Here's why Miami and Phoenix are about to surpass Philly's population

However, the region is growing at a slower rate than the nation, and at a much slower rate than some fast-growing cities in the South and West. In addition, the local population gains are masking an unpleasant fact: When people decide to move, far more are moving out than moving in.

The city's population grew by a scant 2,908 from 2015 to 2016, continuing a general slowing in annual growth since 2009, when the city grew by almost 15,000 residents. All the same, last year's increase extended Philadelphia's growth streak to 10 consecutive years. In the 36 years before that, the city's population grew just twice.

What is holding Philadelphia back just may be the school system, said Cheryl Carleton, an economics professor at Villanova. Philadelphia has been able to attract younger residents with improvements to neighborhoods like the Northern Liberties and Fishtown. But "it is harder to keep them once their children are school age because the schools are so bad," Carleton said.

To move forward, the city also needs to improve public transportation and look to attract business sectors other than education and health care, which are dealing with budget issues, she said.

As a whole, the population of three South Jersey counties shrunk very slightly (-322) in 2016. Camden County has lost residents every year since 2009, and Burlington County has had a decline in three of the last four years.  Gloucester County has grown every year since at least 1991, but added only 224 people in 2016. From 2000 through 2009, it added an average of 3,400 people annually.

Across the river, Philadelphia's suburban counties (Bucks, Chester, Delaware, and Montgomery) grew by a little over 4,000 residents in 2016, a minuscule 0.2 percent. Chester County, one of the fastest-growing counties in the state, added only 1,059 residents in 2016, less than half the growth in 2015 (+2,354).

Brian O'Leary, of the Chester County Planning Commission, cited significant developments in Phoenixville, Great Valley, Exton, Kennett Square and Malvern that are adding more housing units.  But he noted, "I don't think there is any broad trend."

David Elesh, a sociology professor at Temple, said the concentration of growth on the Pennsylvania side of the river may indicate that New Jersey is still suffering from the Great Recession.  "Relatively speaking, it saw more foreclosures and short sales than Philadelphia that probably contributed to slow economic growth," he said in an email Wednesday.

Philadelphia continues to be a big loser in terms of net domestic migration. That's the difference between the number of people moving in vs. moving out, and economists say it's an important indicator of an area's relative economic opportunity, or "pulling power." Last year, 13,622 more people moved out of the city than moved in. The net domestic migration loss has worsened since 2010, when the imbalance was relatively small.

In many ways, Philadelphia has become a more desirable place to live and a trendy destination. The city landed hosted Pope Francis and global Catholics in 2015, the Democratic National Convention in 2016, and will host the NFL draft next month. Yet the only reason Philadelphia has grown at all in the last 10 years is because there are more births than deaths (what demographers call natural increase), and because the city remains a destination for immigrants.

Each of Philadelphia's suburban counties also had a net domestic migration loss in 2016. In most cases, that drop was offset by an influx of immigrants and births outpacing deaths. Bucks was the only local county in which deaths outpaced births — a sign of a population that is skewing older or has higher mortality rates.

In Philadelphia's case, that something would be the city of Phoenix and the Miami metro area.

Despite growing for 10 consecutive years, Philadelphia's days as the nation's fifth-largest city are numbered. The new census data released Thursday covered estimated populations in counties and metro areas. Since the City of Philadelphia and Philadelphia County are one and the same, we have a new figure for Philadelphia's population in 2016: 1,567,872 residents.

When the estimates for the nation's cities are released in July, it is all but certain that Phoenix will leapfrog Philadelphia to claim the No. 5 ranking. As of 2015, Phoenix's population was only 4,400 behind Philadelphia's, and, in the previous four years, Phoenix had an average population gain of over 24,000 residents. The Census Bureau also reported that Maricopa County, which includes Phoenix, added 222 people per day in 2016.

Then again, the specter of Phoenix passing Philadelphia may sound familiar. In 2007, the Census Bureau proclaimed that Phoenix had surpassed Philadelphia in population, based on its estimates. But when the full count from the 2010 Census were released, Phoenix's prior estimated gains were overstated, and Philadelphia remained at No. 5.

In terms of greater metropolitan areas, Philadelphia has been at No. 7.  Last year it was surpassed by the fast-growing Washington metropolis. And next year, the Census estimates we will be surpassed again, by metropolitan Miami.  The 2016 figures show Miami only 4,100 people behind Philadelphia's 6,070,500 population.  And since the Miami region added an average of 5,400 people per month in 2016, compared to the Philadelphia region's 680, it's likely as you read this that Philly has already dropped to No. 8.

Of course, we'll have to wait for next year's Census data to be sure.

And there's always the possibility, as Elesh points out, that climate change will tip the scales back in our favor.

"Perhaps as climate change increasingly affects the quality of life in places like Miami, which will see increased flooding and hotter and more humid summers, or Phoenix, where the summer already is seeing average highs above 100 degrees," Elesh said, Philadelphia might start to fight back.