CONGRATULATIONS to Mayor Nutter on naming an invigorating new sustainability director for Philadelphia. Katherine Gajewski is a splendid choice to realize the mayor's ambition that Philadelphia become "the greenest city in America."

Gajewski delivers the essential quality needed to move forward on this most important of issues: She has the full faith and credit of the mayor. (Her appointment also lays to rest the nagging question of whether the director need wear a tie.)

Now the crucial question turns to the citizens of the region, the private sector of this most private city. Because the mayor's challenge is not that Philadelphia be the greenest government but the greenest city - and region.

While there remains much to do, let's review what our government has achieved. In many ways, it's leaped past us and, for the first time in generations, the private sector must scramble to catch up to leadership in City Hall. Who'd ever have thought that possible, besides Michael Nutter?

Take Greenworks, the policy framework to guide the city and its partners through 2015. It consists of five goals: Energy, environment, equity, economy, and engagement. These goals organize 15 specific targets and 169 mutually reinforcing initiatives to achieve them.

These targets are extraordinarily ambitious. But they have been embraced by this administration. Just watch the video at www.phila.gov/green/greenworks to see what I mean.

Greenworks, especially when linked with the new Metropolitan Caucus that the mayor has established with the region's other local elected officials, provides unprecedented public leadership on these issues.

So, they've done their part, and now I want to talk about us.

With Greenworks, this government has given us an anvil to hammer on - but we have to pick up that hammer and start to forge change.

Greenworks is a tool for evaluating every decision this government makes. As I used to say about Mayor Street, in a very different context: It's not about the mayor, it's about us. I left government because this is where the action is - where it must be, and where it should be.

The sustainability constituency rightly feels a certain ownership of this administration, but they need to play it much bigger than they have. Local green advocates either fail to recognize or fear to exercise their full potential power. They need to speak out on every issue confronting the city, especially the budget and our relationship with Harrisburg and Washington.

I'm not an environmentalist, that's not what brings me to this table. The future of cities is my real interest. Sustainability is a means to that end, and that's why I need the green movement to bring its best game. Sustainability is the most pro-urban organizing policy device we've had in a long time.

And that's because only a national interest can ever trump the old local divisions of city-suburb-rural that keep us barefoot and ignorant. It was Manifest Destiny that built the transcontinental railroad and the Cold War that built the interstate highway system. In the 21st century, is there a national interest that can harness American capitalism to build a whole new infrastructure of place?

Yes - it's the politics of sustainability.

The challenges of climate change and energy independence can harness American capitalism to redevelop metropolitan areas through green building practices, sustainable food production, transit- and pedestrian-oriented development, and renewable energy generation and consumption.

That's why Philadelphia must lead U.S. cities in unleashing a sustainability movement that's as strong as possible: to compel a set of national actions equivalent to the closing of the frontier and the construction of the interstate highway system.

So, it's huge. But Greenworks is also doable. Regional collaboration based on shared interests and self-interest, and Washington's new appetite for change fueled by enormous new federal resources, all conspire in our favor.

Pick up the hammer.

Mark Alan Hughes was Philadelphia's first director of sustainability. Look for his new column in the Daily News in September.