Under Bingaman's proposed legislation, the price of carbon would top out at $12 a ton in the first year, and would be capped at an increase of 5 percent above inflation each subsequent year. This would effectively decouple the price of carbon from the emissions reductions targets -- which makes the bill a non-starter as far as greens are concerned. He also wants all allowances to be given away rather than auctioned for the first five years, and wants the legislation to cover only petroleum, natural gas, and large coal-burning facilities (no transportation sector).

Not surprisingly, Bingaman's bill won accolades from the coal and nuclear industries, with groups like American Electric Power, Duke Energy Corp., Edison International, Exelon Corp., PNM Resources, PPL Corp. and NRG Energy Inc. backing it.

EPW chair Barbara Boxer, who is the floor manager for the Climate Security Act, has said repeatedly that she opposes the safety valve provision. The outline of her manager's amendment to the Climate Security Act that's been circulating on the Hill does include cost-containment language, though not a "safety valve" per se. "If the price of carbon allowances reaches a certain price range, there is a mechanism that will automatically release additional emission allowances onto the market to lower the price," reads the outline of the amendment Boxer has circulated. But it adds, "The additional allowances are borrowed so that the environmental integrity of the caps over the long term is protected." The outline doesn't include a dollar figure indicating when that mechanism would kick in.

It's unclear whether this concession will win the support of Bingaman and other fence-sitters in the Senate, but it's likely to be a source of anxiety for enviros. The bill already has several cost-containment measures built in, including a "bank and borrow" system that allows industries to save their allowances for future years and borrow from future allowances to meet their emissions goals. It also creates a "Carbon Market Efficiency Board" that is allowed to grant lenience on the rules on borrowing future credits if the price of carbon becomes too high. (It's unclear what the "off-ramp" would add to this system.) On top of that, Lieberman-Warner auctions only a quarter of the allowances in the beginning and ramps up to 73 percent by 2036 -- meaning a good portion of the credits in the first years are already going to be handed out to polluting industries.

Here's the list of witnesses for tomorrow's hearing:

• Brent Yacobucci and Larry Parker, specialists in energy and environmental policy for the Congressional Research Service
• Howard Gruenspecht, deputy administrator, Energy Information Administration
• Brian McLean, director of the Office of Atmospheric Programs at the Environmental Protection Agency
• Peter Orszag, director of the Congressional Budget Office

(Gruenspecht is a Bush I holdover who battled climate action during the Clinton administration, along with Orszag.)

Grist will have more on what's sure to be a fascinating noteworthy hearing tomorrow.