THIS IS Philadelphia's moment. Once again.

Economic, social and environmental forces - not to mention a new president - are transforming the nation and the world with breathtaking speed.

They are also fueling an almost about-face in the fortunes of the Philadelphia region, propelling it to the forefront of the coming change. A renewed focus on metropolitan regions, especially older, denser core cities and their surrounding suburbs, like this one, has been building for the last few years. In recent months, the focus has become sharper and the future clearer.

What this new focus means, and how it will reshape the city and region is still unfolding. But the moment to grasp is right now.

For decades, the government's urban policy focused on dealing with, or ignoring, the cities' problems: reducing crime and grime and addressing poverty. In the process, the core city became estranged from even its close-in suburbs, not to mention the sprawling developments further out. Cutthroat competition for jobs and public money ensued. Arguments over the responsibility for dealing with problems like homelessness and crime increased, spawning corrosive contempt on both sides of the city line.

Now, the region's dense housing stock, existing infrastructure and its role as an incubator of innovation and creativity could become the shared rebuilding blocks the new economy needs.

That is, if the family feud can be put aside in favor of collective decision-making among city, suburb and exurb in both Southeastern Pennsylvania and South Jersey.

When gas prices spiked last summer, so did the demand for alternative transportation.

But only metropolitan regions had the robust transit systems already in place to meet the need - and public-transit ridership rose to its highest point in more than 50 years. That's just one of the examples of how the shifting demands of the global economy are well-matched by what cities have offered all along. There are others: the increasing attractiveness of a more compact life - street grids and sidewalks, dense housing mixed in with shopping and entertainment that make an active, independent lifestyle for kids and seniors more accessible.

The very legacy that drove people away for most of the last 50 years now gives Philadelphia a head start on restructuring to match the new realities of a burst housing bubble, expensive energy and promising new technologies. The essence of what makes a city - density, diversity and access - is what will save the economy and, ultimately, the planet.

Early visions

Just last month, the U.S. Census Bureau found that the boom towns of the last few decades are starting to go bust, as U.S. migration, especially to the suburbs and Sun Belt, fell more sharply than at any time since the Great Depression.

A decade ago, John Norquist, then mayor of Milwaukee, saw it coming. In his 1998 book, "The Wealth of Cities," Norquist, now president of the Congress for a New Urbanism, predicted the massive case of buyer's remorse that would follow the proliferation of highways and the sprawl that followed.

He was right too soon, but now, without really noticing it, the U.S. has become a metro nation. According to a landmark study by the Brookings Institution in 2007, metropolitan areas now represent 12 percent of the nation's land, but 65 percent of its population, 65 percent of its college graduates and 76 percent of its high-paid knowledge jobs.

As the report highlighted, the key assets for prosperity - innovation, infrastructure and human capital, and quality living places - are all concentrated in metro areas. Now the nation's reduced circumstances are forcing us to pay attention.

Policy catches up

with reality

That metropolitan areas are "hot" may come as a surprise to some. After all, recent decades have seen public policy weighted toward providing incentives to spreading out: favoritism toward highways rather than public transportation and the blocking of higher environmental standards that made it cheaper to build new than renovate. New and widened highways decanted the cities into increasingly isolated pockets, spreading out people and thinning the lines of communication.

In fact, during last year's presidential campaign, one side maintained more than once that the "real America" can be found only in small towns and farming communities.

Maybe that's why Barack Obama won. While he didn't create the renewed and intense re-focus on metro regions, his urban-policy platform recognized the need to move on from the romantic nostalgia of the past and a commitment to the now.

Obama is the first truly urban president in decades, maybe a century. When he goes on vacation, he doesn't clear brush or ride a mountain bike on remote trails. He plays basketball, the quintessential city game, and heads out to explore the restaurants, stores, schools and activities of his new home, Washington, D.C.

It's powerful symbolism, but it's backed with an actual metropolitan agenda, shaped in large part by Valerie Jarrett, one of the president's top advisers, a veteran of Chicago city government, and underscored by Vice President Joe Biden, once widely known as "the senator from Amtrak," so committed is he to taking and funding public transit. A new White House Office of Urban Affairs, led by a former Bronx borough president, Adolfo Carrion, is charged with "connect[ing] the dots across the wide variety of issues that touch on metropolitan regions and their economics," as Governing Magazine put it.

And this metropolitan moment comes lubricated with lots of money. The American Recovery and Reinvestment Act (the federal stimulus package) is already pouring $16 billion into Pennsylvania on projects that are "shovel-ready," physically and bureaucratically. The 2010 Obama budget includes hundreds of billions more in long-term investments in education, health and energy.

But there's a danger here. The point of a stimulus is to spend money fast, which means it must be administered through existing governmental infrastructure: different agencies - the slang term is "silos" - for roads, subways, housing, public parks and water supply, each with thousands of employees, thousands of eligibility requirements and thousands of regulations to follow, some that overlap and some that conflict. The "silo effect" means lots of wasted energy and resources and is as inadequate for solving 21st century problems as it was for dealing with those of the 20th.

The practice of "siloing" has significant effects locally, too. The region's government and business leaders have been gamely trying to champion the idea that the city and region must not be disconnected from each other but must solve problems collectively, and gain power by consolidating resources.

Now Mayor Michael Nutter, along with about a dozen suburban county commissioners, are renewing efforts to finally make it happen. With a new model for collaborative decision-making, the region will be poised to take advantage of the extraordinary opportunities - and dollars - of this moment. Without it, Philadelphia will fall behind yet again, and may never catch up.

Whether we're ready or not, new government policies directed at reducing carbon emissions will radically change the way we do business. "Cap and trade" policies, granting credits for limiting emissions and creating a market for trading the credits, will be a powerful incentive for collaboration.

This section looks at some of these ideas - in transportation, energy, sustainability and planning - that people will be talking about and, we hope, acting on, in the coming months. This may be metropolitan Philadelphia's moment, but it's only a moment. It shouldn't be squandered. *