Yesterday, Mayor Nutter announced that Hughes, with a Ph.D. in regional science, will be the city's new sustainability director. Editorial Page editor Sandra Shea asked him about his new job.

Q. What the heck is "sustainability" anyway? We keep hearing the term, and have some vague sense it's connected to the environment.

A. For the past couple of decades, the standard definition has been "the meeting of present needs in ways that don't compromise the needs of future generations to meet future needs."

In practice, we talk about things that are more or less sustainable - things that deplete resources that cannot be renewed, like energy or kinds of building materials.

One of the great insights about sustainability is about closing loops: Things we used to think of as waste can be inputs into other activities. Take our Water Department, where sewage can actually be transformed into methane. That closes two loops: It prevents methane escaping into the atmosphere (dangerous for its greenhouse-gas effects) and also converts it into a natural gas for power.

Is there a founding father, or a godfather, of sustainability?

A. Actually, the godmother is Rachel Carson [author of "Silent Spring"]. But there's also a long tradition in Pennsylvania that began with Gifford Pinchot, Teddy Roosevelt's first Park Service official and later governor of Pennsylvania. He and Joseph Rothrock are two people who pushed for legislation on conservation. This is about thinking about the future when you're making decisions about today.

Q. There's no sustainability director at the state or federal level. What good will a city-level post do?

A. Let me tell you how great your government is: Did you know we have a carbon inventory in the city calculated according to internal best practices? We know what our carbon footprint is.

And, last year, we completed a local action plan with 28 specific elements for government to reduce our greenhouse-gas emission. Last year, Mayor Street signed an executive order implementing it. Did you know that in city government, greenhouse gas emissions have fallen 7.5 percent since 1990 levels?

There are 22 city agencies that have formed a sustainability working group that's been collaborating for years on these issues. This will be the foundation for the mayor's office for sustainability.

We have resources and ideas. What we need is someone to pay attention to the knitting . . . to coordinate and make priorities for all these great ideas. My job will be converting one-sentence ideas into action plans with budgets and tasks and personnel and timetables.

Q. Should citizens expect to hear from you, or is this all going to go on inside City Hall?

A. There's a place for everyone at the sustainabilty table. Even the Philles are committed to sustainability. They are buying enough wind energy to power all their operations at Citizens Bank Park. But we want individuals to pursue their own interests. Turning off your lights is an example. It's not a cost, but it lowers your cost. Spend a little money and get it paid back in no time, like by insulating your water heater. Go to Home Depot and buy $5 worth of weatherstripping to close the gap in your front door. But average citizens also must become a relentless constituency for change - and hold our feet to the fire.

Q. Recycling was a big message in the '70s. How do we know this sustainability thing isn't just a fad?

A. Peak oil is the answer. Last time, what happened was a supply interruption through OPEC. This time, it's a supply depletion. That is the game-changer. This time, it's about gasoline at $4 and $5 a gallon. Gas consumption in the U.S. is the single largest consumption item in the world, and it accounts for one out of 10 barrels of oil consumed globally. This isn't about mantras, it's about prices. *