President Bush, for the first time, mentioned it during a State of the Union speech Tuesday when he said his energy policies would help confront "the serious challenge of global climate change."
A day earlier, 10 major corporations, including PG&E, Bank of America, Dupont and General Electric, joined with national environmental groups such as the Natural Resources Defense Council and Environmental Defense to call for a nationwide cap on greenhouse gas emissions.
The central debate is no longer whether the earth is warming or whether man has a hand in it.
It is what the impact of climate change will be and what should be done about it.
Now, the debate gets thorny.
Because the extent of future warming is uncertain, it is difficult for policymakers to decide on appropriate responses.
California policymakers have responded aggressively with laws and regulations to curtail tailpipe emissions and commit the state to reducing greenhouse gas emissions.
The federal government so far has done little by comparison and has opposed California's tailpipe regulations, contending they tread on its authority over vehicle mileage standards.
But some major corporations are urging the federal government to cap emissions because they say it makes sense for the economy and the environment to start cutting emissions soon. The companies include several who face significant financial burdens if greenhouse gases are regulated
"Each year we delay action to control emissions increases the risk of unavoidable consequences that could necessitate even steeper reductions in the future, at potentially greater economic cost and social disruption," the U.S. Climate Action Partnership said this week.
the companies formed the alliance with environmental groups out of a mix of social responsibility and long-range business planning, said PG&e spokesman Shawn Cooper.
"All of the companies in some form or another recognized that global warming is real, that we are part of the cause and that something needed to be done," Cooper said.
They decided it made sense to take the initiative so they could have a hand in solutions.
"It's like your budget. If you have trouble with your budget, it's better to deal with it in January than in November," Cooper said. "By November, it's too late."
The partnership called for a mandatory cap on emissions and a system that would allow companies to buy and sell pollution "credits."
In April, the state Air Resources Board is expected to consider the first set of regulations under a law that commits the state to cutting emissions by 174 million tons. That law requires California to cut emission to 1990 levels by 2020.
The first rules are expected to include limits on the carbon content of gasoline and prohibiting retail sales of car refrigerant. The full plan to bring emissions back to 1990 levels is expected next year.
Meanwhile, state regulations that would cut greenhouse gas emissions from new cars sold in California beginning in 2009 are tied up in court. Those regulations are expected to curtail new cars' carbon dioxide tailpipe emissions by 30 percent by 2016. The fate of that lawsuit is tied to a pending U.S. Supreme Court ruling in a separate case in which states are trying to force the federal government to regulate carbon dioxide emissions.
The state's greenhouse gas regulations are likely to increase the cost of energy and new cars, but state officials say energy efficiency improvements could offset the added costs.
"It's not at all clear that there will be a pocketbook impact," said Chuck Shulock, the air board's program manager for greenhouse gas emissions.
The federal government also appears poised to seriously consider greenhouse gas reductions.
In his State of the Union speech, Bush called for a 20 percent reduction in gasoline consumption, increased use of alternative fuels and reforms in the way mileage standards are set.
The November election that turned control of the Senate to Democrats by the slimmest of margins led to a 180-degree philosophical turnabout on its key environment committee.
Sen. James Inhofe, R-Okla, who called catastrophic global warming a "hoax," had led the Energy and Public Works Committee.
After the election, the chair went to Sen. Barbara Boxer, D-Calif., who has called warming a serious threat and called for slashing emissions by as much as 80 percent by 2050.
Boxer has scheduled a hearing next week on how to approach climate issues. She plans several more.
"I'm going to take the pulse of the Congress and see what the possibilities are," Boxer said in an interview Friday. "California is certainly a model for me."
"If we get a really good bill out of the Senate, it will make waves," said Boxer, adding that she wants to push the issue into the 2008 presidential campaign.
"People are clearly really concerned," she said.
There are numerous proposals.
For example, California's other senator, Dianne Feinstein, plans five separate bills to address fuel efficiency, power plant emissions and other greenhouse gas sources.
Still, new federal laws to address climate change face significant hurdles. In the Senate, Democrats such as Boxer and Feinstein will have to attract significant Republican support, even if they get the full support of their own party.
In the House, the legislation would probably have to pass the Energy and Commerce Committee, now led by Rep. John Dingell, a Michigan Democrat who might be hesitant to move legislation that harms the auto industry or its workers.
For a variety of reasons, global warming has taken a much higher profile in recent months and years.
Former Vice President Al Gore released an attention-grabbing documentary. Polar bears were proposed for protection because of melting habitat. Scientists became more vocal. Hurricane Katrina, though not directly attributable to global warming, raised awareness of the links between warmer oceans and hurricane intensity.
Even the most skeptical of climate change, by and large, agree it is occurring.
The Competitive Enterprise Institute, a Washington policy group that is among the most vocal critics of climate change responses, said in a recent news release that it "does not deny that the global mean temperature is rising and that human activities may be responsible for some or all of the warming."
Asked if anyone anywhere credibly maintains that the earth is not warming or that humans are not responsible for some of it, the group's director of global warming and energy policy could not name a single source, though he said that there were a few out there.
Myron Ebell contends, however, that models of future climate grossly exaggerate the threat.
"The rate of warming is extremely modest, and the second point is I don't think there is any real scientific reason to think that the rate of modest warming will increase," he said.
Computer models forecast an estimated increase of 2 degrees to 5 degrees in California by mid-century.
As sophisticated as those models are, they are inexact. The climate is big and complicated, and trying to project emission levels of greenhouse gases, how various phenomena will play off each other and then forecast how that will affect a specific place is a massive scientific undertaking.
Still, many climatologists say they now have enough information to act.
"No matter how much predictive capacity we have, until it starts to happen, people aren't going to believe this," said Kelly Redmond, regional climatologist at the Western Regional Climate Center in Reno. "To some extent, if we wait until it happens, it's too late in some aspects of this."
"It's very clear that we're going to see more warming, but is there a worse outcome?" Redmond added. "The time to be thinking about that is about now ... to improve the odds of ending up in the middle range rather than the higher range."
Still, no matter what is done now to curtail emissions, climatologists say further warming is inevitable.
Carbon dioxide, the chief greenhouse gas, is long-lived. The gas would remain in the atmosphere for decades even after immediate, drastic reductions.
And the earth has absorbed energy that has yet to show up in the thermometer.
"Even if we were to stop all fossil fuel emissions today, the earth would still warm, probably by about 1 degree Fahrenheit, because greenhouse gases in the atmosphere have already increased and the earth's radiation budget is still not in balance," said Daniel Cayan, a meteorologist at the Scripps Institution of Oceanography at UC San Diego.
Cayan, Redmond and other climate scientists say that the sooner emissions are curtailed, the better chances are that the worst forecasts will not come true.
Mike Taugher covers natural resources. Reach him at 925-943-8257 or email@example.com.
Ten things you can do to reduce your personal greenhouse gas emissions.
Replace the most frequently used light bulbs in your home with more efficient, compact fluorescent bulbs. If every household in the United States did this with the five most used bulbs in the home, it would prevent more than 1 trillion pounds of greenhouse gas emissions.
Information on choosing compact fluorescent bulbs:
Heat and cool your home more efficiently. Raising the thermostat just 2 degrees in the summer and lowering it 2 degrees in the winter can save as much as 2,000 pounds of carbon dioxide per year. Wrapping your water heater in insulation and setting it at 120 degrees Fahrenheit, buying more efficient appliances and replacing air conditioner, furnace and heat pump filters will also help.
Information on saving energy at home:
Maintain your car to improve fuel efficiency. Keeping your tires properly inflated by checking the pressure regularly can increase your fuel efficiency by 3 percent. Keeping your vehicle's engine tuned will also improve efficiency. And the way you drive can have a big impact on how much gas you use.
Information on maintaining your car:
Buy a more fuel-efficient car. For people with a long commute to work, driving a more efficient car can make a big difference in personal greenhouse gas emissions and can save money in the long run. For the average person, switching to a car that is just 3 miles per gallon more efficient can cut 3,000 pounds of emissions a year.
Information on fuel economy, including greenhouse-gas emissions ratings for cars:
Recycle and use recycled products. Recycling all of your home's newsprint, glass, metal and cardboard could save 850 pounds of emissions. Recycled paper products require 70 percent to 90 percent less energy to make and save trees.
Information on recycling:
Install a solar energy system on your home. This can be expensive, but California has incentives for homeowners to offset the cost. The state goal is to produce 3,000 megawatts of new solar energy by 2017.
Information on home solar energy:
Buy locally produced foods. The average meal in the United States travels 1,200 miles before arriving at your table. Buying locally can save fuel. Avoiding frozen foods, which require 10 times more energy to produce, can also help.
Information on local farmer's markets:
Use less water, especially hot water. Installing low-flow shower heads can save 350 pounds of carbon dioxide per year. Washing clothes in cold or warm water instead of hot can save 500 pounds of carbon dioxide per year. Using less cold water can save energy used by municipal purification and distribution systems.
Information on saving water:
Turn off and unplug unused electronic devices. Turning off the TV, DVD player, computer and other electronic devices can save energy and money. Buying more efficient devices can also help. Unplugging electronics can also save. As much as 40 percent of all the electricity used to run your home electronics is used when they are turned off.
Information on efficient electronics:
Buy green energy. Purchasing renewable energy such as wind, solar or geothermal systems will increase demand for green energy and reduce greenhouse gas emissions. California has a number of green energy options and incentives, but if those aren't available to you, you can offset your emissions at www.nativeenergy.com.
Information on buying green power and California's renewable energy incentives:
Sources: EPA, USDA, climatecrisis.net, Go Solar California, Environmental Defense, American Council for an Energy-Efficient Economy