If all the residents of North America and Western Europe cut their per-capita energy consumption in half over the next 20 years (not likely) and the rest of the world held per-capita consumption steady at their current, frugal levels (also not likely), total energy consumption will remain the same. A 50-percent reduction in the developed world will not be sufficient to outweigh population increases in the developing world, even if the increasingly affluent residents of developing countries don’t increase their energy consumption.

Someone’s going to object to my evidence. Maybe it will take 75 years to reach 10 billion population. Maybe the planet can accommodate 12 billion frugal human beings. The rate of population growth is not the issue. Any growth at all creates the same ultimate dilemma. Sure, we might figure out ways of accommodating 10 or 15 or 20 billion people in a crowded world. But why would we want to?

If ultimately we must control our population, why not plan for a rich, healthy planet?

What if we decided, by mutual consensus, that a stable worldwide population of 4 billion people is our goal? Could we then live on a planet with clean air and water, plenty of food for everyone and the environmental resilience necessary for us to prosper through the inevitable environmental fluctuations – the next ice age, for instance? Could we restore habitats now teetering on the brink of destruction?

Couldn’t we create a sustainable healthy planet just because we decided to?

We could take this philosophically, I suppose. A few decades or a few centuries after we disappear there will be a healthy planet here. Or we can see it fatalistically. The damage we are doing is part of a natural process. Our awareness doesn’t change that essential fact. We can even salve our guilty consciences by resorting to the geologic perspective. Eventually this planet will suffer some sterilizing galactic calamity. Scientists tell us our sun will, eventually, burn out.

But it’s not our nature to sit around complacently waiting for the asteroid, not while we have this miraculous opportunity to preserve and enhance our planet. Just as we once visualized the first irrigated field, invented the first wheel and dreamed of machines that fly, we can visualize the earth as a beautiful and productive garden where millions of species thrive. Then we can build it.

Unfortunately, it seems to me that we, as a species, are training our attention on the middle of the decreasing-circumference curve in which we find ourselves. We are not visualizing the successful outcome – a healthy planet. Conservation has captured the human imagination lately and some great new inventions have come from this new fascination – the gas-electric hybrid engine; photovoltaic solar energy, wind-powered electric turbines, the hydrogen fuel cell. This is cool stuff. But it’s stopgap stuff.

The best product of our fascination with conservation is that it has captured people’s imaginations. And it’s the key component in a new human philosophy that values other living things. If we consume less, we leave more room for our biological neighbors. That’s a great thing.

On the other hand, short-term thinking distracts us from the underlying problem. At current rates of population growth, there will be 10 billion people on the planet in about 60 years. When there are 10 billion people on the planet it won’t matter what they drive or if they’ve all committed to vegan diets. The planet will be under human assault in a battle where everyone loses. We could hit that guardrail.

In one sense it’s a terrific time to be human. We’re here to meet our biggest challenge so far – bigger than bipedal locomotion; bigger than the domestication of plants and animals; bigger than the invention of the wheel. We’re here to confront our own biology, the essential nature that tells us to keep reproducing and expanding. If you could view the entirety of human experience from the dawn of our evolution to the present, if you could pick the human century you’d like to witness first-hand, you might choose this one. I think I would. I would want to watch us tackle this problem.

The suffering, if we don’t get it right, will not be humanity’s alone. Already we’ve destroyed thousands of species. In just the last few years Africa’s Western Black Rhinoceros, Europe’s Pyrenean Ibex, Costa Rica’s Golden Toad and North America’s Pearly Mussel have, so far as we can tell, passed into oblivion as humanity has destroyed their habitats. The scientists of the World Conservation Union estimate that 99 percent of recent extinctions and currently threatened species have been or will be destroyed by human activities. Conservation International reports that, as of the middle of 2008, a plant or animal species was becoming extinct every 20 minutes.

Extinction is normal, of course. The vast majority of species that ever lived seem to have disappeared somewhere along the line. What’s not normal is the rate of extinction. The rate of extinctions has been accelerating since the beginning of the 20^th century and we’re responsible.

It’s no great tragedy that any particular species becomes extinct, unless of course it’s us. Generally, it has been part of nature’s way and each extinction opens opportunities for other species.

The greater tragedy is the fact that we’re taking a healthy, resilient and rich natural habitat – the only planet we know where life thrives – and degrading its ability to support life. New species can’t evolve fast enough to replace the diversity we’re destroying, even if we hadn’t made the habitat inhospitable. We’ve inherited the best planet in the known universe, only to squander it. And if we don’t change course soon, the planet could very well end up unfit for human habitation or at the very least damned uncomfortable.

Now is the moment when our uniquely objective perspective and our enterprising intellect are engaged in what may be the most important challenge faced by our species so far.

Other species have damaged their habitats or lost them to environmental disaster. The dinosaurs, the Saber-toothed Tiger and the Woolly Mammoth died out. Many species routinely go through periods of catastrophic population collapse and reestablished themselves in some new biological equilibrium. Lemmings spring to mind. But none of them, so far as we know, are consciously aware of the natural forces at work. They couldn’t conceptualize the fact that their own reproduction, their natural consumption and expansion, played a part in causing the pain of their population’s collapse.

Nature has lots of tools at her disposal for controlling species that cause habitat damage. Famine and disease are her most potent weapons, effective and unpleasant.

We, on the other hand, can conceptualize our effect on the environment and we might, if we wish to, avoid the suffering Nature will inflict.

And we could restore the astonishing garden into which we were born – the Earth.

I can’t think of a more inspirational goal.