It really may be impossible to mock liberals. No matter how absurd a proposal seems, a liberal can always pretend it’s serious and even take it one step further.

Say all human beings agreed to leave the planet by blasting themselves into space. Environmentalists would complain that the rocket they left in was damaging the planet with its toxic emissions.

Luckily for libs, though, author Alan Weisman has come up with a neater way for us to leave the planet. Not with a bang, but with a whimper. And that whimper would be the sound of billions of people around the world choosing not to procreate. His book is called “The World Without Us,” and Slate magazine calls it a “strangely comforting vision of human annihilation.”

Comforting? Speak for yourself. I kind of like it here.

Oh, but things will just get better and better when we’re gone, the book claims.

“Nature lovers can take solace in the idea that the planet will thrive once we’ve finally destroyed ourselves with global warming,” Daniel Engber writes in Slate. “Freshwater floods would course through the New York subway system, ailanthus roots would heave up sidewalks and a parade of coyotes, bears and deer would eventually trot across the George Washington Bridge and repopulate Manhattan.”

Two observations here: If it’s such a great idea to pump water through the subways, why are we fighting global warming? We ought to be speeding it up to raise ocean levels and inundate our costal cities so much more quickly. Also, who’s going to maintain the George Washington Bridge so the animals can stroll across it? As we saw in Minnesota, even the bridges we humans are actively trying to maintain sometimes collapse. Surely they’d all fall apart pretty quickly if we weren’t around to repair them.

The Slate piece starts with the assumption that greenhouse gasses will destroy humanity. If that’s true, the first thing to do would be to kill all the cows. After all, as a U.N. report last year noted, “Cattle-rearing generates more global warming greenhouse gases, as measured in CO2 equivalent, than transportation.”

But the Slate story misses the implications of its own observations. “Our kids won’t spew as much greenhouse gas as we do—automobiles, appliances, light bulbs, and everything else will become more efficient in coming generations,” Daniel Engber writes in passing.

Well, of course they will. Because humans, the planet’s greatest resource, will make everything more efficient. Unless, of course, we follow the author’s advice and stop reproducing. In that case there won’t be enough humans to solve the planet’s future problems the way we’ve solved its past problems. It wasn’t so long ago that the Cuyahoga River was on fire and Lake Erie was dead. Today they’re cleaner than ever. Once, TV stations across the nation ran an ad featuring an Indian crying as a passerby littered. “People start pollution, people can stop it,” the ad warned. Well, we did. We’ve cleaned up litter, cleaned up the water, cleaned up the air. And become more fuel efficient in the process.

For example, just a few hundred years ago we got most of our energy from wood. A scientist projecting humanity’s future in 1786 would have predicted disaster. Americans were hacking down forests at an alarming rate to create farm land, generate fire wood and build ships. Then we discovered coal and petroleum. Where once we cut down trees and killed uncounted numbers of whales (for oil), we now plant trees and protect whales.

We’re the only species on the planet that’s even capable of thinking about protecting the world around us. If a pride of lions could kill every zebra in Africa, it would, and there wouldn’t be any more zebras. The lions would never get together and say, “Hey, there are fewer and fewer zebras these days. Let’s kill some gazelles instead and let the zebras repopulate in a protected area.”

Or consider our predecessors, the dinosaurs. They seem to have died off when a meteor hit the planet. Of course, they never saw it coming and never knew what hit them. They simply disappeared.

We humans might at least be able to track the rock that would destroy us, and maybe even take a shot at it. We might fail, as in the 1998 Tea Leoni movie “Deep Impact.” Or succeed, as in the 1998 Bruce Willis movie “Armageddon.” But at least we’d have a shot.

Without humans around to protect them, lions, tigers and bears will face extinction when a meteor inevitable drifts in this direction. Ironically, though, while most other life might be killed off, human adaptability might just allow us to survive even a direct meteor hit. Don’t bet against us.

What’s amazing is that humans can be smart enough to study our environment and write books, yet many of us aren’t smart enough to realize that humanity will be the solution to -- not the cause of -- our planet’s problems.