*I've just realised that I first used this image back here in response to Sam's analogy.
To push this picture a little further, perhaps we could imagine that we are driving on a narrow road on the side of a tall cliff. We've been driving too fast and are out of control. The outcome could involve sailing through the barrier over the precipice or crashing into the cliff-face that rises above us on the other side. It seems to me that worrying purely about the economic costs of ecologically responsible action is a little like obsessing over not crashing into the cliff face. Sure, crashing into a cliff face would be bad, and in normal circumstances you want to avoid it. But in our case, already speeding and out of control, better the cliff face than over the edge of ecological destruction. Why better? Because economic damage might last years or decades; ecological damage might last decades, centuries or millennia.
Perhaps a simpler image is speeding along a two lane road and finding a stopped or very slow truck up ahead. We can slam on the brakes and probably slide into the back of the truck, in which case, the sooner we hit the brakes the better. Or we can swerve into the other lane and risk a head-on collision with oncoming traffic. Since we can't see around the truck, we just have to hope there is a gap. In the past, it has usually made sense to swerve and keep accelerating, but traffic is increasing.
Perhaps this analogy is reaching its limits. The point is, we often pay too much attention to the wrong threat.
Image by BuenosAiresPhotographer.com, used under creative commons license.