Two giants of California’s energy debate squared off at a Commonwealth Club forum in San Francisco last night.

Chevron CEO Dave O’Reilly and Sierra Club Executive Director Carl Pope fielded questions from moderator Alan Murray of The Wall Street Journal and a sometimes impassioned audience, about renewable energy opportunities, reducing greenhouse gas emissions, and predictions for the future of the United States’ energy economy. Several questions also concerned Chevron’s high profile court battle in Ecuador and the oil company’s presence in Richmond, the Bay Area city where a major Chevron refinery dominates the skyline–and some say, local governance

In what was less of a debate than a discussion, Pope and O’Reilly agreed that the United States needs to make major changes towards greater energy efficiency and that the country must begin to rely more on renewable energy sources.

Their views diverged significantly, however, on the timeline for such changes. While Pope supports a 90% reduction of greenhouse gas emissions from today’s levels by 2050 and says he believes this goal possible, O’Reilly projected that by 2050, the United States will have reduced its GHG emissions by no more than 20-25% from today’s levels.

O’Reilly said that even if the U.S. replaced the entire transportation system with a zero-emissions system, the country would reduce GHG emissions by just 34%–and that doing likewise with the nation’s power generation would reduce GHGs by another 40%.

“So we have to ask ourselves, can we replace our entire energy system–transportation and power–in just a few short decades?” said O’Reilly. “I think the transition is going to take some time.”

According to O’Reilly, his company is already the largest provider of geothermal energy in the world and yet only 2% of Chevron’s income currently comes from renewable energy.

“The challenge of scale demands that we acknowledge that conventional energy sources will remain indispensable for decades,” said O’Reilly. “We must be realistic. For the foreseeable future we need to develop it all: conventional as well as non-conventional energy, as well as renewables and alternatives.”

When asked what his prediction was for how much of Chevron’s income would come from renewable energy sources by 2050, O’Reilly said he thought the number would be about 10-15%.

Pope responded, “The world will have room in 2050 for a very small company, 90% of whose energy comes from fossils [fuels]. The world will not have room, or tolerance, in 2050 for a big energy company [that does], so if Chevron wants to be successful, I think Chevron’s going to need to change those numbers.

Pope also called on Chevron to “come to the table” with local communities in which Chevron operates, such as Richmond, CA, and he proposed that all oil companies donate 10% of their profits to a global fund to clean up areas of the world damaged by the petroleum industry.

Not surprisingly, Pope and O’Reilly agreed that the highest priority for reducing GHG emissions is to replace coal with natural gas or another less carbon-intensive energy source, and while on stage, the men shook hands on an agreement to lobby the issue together in Washington.

KQED will broadcast the entire Commonwealth Club event at 8 p.m. on Friday, June 19, with a rebroadcast at 2 a.m. the following morning.