The credit crunch reveals at least one important thing about major policy decisions: once they are taken, one way or another, it becomes impossible to fully evaluate what the world would have been like without them.
Plenty of people claim that it was terribly necessary to bail out the banks and car companies, cut interest rates, and wreck the public finances by splashing out on every sort of tax cut and government spending program. They argue that doing so made this into a minor recession compared to a decade-long global recession. Having taken those actions, our ability to know what the world without them would have been like is very limited. The unknowable costs of inaction can always be used to respond to examples of cases where action currently seems unnecessary: “Yes, it was regrettable for the investment bankers to pay themselves off with taxpayer money, but the alternative to all this would have been a terrible global depression!”
The same will eventually be true of climate change mitigation policy. Say we eventually make it a real priority – setting a high price on carbon and really focusing on de-carbonizing our infrastructure. At some point in the distant future, we will look at our efforts and at how much climate change occurred. Provided it was a non-catastrophic amount, we will be in the same situation as we are now in relation to the credit crunch. We will be unable to know how bad things would have gotten if we had not taken action,
That being said, the stakes are enormously higher with climate change than with the credit crunch. While global economic turmoil would hurt, climate change risks destroying or severely degrading the capability of the planet to sustain human life. The risks associated with allowing it to occur to an extreme extent are practically incomparable.
It should also be noted that the credit crunch bailouts served a purpose that we must look on with increasing suspicion: maintaining economic growth. While we should certainly hope that human welfare will continue to improve, it is tautologically the case that we will eventually need to move to an economy in a steady state, when it comes to the resources it extracts from the atmosphere, lithosphere, hydrosphere, and biosphere, and in terms of the wastes it ejects into those places.