People often ask: I want to get greener, what should I do? New light bulbs? A hybrid? A solar roof? Well, all of those things are helpful. But actually, the greenest thing you can do is this: Choose the right leaders. It is so much more important to change your leaders than to change your light bulbs.

Why? Because leaders write the rules, set the standards and offer the tax incentives that drive market behavior across a whole city, state or country. Whatever any of us does individually matters a tiny bit. But when leaders change the rules, you get scale change across the whole marketplace. And the energy-climate challenge we face today is a huge scale problem. Without scale, all you have is a green hobby.

Have no illusions, everything George W. Bush wouldn't do on energy after 9/11 - his resisting improved mileage for cars and actually trying to weaken air-conditioner standards - swamped any good works you did. Fortunately, the vacuum in the White House is being filled by leaders from below.

Take the New York City taxi story. Two years ago, David Yassky, a City Council member, sat down with one of his backers, Jack Hidary, a technology entrepreneur, to brainstorm about how to make New York City greener - at scale. For starters, they checked with the Taxi and Limousine Commission to see what it would take to replace the old gas-guzzling Ford Crown Victoria yellow cabs, which get around 10 miles a gallon, with better-mileage, low-emission hybrids. Great idea, only it turned out to be illegal, thanks to some old size regulations designed to favor Crown Vics.

Recalled Hidary: "When they first told me, I said, 'Are you serious? Illegal?' " So he formed a nonprofit called SmartTransportation.org to help Yassky lobby the City Council to change the laws to permit hybrid taxis. They also reframed it as a health issue, with the help of Louise Vetter, president of the American Lung Association of the City of New York.

"New York City has among the dirtiest air in the U.S.," Vetter said. "When it comes to ozone and particulate matter, New Yorkers are breathing very unhealthy air. Most of it is tailpipe emissions. And in New York City, where asthma rates are among the highest in the nation, the high ozone levels create very serious threats, especially for kids who spend a lot of time outdoors. Converting cabs from yellow to green would be a great gift to the city's children."

Matt Daus, who heads the taxi commission, which is independent of the mayor, was initially reluctant, but once he learned of the health and other benefits, he joined forces with Yassky and Hidary, and the measure passed the City Council by 50-0 on June 30, 2005. Since then, more than 500 taxi drivers have converted to hybrids - mostly Ford Escapes, but also Toyota Highlanders and Priuses, and others.

On May 22, Mayor Michael Bloomberg, one of the greenest mayors in America, decided to push even further, insisting on a new rule, which the taxi commission has to approve, that will not just permit but require all cabs - 13,000 in all - to be hybrids or other low-emission vehicles that get at least 30 miles a gallon, within five years.

"When it comes to health and safety and environmental issues, government should be setting standards," the mayor said. "What you need are leaders who are willing to push for standards that are in society's long-term interest." When the citizens see the progress, Bloomberg added, "then they start to lead." And this encourages leaders to seek even higher standards.

I asked Evgeny Freidman, a top New York City fleet operator, how he liked the hybrids: "Absolutely fabulous! We started out with 18, and now we have over 200, mostly Ford Escapes. Now we only put hybrids out there. The drivers are demanding them and the public is demanding them. It has been great economically. With gas prices as they are, the drivers are saving $30 dollars a shift." He said drivers who were getting 7 to 10 miles a gallon from their Crown Vics were getting 25 to 30 from their hybrids. The cost of shifting to these hybrids, he added, has not been onerous.

Now Hidary is trying to get law firms and investment banks, which use gas-guzzling Lincoln Town Cars - 12,000 in the city - to demand hybrid sedans only.

This is how scale change happens. When the Big Apple becomes the Green Apple, and 40 million tourists come through every year and take at least one hybrid cab ride, they'll go back home and ask their leaders, "Why don't we have hybrid cabs?"

So if you want to be a green college kid or a green adult, don't fool yourself: You can change lights. You can change cars. But if you don't change leaders, your actions are nothing more than an expression of, as Dick Cheney would say, "personal virtue."