Posner and Becker on Carbon Offsets

At the Becker-Posner blog they are discussing carbon offsets. Posner's comments are here while Becker's discussion is here. Posner concludes his comments with:

Against this it can be argued that the carbon-offset movement is increasing the public awareness of the global warming problem, which may lead to other voluntary efforts to reduce carbon emissions, such as switching from SUVs to more fuel-efficient vehicles, or may exert pressure on politicians to support the regulation of carbon emissions. I am skeptical. I think very few Americans are prepared to incur substantial costs to deal with a problem that is so afflicted by uncertainty about its imminence and magnitude as global warming. They will avoid cognitive dissonance by exaggerating the practical efficacy of largely symbolic gestures, such as purchasing carbon offsets.

Becker concludes in favour of a cap and trade system:

In our complicated and interdependent global economic system, opportunities to create carbon offsets can be readily produced by both companies and governments without any significant affect on the scale of emissions. Mainly for this reason, but also because of the reluctance of most individuals to voluntarily pay significant costs for acting "green", a cap and trade system, despite its many flaws, is a far preferable direction to develop in order to cut down on carbon emissions.