We greens spend a lot of time obsessing about how life as we know it is likely to end: in a slow, painful miasma of greenhouse gases; in the violent cross fire of a nuclear gang war; in mass ignominy, dead and bug-eyed in our folding chairs after endless rounds of fruitless policy discussions. But what the heck do we really know? Before the car was invented, people worried that the whole world would eventually be knee-deep in horse manure. Really, they did.

Keep it handy, just in case.

Photo: iStockphoto.

Environmentalism, by definition, is about life and death. But what kind? Depends who you ask. I talked to a few people to identify the most common end-of-the-world, planet-busting scenarios. Then I talked to a few more to find out how likely it is that those things will happen.

My point is not to make you throw in the towel, but to learn to accept the things we cannot change — and continue to work at a frenzied but life-affirming pace to change the other ones, before they do us in.

Nanotechnology, the manipulation of wee things like molecules and atoms, may have enormous, positive implications for the planet. But big-frontal-lobes like nanotechie Eric Drexler and Sun Microsystems cofounder Bill Joy have predicted that self-replicating nano-robots will eventually run amok, converting everything into “gray goo.” You think it’s funny? Try this on for size: Adidas has introduced a running shoe with a microprocessor capable of performing 5 million calculations per second so you can avoid shin splints. You’re gonna be in deep goo indeed when your smelly trainers hack into NORAD from the bottom of your gym bag.

The bad news: Some critics — such as the Canadian nano-watchdog ETC Group, authors of the controversial paper The Big Down — fear that we will goof and alter life irrevocably. Thus, they advocate putting the brakes on nanotech.

The good news: According to Christine Peterson, founder of the Foresight Nanotech Institute, nano-scientists are developing neato green stuff like cheaper, stronger solar cells, and are working toward earth-loving goals such as zero-waste manufacturing and new chemical-remediation techniques.

Big-Ass Impact

Something to think about during yoga: the earth resides in a swarm of 300,000 or so asteroids that travel around the sun with us like pesky gnats. The probability of a weighty asteroid hitting our planet is slight, but such an impact could be substantial. According to the B612 Foundation, a group of scientists aiming to alter the orbit of asteroids on humanity’s behalf, a large (one-kilometer diameter) Near Earth Asteroid would explode with the energy of 70,000 megatons of TNT if it hit our planet. Holy vinyasa! While 65 percent of the one-km NEAs have been identified as non-threats, 35 percent remain an unnerving mystery.

The bad news: These kindly scientists need cash, international cooperation, and leadership. “No one is responsible for protecting earth from asteroid impacts,” explains Rusty Schweickart, chair of the B612 Foundation (the name comes from the title character’s asteroid home in The Little Prince).

The good news: According to the B612 folks, we now have the capability to anticipate and prevent an impact. They even designed a space tractor to tow or push away an NEA. And according to Near Earth Asteroid Tracking — a celestial observatory funded by NASA to study asteroids and comets that goes by the happy acronym NEAT — big asteroids impact the earth only once every 1,000 centuries on average.

Big-Ass Eruption

Krakatoa and Mount St. Helens are mere pimples compared to supervolcanoes — biggy-sized pustules capable of spewing enough magma, dust, and chemicals into the atmosphere to alter life on a global scale. Yellowstone National Park, that suppurate land of wolves and geysers and snow machines, is the caldera of a supervolcano, a source of wild internet rumor and Pompeii-ish dread. According to the Yellowstone Volcano Observatory, another catastrophic caldera-forming eruption would likely alter global weather patterns and, um, “human activity.” Can you hear the distant drums? Oh my God, people, that’s my heart beating!

The bad news: According to the U.S. Geological Survey, hazardous volcanic activity will continue, and because of increasing population, development, and air traffic, human exposure to it is increasing. Oh, and when it comes to preventing a volcanic eruption, we can’t do diddly.

The good news: The ash from an eruption makes for one heck of a pretty sunset … no, seriously: Programs like the USGS National Volcano Early Warning System can assess the hazards and alert those at risk. “It’s not as if something is going to go kaboom in the middle of the night and nobody is going to know about it,” says Tom Murray, a USGS scientist based in Anchorage. And although Yellowstone sits above a hotspot, YVO has not detected evidence of an imminent eruption.

Ready, set, glow.

Photo: iStockphoto.

Nukular Annihilation

Remember when Sting hoped the Russians loved their children too? Join me now in hoping that the uranium-enriching nations of France, China, Great Britain, India, and Pakistan (and possibly Israel, North Korea, and Iran) are all concerned with posterity. Oh, and speaking of generations, you’re probably wondering what time it is on the Doomsday Clock created by the Bulletin of the Atomic Scientists in 1947 as a symbol of nuclear danger. Answer: It has been seven minutes to midnight since 2002. The clock will be re-calibrated in 2006, taking into consideration not only nukes but also other threats to humankind, such as biological weapons.

The bad news: Some people out there are seeking the biggest, baddest weapons of mass destruction they can get, and may not be concerned with Bulletin of the Atomic Scientists-type publicity stunts. (Of course, there are those — and I won’t name names [IRAN] — who claim they’re just gathering the ingredients for civilian power.)

The good news: According to Kennette Benedict, executive director of the Bulletin, the U.S. and Europe could turn back the clock if they collectively spent a mere $10 billion in the next few years to secure the remaining bomb-making nuclear material still in Russia. “It’s a doable thing,” she says.

The Rapture

Some evangelical Christians believe that, at a conveniently unspecified moment — perhaps during Nip/Tuck — true Christians will be transported to heaven in an event known as The Rapture. According to the Rapture Index (a thinly disguised blog of superstition and hate as far as this reporter can tell), you’d better throw some extra granola bars in your heavenly go-box, because we’re at “fasten your seatbelts.” Among the Rapture Index’s somewhat redundant categories are climate, wild weather, and floods, all top scorers at five points apiece. “Beast government,” meanwhile, brings in a disappointing three points.

The bad news: End-timers tend to believe “things are getting worse and worse and there’s nothing human beings can do about it,” says Bruce David Forbes, a religious studies professor at Morningside College. “If you have that view, why would you try to improve anything in the world?”

The good news: Not only do many mainstream Christians not believe in the Rapture, it’s also as likely to happen as, say, Pleistocene rewilding. As Forbes, coauthor of Rapture, Revelation, and The End Times: Exploring the Left Behind Series, points out, history is littered with people who thought they knew when the end was near.

Coming to a Boil

Last but not least is everyone’s favorite: the death-by-carbon-emissions scenario. But exactly how does global warming kill? Will we get swept up in a swirl of chaotic weather, drown in a pool of melted ice sheets, or succumb to a bevy of hot-weather-loving diseases? All of the above. Maybe. According to Susan Joy Hassol, one of the lead authors of the Arctic Climate Impact Assessment, we’re already committed to about another 1 degree Fahrenheit of global warming in this century, which our species could likely adapt to, albeit at some cost. But a worst-case scenario — in which we lollygag on coming up with energy alternatives and instead burn all the oil and coal we can scrape out of the earth, thus raising average global temperatures 5 to 10 degrees F — involves, at least by my interpretation, the following Rapture Index categories: floods, plague, wild weather, oil supply/price, global turmoil, beast government, and apostasy.

The bad news: While the rest of the world is trying to deal with this issue, the U.S. sorely needs a national policy that limits CO2 emissions. “We’re still speeding in the wrong direction,” says Hassol.

The good news: By taking the necessary measures to address global warming, the U.S. could also decrease our dependency on foreign oil, clean up our air, improve our health, and boost our economy. “We can slow the rate and magnitude of global warming,” says Hassol. “We have the technologies and we know what we have to do.”

The really good news: We might even do it.