The head of oil company Chevron Corp said efforts to cut U.S. carbon emissions by 80 percent by 2050 were unrealistic because so much current energy infrastructure would have to be replaced.

Highlighting the scale of the task, CEO David O'Reilly said making every U.S. vehicle carbon free would only cut out 34 percent of greenhouse gases, while a completely zero-emission power generation system would only eliminate 40 percent.

We'll be lucky if we can get 20 percent or 25 percent by 2050, O'Reilly said in a largely cordial debate on Wednesday night with Carl Pope, executive director of environmental group Sierra Club. I hope I'm wrong about it, but I'm just looking at capital stock turnover.

O'Reilly cited the small fraction of hybrid cars on the road, after years of investment, as an indication of how long it would take to change. But Pope, pulling a cell phone out of his pocket, argued technology could drive change faster than people often believed.

Pope, who has held his position at the San Francisco-based environmental group since 1992, pointed to California's reduction of electricity use by half between 1973 and 2003 as an example.

O'Reilly, who has run San Ramon, California-based Chevron for nearly a decade, applauded the achievement, but said the wider transition of heavy industry to other states and countries meant much of the usage had simply moved elsewhere.

You just can't draw a circle around the state of California and draw that conclusion without looking at the total load, O'Reilly said.

Both men agreed that moving away from coal toward natural gas in power plants would be one of the quickest ways to reduce carbon output, and promised to talk privately about potentially lobbying together in Washington D.C. about a perceived congressional bias toward coal.

Pope was particularly critical of the current climate bill in Congress for not making coal share more of the burden.

The coal industry sent oil and gas its share of the dinner bill, he said, though he later added that he wanted the bill to move forward through Congress so it could be improved.

In the debate hosted by the Commonwealth Club, a San Francisco-based public affairs forum, both men agreed that a carbon tax was a far simpler way of reducing consumption, with O'Reilly calling the legislation unnecessarily complex.

When people see complexity, when they look at what happened in the financial system with all the complexities that developed there, I think they get distrustful, O'Reilly said.

Pope criticized U.S. energy regulation for not forcing utilities to buy more low-carbon electricity.

Well, if you can get the government to move faster, then good luck, O'Reilly said.

Pope replied, to applause: It would help if you would get out of the way.

After a brief discussion of a $27 billion pollution case Chevron is fighting in Ecuador, which has received much attention in recent months, a handful of audience members wearing shirts that said 'Chevron's $27 billion secret in Ecuador' stood up in protest.

It's not a secret, O'Reilly said.

(Reporting by Braden Reddall; editing by Simon Jessop)