You thought old Mossback was an anti-density grump, but I'm not in the same league as Oregon's Les Knight, head of the Voluntary Human Extinction Movement. Maybe you've heard of the guy before, but I just read about him in Alan Weisman's fascinating book The World Without Us, an extended examination of what the world would be like if all people vanished tomorrow. It's a great thought experiment, but for some people, that world would be a dream come true, even if they weren't around to see it.

Weisman's exploration isn't really about the future in any meaningful sense, partly because the premise of a Rapture-like disappearance of the human race is highly unlikely, and partly because the time frames for imagining what would happen are so long – Weisman spends much of his time looking across centuries and millennia, in fact all the way to the end of the planet in 4 billion or 5 billion years, when the sun goes red and burns us to a crisp.

It's really a book about today, because it looks at our lasting impact on the Earth. It turns out the long-lasting traces we will leave are largely chemical: an altered atmosphere, radioactive substances with half-lives in the millions of years, plastics, dioxins, and PCBs throughout the food chain.

The collapse of our species' physical presence would be somewhat apocalyptic in places: More than 400 nuclear power plants would eventually melt down, Manhattan would collapse into flooded subway tunnels, chemical and gas plants would release terrible fumes into the atmosphere, dams would breach. But despite those nightmares, the planet would begin to heal rapidly from our depredations: Neighborhoods would quickly be consumed by forest (if you check out the book's Web site, be sure to watch the animation about what will happen to your abandoned house over the next 500 years), species would recover, the climate would, perhaps slowly, begin to regain some equilibrium.

Knight, a Portland schoolteacher, sees voluntary human extinction as the way to save the biosphere. Unborn children have a much bigger positive impact on the environment than the most avid of recyclers. The best gift we could give the planet, he says, would be to phase ourselves out. Killing people doesn't work, he argues, because, first, it's immoral and, second, it's counterproductive (or counter-reproductive?) because wars tend to increase the birth rate. An epidemic that killed 99.99 percent of the population would leave enough people – around 650,000 – to repopulate the planet at its current level over the next 50,000 years.

The humane, dignified, and moral way to bow out (and it's virtually certain that we will eventually become extinct, anyway) would be if mankind turned into one of those Shaker religious sects that practiced strict celibacy and stopped accepting converts. Eventually, the Shakers passed gracefully out of existence and left behind lots of very cool furniture. Allowing for increased life spans, if everyone stopped having babies today, people would be gone in 150 years or sooner, Knight projects. Knight lays out his views in this 2006 interview in Radar. You can decide for yourself whether he's nuts or not.

In the book's final chapter, Weisman plots a middle course and looks at what would happen if we adjusted the birthrate downward. No extinction, but a rollback to a more sustainable population level, one that could live within its means.

The Earth's population is around 6.5 billion people, headed for 9 billion by mid-century. He reports that one set of calculations, using the projected life expectancy for 2050 and cutting the 2004 birthrate of 2.6 births per female to one birth per, shows that the world's population could be reduced by a billion people in about 50 years. By 2075 it would be down to 3.43 billion. By 2100, we'd be at 1.6 billion, or 19th century levels. The result would be a kind of low-density Eden. Writes Weisman:

At such far more manageable numbers ... we would have the benefit of all our progress plus the wisdom to keep our presence under control. That wisdom would come partly from losses and extinctions too late to reverse, but also from the growing joy of watching the world daily become more wonderful. The evidence wouldn't hide in statistics. It would be outside every human's window, where refreshed air would fill each season with more birdsong.

Lovely. But would it last? Even without humanity poisoning the planet and using up the atmosphere and resources at gluttonous levels, we're likely to get smacked by an asteroid or some such. We're about due. Two hundred and fifty millions years ago, the Permian Extinction saw 95 percent of all living things wiped out. That led to an era when clams – yes, a monarchy of mollusks – ruled the planet, but eventually it led to fuzzy little rodents and eventually to us.

Who knew we'd grow up to be the first species to menace the planet itself? And who says that even if we become extinct that there won't be some other new kid on the block in another few hundred million years, ready to screw things up all over again? I'm sure Gaia takes a worldly view of such things.