growth – we have no hope of stopping before we plummet into the chasm ahead. Until we stop stoking the engine, there is no reason to think that pulling on the brakes is going to stop this runaway locomotive.

Would you fly in a plane that was designed by someone who didn’t understand the laws of aerodynamics? Would you step off a cliff and expect not to fall? Only if you were ignorant or a fool, and we can’t claim to be ignorant any longer.