The threat highlights the lingering gap between society's understanding of the dangers posed by global warming and the policy in place to address them.

While California adopted landmark anti-global warming legislation last year, it is still being implemented. The state aims to reduce greenhouse gas emissions to 1990 levels by 2020 and is expected to create a market for carbon emissions. The system, known as a cap-and-trade program, would set a limit for carbon emissions and give polluters a certain amount of tradable credits. Polluters who exceed their limits would have to buy potentially expensive credits, encouraging them to instead reduce emissions.

But that system has not been designed yet. In the interim, government agencies are left to reconcile the need to fight warming with the uncertainty of how best to do it.

"Nobody knows what to do," said Mary Teresa Sessom, Lemon Grove's mayor and Sandag's chairwoman. "There is no policy, no comprehensive global warming policy, that anyone has put forth that people are willing to throw money at."

Michael Hix, Sandag's principal transportation planner, said the agency is "scrambling" to respond to the attorney general. But he stopped short of promising a comprehensive answer in the transportation plan, which is due for adoption later this year.

Hix said the agency aims to at least quantify the additional carbon dioxide that will be generated as a result of the plan. But the plan will likely not incorporate the suggestions the attorney general offered. Hix said those could be included during coming years. The transportation plan is next scheduled for revision in 2011 but can be amended in the meantime.

"They're wanting much more information and to much more directly deal with the impacts," Hix said of the Attorney General's Office. "Everyone is trying to figure out how to do that in a region that's still growing."

One option has been hotly debated before in San Diego: Public transit. If more residents rode buses or trains to work, they'd reduce congestion and cut carbon dioxide emissions.

"We need a combination of everything," said Kassie Siegel, climate program director at the Tucson-based Center for Biological Diversity. "It doesn’t mean we're going to eliminate cars on the roads tomorrow. But we need to accommodate new growth with public transportation, not new highways."

But improving public transit in car-happy San Diego is a challenge. Despite high gas prices, commuters have increasingly passed up carpools and driven solo to work. In search of affordable housing, workers are moving farther away from their jobs, driving farther and longer each day.

"We're all in love with our cars," said county Supervisor Dianne Jacob, who represents a large swath of rural San Diego County and has opposed increased transit spending. "I just don’t see in the future that our behavior is going to change a whole big bunch. In theory, it may sound good to get people out of their cars and onto buses, but in practice it is not going to work for a majority of San Diegans."

Then there is the question of funding. Lani Lutar, San Diego Taxpayers Association president, said she is concerned the attorney general is giving Sandag an unfunded mandate.

If Sandag adopted the recommendations, "where is that funding going to come from?" Lutar asked. "How will it impact project priorities? And what will this mean for taxpayers, if they don't have the funding available? If they attempt to seek more funding from taxpayers, you need to evaluate whether that's the most fiscally prudent option based on the science available."

Hix said Sandag is trying to boost public transit, including lanes on Interstate 15, for example, that can be shared by carpools and buses.

Alan Hoffman, a local transportation consultant, said those lanes will do little to encourage the public to ride buses. Instead, he said, those lanes should be dedicated solely to buses, preventing them from getting stuck in the same gridlock as commuters in cars.

"Sandag's thinking was based on the notion, which sounds good on the surface, of how you increase the carrying capacity of the freeway system," Hoffman said. "The only problem is that it doesn't create a system that allows transit to maximize ridership."

Though bus-only lanes have been considered along El Cajon Boulevard, they're unlikely to materialize in the near future, said Rob Schupp, spokesman for the Metropolitan Transit System, in part because of resistance from local businesses concerned it will eliminate curbside parking.

"It's just difficult to design a system that makes everybody happy," Schupp said. "So you begin to find solutions that make people less unhappy."