Fun with Densities

Albuquerque from Volcanoes on West Mesa

I’ve lived in both the Southwest and the Northeast, and one thing I’ve found very striking is just how different they are physically in just about every conceivable way.  The differences are so extreme, in fact, that I’ve found it basically impossible to explain.  This comes up pretty often, especially in the Northeast.  When people find out that I’m from New Mexico they often ask what it’s like and when I say it’s really different they ask me to explain further.  I’ll generally give some examples, but they don’t really convey the extent of the differences.  I don’t often say that actually going to a place is necessary to talk about it, but this is one case in which you really have to see both regions to understand the differences.  I think the difference is actually between the East and West, separated roughly by the Great Plains (which are somewhat transitional), but it’s the Southwest and Northeast specifically that I have the most experience with and so it’s those two that I’ll be talking about here.

One of the many ways in which the two regions differ is in population density.  This flows rather obviously from some other differences, especially climatic.  It’s tricky to discuss this, though, because density is a function of both population and area, so where you draw the lines around the area you’re talking about makes a huge difference.  This makes intuitions about density very unreliable in many cases.  To take a concrete example, I was recently talking to some of my fellow graduate students in city planning here in New Jersey about the differences between New Jersey and New Mexico.  I was talking about how I’ll probably eventually go back to the Southwest after I finish my degree here, and one colleague asked what I was going to do when New Mexico filled up with people, like New Jersey had.  I said that would never happen.  The other students seemed shocked and pressed me to explain.  I could have talked about resource constraints, but I decided to focus on land area instead.  Basically, I said that you would run out of people before you ran out of land.  In practice, of course, you would run out of water first, but assuming you had the resources to bring in enough water to build up the whole state at New Jersey’s density the resulting population would be so high that there’s no way that could ever happen in reality.  The colleague who had asked the initial question then clarified that he didn’t mean the whole state would fill up, just the part that was already built up to some extent.  There I was less sure but said it would probably not happen either, and that’s where we left it.

I decided to take a more rigorous look at this, using 2000 Census data.  First I calculated the number of people that would fit in New Mexico’s entire land area if it had New Jersey’s overall population.  Then I looked at just the part of New Mexico that the Census Bureau designates as “urban”; this is part of a general division of the whole country into “urban” and “rural” that exists separately from the designation of metropolitan areas and I think most closely corresponds to the distinction between the parts of the state that are already built up and those that are not and are unlikely to be developed to the same extent in the future.  Then, to provide more context, I did the same calculation for both the whole state and the urban part using the density of the urban parts of New Jersey from the same classification.  To provide some context and a plausible maximum, I then did the calculations using the density of Manhattan, which is an easily recognizable reference point for a very dense urban place.

Manhattan Skyline from Newark Airport

I think the results basically support my arguments, but they’re also interesting on their own.  New Mexico has a total land area of 121,356 square miles and a population of about 2 million, resulting in a statewide density of about 15 people per square mile.  New Jersey, in contrast, has a land area of 7,417 square miles and a population of more than 8 million, giving a statewide density of 1,134 people per square mile and making it the densest state in the country.  This comparison also points out how at least in a physical sense “state” is not a very useful category to use for comparisons, but let’s set that aside for now.  Using just those numbers, if New Mexico had the population density of New Jersey it would have over 130 million people.  Since the population of the entire US is about 300 million, I feel pretty confident in predicting that New Mexico will never have that many people.

It turns out that the urban part of New Mexico actually has a slightly higher population density than New Jersey as a whole (1,814 people per square mile), so if we’re just talking about when the settled portions of New Mexico will fill up to the same extent as New Jersey it’s already happened.  That’s not really a fair comparison, though, since the New Jersey number is being dragged down by the rural parts of the state (which are more extensive than people elsewhere often realize).  The density of the urban parts of New Jersey is 2,846 people per square mile, which when applied to the whole of New Mexico results in a population of 345 million.  I’m very confident in predicting that that will never happen.  If applied only to the urban parts of New Mexico, however, the New Jersey urban density results in a population of 2.1 million, as compared to a 2000 actual population of 1.4 million, which is a big increase but quite plausible if the state continues to grow as it has in recent decades (which may or may not actually happen).  So I concede on that point.

Manhattan has a density of 66,951 people per square mile.  If applied to the whole of New Mexico, this would result in a population of over 8 billion people, considerably higher than the current population of the entire world.  If applied just to the urban parts of New Mexico it would result in a population of about 50 million.  There’s no way either of those is ever going to happen, of course, but it’s interesting to see the numbers.

So there you have it.  These numbers in isolation don’t actually say very much, of course, but they do hopefully give some sense of both the differences between the Northeast and the Southwest and the importance of area as well as population in calculating densities.

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