The Benefits of Density

In the 1970s, Jane Jacobs argued that we could minimise our damage to the environment by clustering together in high-rises and walking to work, and this point has been eloquently argued by David Owen in his book Green Metropolis (2009). We maximise our damage when we insist on living surrounded by greensward. Lower densities inevitably mean more travel, and that requires energy. While larger living spaces certainly do have their advantages, large suburban homes also consume much more energy. Anyone who believes that global warming is a real danger should see dense urban living as part of the solution. Over the next fifty years, China and India will cease to be poor rural nations, and that’s a good thing. They – like the United States and Europe before them – will move from rural to urban living. If billions of Chinese and Indians insist on leafy suburbs and the large homes and cars those suburbs entail, then the world’s carbon emissions will soar. The critical question is whether, as Asia develops, it will become a continent of suburban drivers or urban public-transit users.

Matthew Kahn and I have put together a carbon inventory of new housing throughout the United States. We wanted to determine the amount of carbon emissions that come from building a typical new home in different parts of the country, so we based our estimates primarily on homes built over the last two decades. About 20 per cent of the US’s carbon dioxide emissions are related to residential energy use, and almost another 20 per cent are associated with our motor vehicles. An average family in the United States buys about 1,000 gallons (3,785 litres) of petrol 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 petrol usage among various people over various periods of time comes from total distances travelled, not from fuel efficiency. Cars now average about 22 miles per gallon (9.35 km per litre), and the big difference is whether you drive 300 miles per year or 30,000, which depends on whether you live in a city or a suburb. Kahn and I found that area density and distance to the city centre are both strongly associated with petrol usage. The average household living in a census tract with more than 10,000 people per square mile (3,861 per km2) uses 687 gallons (2,600 litres) of petrol per year, while the average household living in an area with fewer than 1,000 per square mile (386 per km2) (about one household per acre / 4,000 m2) uses 1,164 gallons (4,406 litres) of petrol 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 from school. The density of shops 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 25-minute drive each way. Holding family income and size constant, petrol consumption per family per year declines by 106 gallons (401 litres) as the number of residents per square mile doubles. These facts remind us that mass transit isn’t the only way to lower petrol consumption. If people lived in denser areas, they’d travel far smaller distances and use much less petrol, even if they still drove to work. 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 petrol burners. For example, the New York City Transit system uses 42 million gallons (159 million litres) of diesel fuel and 14.8 billion MW 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 of the 9 pounds of carbon dioxide emitted in an average car trip.

Kahn and I predicted the amount of petrol that an average family with about US$60,000 (€46,866) of income would consume in every census tract and every metropolitan area in the country. While every other area in our sample was associated with more than 1,000 gallons (3,785 litres) of petrol per year, the average household in the New York metropolitan area was connected with fewer than 850 gallons (3,217 litres) of petrol per year. While people in the United States as a whole are more than fifteen times as likely to drive themselves to work than use public transportation, New York City residents are more than twice as likely to take mass transit than drive to work. Throughout the country, big cities mean less driving. On average, when population doubles, per-household carbon dioxide emissions due to driving decline by almost a ton per year. In almost every metropolitan area, city dwellers consume a lot less petrol 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 300 gallons (1,136 litres) of petrol less per year than its suburban counterpart. Cities are also greener than suburbs because urbanites use less electricity. Bigger, denser cities, where people own smaller homes, use less electricity. The average single family detached home consumes 88 per cent more electricity than the average apartment in a building of five or more units. The average suburban household consumes 27 per cent 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 44 out of the 48 metropolitan areas that we analysed. To form a total estimate of household carbon emissions, we just add together the emissions from driving, electricity, and heating, and add public transit.

So how should we interpret all these data? Simply put, if we wanted to reduce emissions by changing our land-development policies, more US residents should live in denser, more urban environments.

Higher-density construction in the United States and Europe will reduce carbon emissions, but the most important battles over urban development in the coming years will be waged in India and China. About half of US homes in 2000 were built between 1970 and 2000, so let’s assume that about half of America’s housing stock 30 years from now will also be new. If every pro-density effort is wildly successful in the US, emissions from driving and powering these new houses might fall by 50 per cent. That would be a great achievement, reducing America’s household carbon emissions by 25 per cent and the US’s total emissions by 10 per cent. Yet from this momentous shift, world carbon emissions would fall by only 2 per cent. That calculation is not meant to excuse inaction, but rather to make the point that the United States is something of a sideshow in the long-run battle against climate change. The US has trillions of US$ of infrastructure built around the car, and any developed country changes slowly. India and China are changing fast, and they have a lot more people than the US does. If carbon emissions in India and China rose to American per capita levels, the world’s carbon consumption would increase by 139 per cent, even if their population stayed the same. The biggest environmental benefits from supporting higher-density development in the United States may well be in helping to persuade the Chinese and Indians to build up rather than out. Today, the United States is the world’s second biggest carbon emitter; on average, Americans emit about 20 metric tons of carbon dioxide per person per year. The Chinese are producing almost 5 tons of carbon dioxide per person per year and the Indians 1 ton. If the Chinese per capita carbon emissions rose to American levels, this would lead to an extra 20 billion tons of carbon emitted every year, increasing world carbon emissions by 69 per cent.

So it’s essential that we encourage these countries to keep their carbon emissions at the more modest European levels, rather than emulating current American energy use and development patterns. Today, China’s carbon emissions are largely industrial. Like the black smoke that once surrounded Pittsburgh or Manchester, they are the by-product of a great industrial power on the rise. So far, China’s households are remarkably parsimonious energy users. Matthew Kahn, Rui Wang, Siqi Zheng and I did a city-by-city analysis of China’s household carbon emissions, similar to the one that we did for the United States. While the typical household in the Washington DC area generates 43 tons of carbon dioxide per year, the typical Beijing household emits only 3.997 tons – and Beijing is one of the brownest places in China. In more than 60 per cent of the Chinese cities we examined, carbon dioxide emissions per household ran at 2 tons a year or less. Household emissions in Daqing, China’s oil capital and brownest city, are one fifth of emissions in San Diego, America’s greenest city. Chinese household emissions are driven by home heating and electricity. As countries develop, warmth comes first, long before air conditioning. While half of US household emissions reflect personal transportation, only a tenth of Chinese emissions currently come from cars. The relative paucity of driving and air conditioning in China keeps current emission levels low, but we can hardly expect an increasingly prosperous Chinese population to forego the luxuries that Americans take for granted. If anything, the case for air conditioning in India seems even stronger.

A generation ago, both China and India were solidly rural. They did little environmental damage because, like all poor places, they used little energy. Over a fifty-year period, however, they’re achieving the same industrial and urban transformation that took centuries in the West. The result is an inevitable explosion in energy consumption, which is today helping to drive up oil prices and that could produce extraordinary increases in carbon emissions in the future. There is, however, a middle way that combines prosperity and growth with fewer environmental risks. That path involves high-density urban living, not the cars of American exurbs. Growth patterns in India and China offer both hopeful and disturbing signs. On the plus side, the great cities of both nations are enormously dense. Mumbai has more than 50,000 people per square mile (19,305 per km2), about double the density of New York City. Kolkata and Bangalore are above 20,000 per square mile (7,722 per km2). Shenzhen, the rapidly growing metropolis in mainland China, across the water from Hong Kong, has more than 15,000 people per square mile (5,792 per km2). These densities fit well with buses and trains and lifts, but make car usage practically impossible. The world will be safer if China’s future involves hyper-dense places made more comfortable with better public transportation and high-rise residences. But there are also warning signs. Car usage in both India and China is soaring. Chinese car ownership hit 60 million vehicles in 2009, with an annual rate of increase of over 30 per cent. A few more 30 per cent years, and China could have 500 million cars by 2020. Meanwhile, India’s Tata Group made headlines by producing a US$2,500 (€1,953) car, and Tata’s cars could put a billion Indians behind a wheel, if they can handle the traffic jams. A billion Indian drivers will emit a lot of carbon.

There is a powerful whiff of hypocrisy associated with energy-mad Americans – and I’m part of this group – trying to convince Asians to conserve more. One distinguished economist likened it to a ‘nation of SUV drivers trying to tell a nation of bicyclists not to drive mopeds’. My awkward suburban life is certainly no model of green living. The only way the West can earn any moral authority on global warming is to first get its own house in order. As long as the US leads the developed world in per capita carbon emissions, we’ll never be able to convince China and India and the rest of the developing world to do anything other than emulate our own energy-intensive lifestyles. The West also needs to embrace a smarter form of environmentalism. In the first phase of environmentalism, when the objective was just to make people care about nature, the exact policy prescription was less important than raising public consciousness. Today the stakes are higher. We need instead to focus on those proposals that will have a meaningful impact on climate change. Smart environmentalism needs to embrace incentives. Ken Livingstone’s congestion charge showed the power of using prices to get people out of their cars. This can be done in other cities. Fuel efficiency is unlikely to be the only answer, because Jevons’s paradox reminds us that as engines and appliances get more efficient, they will also be used more. If the future is going to be greener, then it must be more urban. Dense cities offer a means of living that involves less driving and smaller homes to heat and cool. For the sake of humanity and our planet, cities are – and must be – the wave of the future.

This text consists of edited extracts from E. Glaeser, Triumph of the City, Pan Macmillan, 2011, reproduced with permission of the author and the publisher.

Copyright © 2011 Edward Glaeser

Ed Glaeser is Fred and Eleanor Glimp Professor of Economics, at the Faculty of Arts and Sciences, Harvard University.