By the time Atlanta’s oldest millennials are in their sixties, the metro’s population will have surged to upwards of 9 million, leapfrogging larger metros of today such as Philadelphia and Washington D.C., according to recent projections by the U.S. Conference of Mayors.

That would make Atlanta the nation’s sixth largest metro, trailing only New York, Los Angeles, Dallas, Houston, and slow-growing Chicago.

And it would slot Atlanta among Sun Belt competitors from Miami to San Bernardino predicted to explode in coming decades, relatively speaking, according to the mayor’s report, which was issued to mark Infrastructure Week (and make the argument that transit and infrastructure spending is crucial).

The U.S. Conference of Mayors foresees Atlanta joining Sun Belt brethren Dallas and Houston with exponential population increases in coming years. Chicago would slip two places by 2046, per these projections:

This week, the AJC contrasts those numbers with projections by the Atlanta Regional Commission that are no less glowing for fans of vibrant economies.

It’s important to note the ARC for decades has considered metro Atlanta to be the 10 core counties around Fulton. This excludes places such as Forsyth and Hall counties, with around 200,000 residents each.

By this measure, metro Atlanta stands at about 4.5 million folks right now. And it’s packed on more than 500,000 people since the depths 2010—not including transplants who’ve arrived in 2017, a leading researcher told the AJC.

Still, the ARC’s projections show a region with few indicators of becoming sluggish—and one that needs to wisely implement changes to its transit systems and community planning.

Past ARC projections have been “really good” in predicting the sheer amount of people bound for the Big Peach, but they’ve been less accurate in forecasting exactly where they’ll settle, the official told the AJC.

In the next 23 years, the ARC foresees:

Atlanta’s 10-county core will pack on about 1.6 million residents;

With 1.35 million people, Gwinnett will be the region’s largest county (Fulton would be second at 1.26 million);

Metro Atlanta’s Hispanic population will reach 1.75 million, more than doubling;

Atlanta residents age 65 and over will more than triple, begging the question: What’ll the first millennial retirement community look like?

If the ARC’s version of 2040 seems impossibly distant in the future, consider that 23 years ago Micheal Jordan played baseball and Kurt Cobain died.

Not exactly ancient history.