A long time. Even the populated areas could be a lot more dense, and we wouldn't have people pushing each other into the oceans.

Hong Kong has a density of 6,620 people per square km. If the US was that dense, we could fit the entire population (317 million) in about 48,000 square km (the US has close to 10 million square km)

Of course, we probably couldn't have that many people living in that dense an area. But we do have a long way to go before we are pushing each other into the ocean.

Hong Kong's population density is actually even more impressive than that when you consider that large swaths of the land are either mountains or protected areas that developers are unable to build on. Many areas in the city are an order or magnitude or more higher - Mong Kok, for instance, has a population density up around 130,000 people per km/sq.

Just for reference purposes:

http://simple.wikipedia.org/wiki/List_of_countries_by_popula...

The chart only shows presence or absence of human settlement. Just because there are people there doesn't mean there are a lot of people there. Outside of big urban areas like SF, LA, NYC, and Chicago, the US isn't a particularly crowded place. I was born in Shanghai, China and went back to visit family about 4 years ago. I also lived in Manhattan for 3.5 years. Times Square cannot even compare to the absolute crush of humanity at Shanghai's Nanjing Street. Believe me, the US won't be filling up anytime soon.

Just have to point out that the population density is hard thing to talk about in the U.S. because the reality is that NYC is far more dense that any other city. We talk about a few cities as though they are equals when in reality it's NYC and then a giant dropoff to everyone else.

Also there's a perception of a few places as being the "big cities" even though population has changed a lot over the years. Perception hasn't changed. So even though SF is definitely dense, there are other places that are equally dense, like Miami, Philadelphia, and Louisville that people don't really think of that way. And then there are emerging big cities (but not necessarily dense) like San Antonio that people perceive as being small.

Western china is also sparsely populated. Heck, much of even eastern f china is mountainous and not suited to dense population. That there are people at all in these mountain villages is quite amazing. Anyways, the us and china are quite similar in the whole. Just china has many more people.

We'll fight each other over fresh water supplies long before we're shoving each other into the sea.

The looming water crisis is a particularly big problem in California, Arizona, and parts of the Southwest. Interestingly, farmland irrigation accounts for a bigger drain on water than almost all of the nearby cities combined.

You can 'create' more water by desalination. The real problem is in energy.

Growing crops with desalinated water costs more for the energy to produce the water than the price of the crops grown. That's why desalinated water tends to only be used for domestic supply. The real problem is energy price.

Given that population growth in the developed world is decelerating, and in many countries has already reversed, this is likely to take very long....

Really? I have the opposite viewpoint. It's amazing how much of the US has zero people living in it.

Remember, if the census block has is filled in, it could only have 1 or 2 people living in it (i.e. a farm).

We have so much dam space in the US it's mind blowing.

A lot of the space is not really suitable for dense inhabitation, tho. (or any at all for a smaller subset)