WASHINGTON — President-elect Barack Obama's plan to create a "green energy economy" is designed to address the problems of energy security, climate change and job creation, but its cost could be greater than policymakers realize, the chief executive of oil company ConocoPhillips said Tuesday.

"We agree that we must reduce the environmental footprint of energy production and consumption. But we must be realistic about the cost of green energy ... (and) about its true potential and how long it will take for commercial-scale supply contributions," ConocoPhillips Chairman and CEO Jim Mulva said in a speech at the National Press Club.

To ensure prosperity, the U.S. economy needs quickly available energy, reasonably priced, he said. By restricting energy development domestically, the country exports dollars to buy oil from abroad, thereby also exporting jobs.

ConocoPhillips, the third-largest U.S. oil company, is a member of the U.S. Climate Action Partnership, a coalition of business and environmental groups calling for a mandatory national framework to slow and eventually reverse the growth of greenhouse gas emissions. The group is issuing recommendations for climate policy later this week.

Mulva leaned lightly toward institution of a cap-and-trade system to cut carbon dioxide emissions over a tax on them, though he added that "they both can work."

A cap-and-trade system _ which allows a company with reduced emissions to sell a credit to another business that needs to exceed the emissions limit to operate _ is more widely understood and has a better chance of working through the political process, he suggested.

But Exxon Mobil Corp. CEO Rex Tillerson last week called for a carbon tax, saying it would be transparent and effective, while a cap-and-trade system would require establishment of new markets to trade carbon credits and new regulators to monitor them.

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Obama's nominee for energy secretary, Nobel Prize-winning physicist Steven Chu, promised Tuesday in his Senate confirmation hearing that he will aggressively pursue policies aimed at addressing climate change and achieving greater U.S. energy independence by developing clean energy sources.

But Chu also views nuclear power and coal as critical parts of the nation's energy mix and said he was optimistic ways can be found to make coal a cleaner energy source by capturing its carbon dioxide emissions.

As the Obama administration prepares to take office next week, Big Oil clearly is seeking a seat at the table as a new energy policy is crafted.

ConocoPhillips "is ready, eager and willing to work with the new administration," Mulva said.

He said a comprehensive policy is needed that would embrace the principles of broad diversity of energy supply, greater efficiency, technological innovation and sound environmental stewardship.

Oil prices have been sliding as anxiety over the weakening global economy has overshadowed Mideast tensions, Organization of Petroleum Exporting Countries production cuts and a winter season expected to bring the coldest weather in a decade. In the past week alone, crude oil prices dropped nearly 30 percent, hitting a low of $36.10 a barrel Tuesday on the New York Mercantile Exchange before rebounding a bit.

"A year or even six months ago, with gasoline prices triple what they are today, energy security was on the A-list of vital issues. So was climate change," Mulva said. "Now, they have taken a back seat, replaced by the new challenges."

Record crude oil prices last summer helped ConocoPhillips' third-quarter profit soar 41 percent from a year earlier, as the company overcame lower oil and gas output caused by two hurricanes. But Houston-based ConocoPhillips has said its 2008 oil production will likely fall short of 2007 levels, in part from the two storms.

And major oil companies are expected to post fairly dismal results for the fourth quarter later this month, depressed by the tumble in oil prices.