I don’t completely understand the concept of global warming because I haven’t spent my whole life studying how climates work and, at this point, I wouldn’t want to start trying. But I get the basic idea. We’ve been dumping so much crap into the air for so long that it’s come time to pay the bill.

I believe that. Here’s why. I used to live in Denver, Colorado. The city is famous for being a mile above sea level, but it actually sits in a bowl. When you drive in from the east, you’re higher and looking down at the city, and in the west, of course, are the Rocky Mountains, so the city sits in a depression that traps all the crap that comes out of every chimney and exhaust pipe. The result is The Brown Cloud. When I lived there, the local radio stations would tell you daily how bad it was.

Some days, I didn’t notice The Brown Cloud at all. The sky appeared to be so clear that it seemed to be a brighter, more vibrant shade of blue than I’d ever seen anywhere else. On most other days, though, I could dimly see a haze that discolored the sky with a vaguely purplish tint, and on really bad days, when a layer of cold air blanketed the city and trapped The Brown Cloud in the bowl around Denver, the sky was a dingy brown, and going outside was unpleasant.

On any day at all that you were up in the Rockies looking east, you could see The Brown Cloud smothering Denver and reaching all the way out onto the prairie.

None of this describes global warming, but living in a city that was buried under a permanent cloud of crap gave me an appreciation of just how fragile the world is, and made me realize that the crap isn’t going anywhere. It’s there all the time. The sky seems huge, infinite even, when we look up at it from the ground, but I’ve gazed across the edge of breathable air from the tops of The Fourteeners, and I can tell you it’s not. It stands to reason, then, that there’s a limit to how much crap we can dump into it.