Environmentalists gave generally good marks to Sen. Dianne Feinstein's new climate bill because they say it commits to immediate reductions in greenhouse gases, with an initial target of returning to 2006 levels of emissions by 2010 and making gentle but steady cuts totaling just more than 7 percent from then until 2020.

Feinstein pointed to signs of warming through-out the world, from more severe storms to masses of ice flowing into the oceans at the poles.

"The clock is ticking on global warming," she said in a statement. "If we do not slow, stop and reverse global warming soon, we will do irreparable harm to the world around us."

Her language is familiar to theorists who have been searching for ways to cut greenhouse gas releases without hurting energy supplies and crippling the economy.

"That is actually a pretty significant reduction, because that's avoiding a fairly significant wedge of increased emissions between now and 2010," said Dallas Burtraw, an economist at the nonprofit, nonpartisan research institute Resources for the Future. "Most economists embrace the notion of slow-stop-reverse as a strategy for carbon policy that would minimize the economic costs of climate policy,"

Many industries heavily reliant on fossil fuels, especially energy, steel and concrete producers, favor a more gradualist approach, such as one proposed by Sen. Jeff Bingaman, D-N.M., which would slow the accelerating growth of emissions annually to an eventual stop and reversal, he says. Industries could move to lower-emitting technologies — say, switching from coal to natural gas or capturing carbon-dioxide for injection into underground reservoirs — as those become economical.

Feinstein's prescription for an immediate cap and then reductions, said Tom Wilson, a climate policy researcher at the Electric Power Research Institute in Palo Alto, "is much more costly according to all the economic studies than to slow emissions growth, then stop it."

"The economy is like a ship and you need to slowly turn it, then the emissions peak and then decline," he said.

Climate scientists say the global temperature is creeping up because of greenhouse gas emissions decades ago and that it already is too late to make cuts as slowly as Feinstein's bill would.

"Capping emissions at today's level is not going to make things better," said Phil Duffy, a senior atmospheric scientist at Lawrence Livermore National Laboratory. "That by itself is not going to make a big dent, because the greenhouse gases do accumulate in the atmosphere. But the political significance is taking some sort of step. It would be useful to have even a small program of that sort to understand how it works."

Avoiding serious global warming impacts means cutting greenhouse gas emissions in half at least in the developed world and then in the developing world by 2050, said David Doniger, policy director for the Natural Resources Defense Council's Climate Center in Washington, D.C.

"You've got to get to it right now," he said. "A slow start means a crash finish."

Feinstein's bill, said Doniger, "has the right idea."

Environmentalists and industry representatives are debating how large the U.S. carbon market will be, what industry sectors should be included and when. Feinstein's bill would define the market as the overwhelming majority of the nation's biggest stationary contributors to greenhouse emissions, from power plants and steel factories to refineries and paper mills.

That market is huge, and exactly how the currency of the market — permits to emit greenhouse gases — is issued could bear heavily on the prices that ordinary consumers pay to power their homes and fill up their cars.

"It's sort of like the railroad land giveaway of the late 19th century that imparted value worth enormous sums of money. That's what's going on now," said Burtraw of Resources for the Future. "Sen. Feinstein's playing a lead role in something as important as social security in decades hence."

Feinstein provides for industrial emitters to buy credits from farmers and foresters growing plants that absorb carbon dioxide or other nations investing in projects to reduce greenhouse emissions worldwide. But she left open, for now, the crucial question of whether the initial greenhouse pollution permits, called allowances, will be given or auctioned to industry, or both.

"The problem with the free giveaway of emission allowances is it dramatically overcompensates the industry," said Burtaw. That's become a problem in the European Union, where a 2-year-old carbon market appears to be providing utilities with windfall profits.