CALIFORNIA political leaders need to get serious about limiting suburban sprawl if they want to help curb global warming.

In the past two years, Gov. Arnold Schwarzenegger has signed legislation and an executive order setting a state target of reducing carbon dioxide and other gases by about 25 percent by 2020 and more than 80 percent by 2050. The California Air Resources Board must now figure out how to reach those goals.

Thus far, air board staff proposals have picked low-hanging fruit -- retrofitting trucks with devices that reduce aerodynamic drag; requiring auto mechanics to ensure proper tire inflation; reducing perfluorocarbons emissions in the semiconductor industry; and allowing docked ships to shut off auxiliary engines by plugging into shoreside electrical outlets.

All laudable goals, but none likely to produce significant savings. The biggest producer of emissions is transportation, which accounts for about 40 percent of California's greenhouse gases. That's where the biggest efforts are needed.

We can, and should, demand more fuel-efficient cars and gasoline blends that pollute less. But the greenhouse gas reduction from those steps will be offset in the long run if we don't do something about the increasing distances we're driving.

It's not just that there are more people in the state and more cars on the road. It's that each of those cars is being driven more because we're moving farther and farther from our jobs. In the Bay Area, the typical resident travels 21 miles a day. That's up from 18 miles a day in 1990.

The solution is simple: We need to drive less. But what will force us out of our cars? We could make it more expensive to drive by, for example, taxing gasoline more and increasing charges for parking. Those are politically unattractive tactics, yet they're needed.

Another large part of the problem stems from our desire to live in single-family houses with full-sized yards, which often forces us to live far from our jobs.

So, if we want to reduce greenhouse gas emissions, we're going to have to change our land-use policies. We're going to have to build residences closer to jobs, in denser communities and closer to public transportation.

In California, the problem of sprawl is exacerbated by tax and planning policies. Proposition 13, the 1978 property-tax-cutting initiative, creates incentives for cities to seek out major sales-tax-producing retail construction rather than residential, which is costly to service. And regional planning in this state is toothless because most land-use decisions lie with individual cities.

But the state has one key area of leverage to discourage sprawl. The state and regional planning agencies, such as the Bay Area's Metropolitan Transportation Commission, control highly coveted transportation funding.

Enter state Sen. Darrell Steinberg, D-Sacramento, who has written a bill that would reward land-use planning that reduces sprawl. Specifically, it would require the California Air Resources Board to set greenhouse gas targets for metropolitan areas. Then regional agencies would have to devise growth and transportation plans to meet those targets. And future transportation funding would be tied to meeting those targets.

The bill has already run into a howl of protests from transportation leaders fearful that projects already approved, such as the fourth bore of the Caldecott Tunnel in the East Bay, would be blocked. In response, Steinberg has amended the bill to grandfather in projects that have already been approved by voters.

Although Steinberg's bill might seem extreme to some, it's the sort of "tough love" that's needed. We can't talk about cutting greenhouse gases by 25 percent in a little more than a decade, or more than 80 percent by 2050, if we don't take significant steps to reduce our driving.

Borenstein is a columnist and editorial writer. Reach him at 925-943-8248 or dborenstein@bayareanewsgroup.com.