As anticipated, Sens. Boxer and Kerry today released their draft cap-and-trade (cap-and-tax) bill (full text PDF, 821 pages). It claims to be more severe than the House version, imposing, for example, "a mandate by 2020 to curb the nation's greenhouse gas emissions by 20 percent from 2005 levels."
The CBO says (PDF) that the House cap-and-tax would hurt the economy by 1% to 3.5% by the year 2050 which is more than the anthropogenic global warming even using "relatively pessimistic estimates" for the AGW damage (only 3% of GDP or so in the distant future: 2100),.
CBO is paid by Congress so, naturally, that comparison does not call attention to the much worse actual cost-to-benefit ratio. Even if one accepts UN physics, (1) "pessimistic estimates," because they are pessimistic, overstate, even by UN standards, the likely AGW damage, (2) the benefit of cap-and-tax is only the fractional reduction it would cause in hypothetical damage done by AGW, not the total hypothetical damage, and (3) the CBO is aware that cap-and-tax, even if fully implemented, merely reduces the rate of growth in CO2 gases. Since US CO2 emissions are only 20% of world-emissions and that fraction is declining as the rest of the world industrializes, the saving from reduced AGW, if there is AGW, would only be a small fraction of 1% of GDP by 2100. Consequently, the distant supposed benefits are much much smaller than the more-near-term loss to the economy due to cap-and-trade itself.
RELATED: The Institute for Energy Research (IER) says Cap-and-trade is a regressive tax. (Source warning: the left objects to IER because IER tilts toward free-market solutions.)