The population growth in cities is such a world-wide phenomenon that the U.N. Population Fund is studying the possible effects.

The Funds' "State of the World Population 2007" report finds that over half the world, about 3.3 billion people, will live in cities by 2008, and by 2030, cities will have approximately 5 billion residents.

Smaller cities, not major metropolises, will absorb the bulk of urban growth, the report said.

That squares with the latest findings from the U.S. Census, which finds that the U.S. population is also growing, even if it's temporarily slowing down on the formation of households, a key factor in housing sales. While some population shifts are taking decades, a few cities, says the Census' new population estimate, are growing fast.

In 1910, each of the 10 most populous cities was within roughly 500 miles of the Canadian border, but in 2006, estimates show that seven of the top 10 -- and three of the top five -- are in states that border Mexico.

This decades-long population shift shows that only three of the top 10 from 1910 remained on the list in 2006: New York, Chicago and Philadelphia. Conversely, three of the current top 10 cities (Phoenix; San Jose, Calif.; and San Diego) were not even among the 100 most populous in 1910, while three more (Dallas, Houston and San Antonio) had populations of less than 100,000.

Leading all states was California, which had seven cities among the 25 fastest growing.

Texas dominated the list of the 10 highest numerical gainers, with San Antonio, Fort Worth, Houston, Austin and Dallas each making the top 10. One reason is jobs. The oil "bidness" is back in "bidness," helping to drive the unemployment rate below the national average for the first time in decades. While the country is happy with 4.5 percent unemployment, Texas' unemployment rate has dropped from 5 percent to 4.1 percent.

Overall, eight Texas cities were among the 25 biggest numerical gainers to lead all states. On the list of the 10 fastest-growing cities were three in the Dallas metro area: McKinney (ranking second), Grand Prairie (sixth) and Denton (ninth). In the same vicinity, Fort Worth just missed the list, ranking 11th.

New York continues to reign as the nation's most populous city, with 8.2 million residents. That's more than twice the population of Los Angeles, which ranked second at 3.8 million.

Phoenix had the largest population increase of any city between 2005 and 2006, adding more than 43,000 residents to reach 1.5 million. Phoenix moved into fifth place of the most populous U.S. cities -- due to favorable weather, businesss-friendly bureaucracies, and outmigration from California, among other reasons.

Including surrounding areas, the "Valley" is about 3.4 million and projected to be 4.1 million by 2010 says the Census and the Arizona Department of Economic Security.

The estimates also reveal that many of the nation's fastest-growing cities are suburbs. North Las Vegas, Nev., a suburb of Las Vegas, had the nation's fastest growth rate among large cities (100,000 or more population) between July 1, 2005, and July 1, 2006. North Las Vegas' population increased 11.9 percent during the period, to 197,567.

North Las Vegas; Miami; Charlotte, N.C.; and San Jose, Calif., each rounded out the list of the 10 biggest numerical gainers.

Florida and Arizona each had two cities among the 10 fastest growing: Port St. Lucie (third) and Cape Coral (fourth) in Florida; and Gilbert (fifth) and Peoria (seventh) in Arizona, both near Phoenix. North Carolina (Cary, near Raleigh) and California (Lancaster, near Los Angeles) each contributed one city to the list.

New Orleans had by far the largest population loss among all cities with populations of at least 100,000 people. The city lost slightly more than half of its pre-Hurricane Katrina population. It fell from 452,170 on July 1, 2005, to 223,388 one year later -- a loss of 50.6 percent. To put the size of this loss into perspective, Hialeah, Fla., which experienced the second-highest rate of loss over the period, saw its population decline by 1.6 percent.

According to the U.N., the world isn't ready for its cities to grow so fast. Proper planning is necessary to prevent the threat of "overwhelming poverty, limited opportunities for youth, and religious extremism," said U.N. Population Fund Executive Director Thoraya Ahmed Obaid to The Associated Press. Among the problems the report finds is that NIMBYism (Not In MY Back Yard) is a global phenomenon and that "current policy initiatives often aim to keep the poor out of cities by limiting migration and cutting lower-income housing."

"Cities see poor people as a burden," Obaid told the AP. "They should be seen as an asset. Investing in them in terms of shelter, education and so on would mean you have a good economic force that can work and create even further economic growth coming from cities."