All three presidential hopefuls would cap emissions at a certain overall level. Anyone wishing to emit more than their cap would have to buy pollution permits from someone emitting less. The total number of permits would be ratcheted down over time. Sens. Clinton and Obama both say they would cut emissions 80% by the year 2050. Sen. McCain aims for a slightly lower 65% target.

Whether these goals can be met without dire economic dislocation is a matter of conjecture.

Last week, at The Wall Street Journal's ECO:nomics conference, optimists abounded. The business executives, entrepreneurs and policy experts who gathered in Santa Barbara, Calif., for the two-day conference mostly thought new technologies could make the task a relatively painless one. Seventy-five percent of those in attendance said the candidates' goals were "feasible"; only 25% said they weren't.

The consensus was that a cap-and-trade system would spark innovation that Silicon Valley Venture Capitalist John Doerr called "the largest economic opportunity of the 21st century."

The optimists may be right. But it's prudent to consider the possibility that they might be wrong.

Let's be clear: a carbon cap-and-trade system is really just a tax, of indeterminate size, by a different name. In effect, it taxes carbon emitters however much it takes to achieve the emissions goal.

Sens. Clinton and Obama have rejected calls from utilities and other businesses to "allocate" the initial polluting rights based on a company's current emissions. Instead, they want to auction off all the pollution rights, making the worst polluters pay a bundle just to keep operating. (Sen. McCain is less clear on this point.)

Duke Energy CEO James Rogers, who was one of the first in his industry to endorse cap-and-trade legislation, complained that for a coal-fired utility like his, the auction approach is tantamount to starting a game of strip poker with no clothes on.

In the end, of course, consumers will pay this bill, as business passes the costs on. The result could be a heavy new burden on people already feeling the pain of stagnant wage growth and soaring energy prices.

For now, that pain may seem pretty hypothetical to most voters. They are willing to give candidates kudos for getting serious about global warming. At the same time, candidates are using the tens of billions of dollars they hope to raise from auctioning off permits to hypothetically pay for other costly campaign proposals.