I’m reading the fascinating book “Triumph of the City” by Edward Glaeser, at the moment. While I don’t find myself necessarily agreeing with everything in the book, its arguments are really well made – in a way that makes you take full consideration of them. In particular, there’s an interesting discussion of why Houston has become such a popular city for people to move to over the past decade or two, which has some useful lessons for Auckland (even if I really don’t want Auckland to follow the Houston model in all ways). But that particular issue is probably best left to a future post.
A section of the book that I find really fascinating focuses on the relationship between different types of urban environments and their environmental impact. It seems to have become broadly accepted that if you want to live a sustainable, ‘green’ lifestyle then the best thing you can do is head to the country, find a reasonably large piece of land and start growing your own food, generating your own power and collecting your own water. Cities are seen as polluting, consuming ‘anti-environment’ places. However, when you look at things in a bit more detail, the reality is quite different:
Using a gallon of gas produces about twenty-two pounds of carbon dioxide, if you factor in the carbon used in refining and distributing gasoline. An average family in the United States buys about a thousand gallons of gas a year, which is associated with about ten tons of carbon dioxide. It may be easier to imagine American families buying more fuel-efficient cars than giving up on car-based living altogether, but historically the bulk of variation in gas usage among various people over various periods of time comes from total miles travelled, not from fuel efficiency. Cars now get, on average, about 22 miles per gallon, and the big difference is whether you drive three hundred miles per year or thirty thousand, which depends on whether you live in a city or a suburb.
There’s an interesting paradox which results from improved efficiency not always leading to a reduction in use of the energy input – one might call it the ‘heat-pump paradox’ where the efficiency of a heat pump doesn’t mean reduced power bills, but rather that people use the heat pump more than they would use a conventional heater – leading to either the same amount of energy use or actually an increase in energy use than before. Over the past few decades we’ve seen the same thing happen with improvements in vehicle efficiency, which has either gone into more powerful cars or been eaten away by increased usage of vehicles. Of course that’s not to say that we shouldn’t bother improving efficiency, but rather than we perhaps shouldn’t completely rely on it.
The book continues:
[Matthew] Kahn and I found that area density and distance to the city centre are both strongly associated with gasoline usage. The average household living in a census tract with more than ten thousand people per square mile uses 687 gallons of gas per year, while the average household living in an area with fewer than one thousand people per square mile (about one household per acre) uses 1,164 gallons of gas per year. The density of one’s home neighbourhood matters because most car trips aren’t commutes downtown. People drive millions of miles to buy groceries, to go out to eat, and to pick their children up at school. The density of stores and schools in an area determines the average distance of those trips. In a city, you often walk to a restaurant. In a low-density area, eating out might entail a twenty-five minute drive each way.
Generally the largest proportion of vehicular trips are for what tends to be termed ‘errands’. Picking up the kids from school, popping down to the store, heading to the shopping centre, visiting friends and relatives – and so forth. I suspect that even in low density cities with very good public transport systems – like Perth, outer New York City and Vancouver, the vast majority of these ‘errand’ trips are undertaken by private vehicle. I’ve had some long and detailed debates on this issue of density and transport choice and generally feel that, while not necessarily impossible, it is certainly very difficult to provide a public transport system that can be attractive for non-commuting trips in low density urban environments.
Indeed, in higher density urban environments even if we did continue to drive for many of our trips – at least those trips would be shorter, as the book goes on to explain:
Holding family income and size constant, gas consumption per family per year declines by 106 gallons as the number of residents per square mile doubles. These estimates suggest that if the average Northeastern household moved from living at one family per acre to five families per acre, then that family would consume 350 fewer gallons. These facts remind us that mass transit isn’t the only way to lower gas consumption. If people lived in denser areas, they’d travel far fewer miles and burn much less gas, even if they still drove to work.
Of course catching public transport is quite green too, particularly when the system is well used:
Public transportation emits carbon too, but most forms of public transit are a lot more energy efficient than driving vast distances in our own personal gas burners. For example, the New York City Transit system uses 42 million gallons of diesel fuel and 14.8 billion megawatts of electricity each year to deliver 2.6 billion trips to its riders. That works out to an average of 0.9 pounds of carbon dioxide per trip – a tenth as much as the nine pounds of carbon dioxide emitted in an average car trip.
New York’s public transport system is obviously incredibly well used, which means it’s very efficient. Along with ensuring we get good value for money from our spend on public transport, it’s also important from an environmental perspective to ensure we run an efficient system. Empty buses not only wastes money, it also unnecessarily pollutes our city and contributes to carbon emissions.
Comparing various cities in the USA highlights the relationship between urban density and carbon emissions:
On average, when population doubles, per-household carbon dioxide emissions due to driving decline by almost a ton per year. Southern cities have particularly high driving levels, and over 75 percent more gasoline usage than New York. Sunbelt cities like Greenville, South Carolina, Nashville, Tennessee and Oklahoma City were built at low densities and have widely dispersed employment, and their residents use more gas.
In almost every metropolitan area, city dwellers consume a lot less gas than suburbanites. Predictably, some of the biggest city-suburb gaps are in older areas, like New York, where the average urban family consumes more than three hundred fewer gallons of gas per year than its suburban counterpart. But some of the largest gaps between cities and suburbs also occur in places like Atlanta and Nashville. It isn’t that central Nashville or Atlanta has so little driving, but that people drive so much in their suburbs. These facts suggest that city density reduces carbon emissions in the older areas of the Northeast, but also in the newer areas that are growing fastest.
It’s not only in relation to transport that density makes a difference, but also when it comes to electricity usage. In the USA, where most power is generated by coal powerplants, reducing energy use in buildings makes a huge difference to carbon dioxide emissions.
Cities are also greener than suburbs because urbanites use less electricity. Electrical appliances account for two thirds of residential energy use. The main factor that explains the difference in energy use among various metropolitan areas is summer heat. Everybody runs refrigerators and appliances, but air conditioning really drives the differences from place to place. The rise of the American sunbelt in the postwar period owes much to the availability of cheap, cool air. Who would want to put up with Houston’s ninety-nine 90-degree days a year without air-conditioning?
Bigger, denser cities, where people own smaller homes, use less electricity. The average single-family detached home consumes 88 percent more electricity than the average apartment in a five-or-more-unit building. The average suburban household consumes 27 percent more electricity than the average urban household. When we standardise for income and family size, we find that central-city residents use less electricity in forty-four out of the forty-eight metropolitan areas that we analysed. More centralised metropolitan areas, such as New York, Boston, and even Las Vegas, use less electricity than more sprawling places, like Dallas or Phoenix.
In a way none of this should come as a surprise. In an apartment building you have more shared walls (ceilings and floor) than in a detached home, so your heating (or cooling) helps your neighbours (and vice-versa) rather than disappearing into the outside world. In terms of transport, it is also fairly logical that denser urban environments mean less travel and a greater likelihood of public transport being used for trips.
Which does make one wonder why the myth of low density semi-rural living being environmentally friendly has persisted.