Something must be done.
This is something.
Therefore, this must be done.
With both McCain and Obama proposing cap and trade systems, it looks like that's what we'll get.
I'd rather just have a straight greenhouse-gas-tax. It's too easy to cheat with cap&trade (as Europe seems to be finding out-- it looks like their system isn't working very well), and cap&trade creates more inertia-- everybody who makes money trading emission permits under a cap&trade system will lobby VERY hard against changing a system that pays their salary. If, 50 years from now, somebody figures out a clever way of either reversing global warming or if it turns out rising temperatures and ocean levels isn't as big a problem as we think, we'll be stuck with an antiquated system that just makes a few rich people richer.
A tax would be simpler and more transparent. Which, I think, is part of the reason it has no chance of being adopted.
I think the other part of the reason is because "tax" is a dirty word. Nobody likes taxes, and politicians bend over backwards to avoid using the "t" word ("It's not a tax, it's a user fee!").
But Cap and Trade has something for everybody! Lefties like the Cap part-- we'll Cap emissions and save the environment! And Righties like the Trade part-- we'll Trade our way to economic bliss!