If most of us are going to live in cities, then we better ask the question, what is it that makes a city sustainable. And the answer that I want to give is twofold. One, is that it is a green city. Green in the literal sense that there are parks and places of people and open areas combined, but also green in the sense that its economic impact on the environment, the ecological footprint of the city is also limited. The other dimension of a sustainable city is that it is resilient. Why resilient? Because we in the age of the anthropocene, in the era of planetary boundaries, in a period where whether we like it or not, we are going to be experiencing more jolts of human induced climate change. Know that the cities are going to be buffeted. If the cities are on the coasts, and sea levels are rising, what that means in terms of vulnerability to storm surges more intense cyclones and other storms, other dislocations, is very great. And so cities need to prepare for those shocks, not as disasters that seemingly come out of the blue, but rather as known, even if unpredictable specific events that need to be prepared for with care. So what is it that makes a city sustainable? Let me mention a few of the crucial points that we need to explore. One is the energy system. Is the city a major contributor to greenhouse gas emissions, especially through its energy use but also through landfills that may emit methane or through a problematic industrial processes that are emitting nitrous oxide? Or is the city efficient, both energy efficient and based on a low carbon clean energy system? What about transport? Cities can be places of incredible congestion, smog, huge waiting times, traffic jams, and of course large amounts of CO2 emissions coming from all those internal combustion engines, burning all of that petroleum and diesel. Or cities can be places of highly efficient transport through very clever integration of walking areas, bicycling areas, as well as various kinds of public transportation, taking the pressure and the demand off of the automobile. And so the nature of the city's transport is a crucial determinant of its sustainability. A city's infrastructure, its water sanitation, its waste management, its ability to recycle industrial wastes and to control industrial pollution are clearly fundamental determinants of a city's sustainability. How effectively the city plans and prepares for the future is obviously decisive. Not one of these issues, energy systems, transport systems, waste management, recycling, open green areas, resiliency to shocks, is something that takes care of itself for something that is solved by a market economy, certainly not a market economy just left to run on its own. Urban resilience and urban efficiency and low ecological footprint, low impact on the natural environment are aspects of city life that must be planned. What does it mean for a city to be unsustainable? Well, first, it means that the city is highly vulnerable to shocks. Shocks again may seem like they're coming out of the blue, but they can be predicted if not in exact timing at least with probabilities of their visitations. Cyclones, droughts, earthquakes in many places, floods and storm surges, landslides, in a few cities volcanoes, are threats to lives, to livelihoods, to the economy and mitigating those threats, anticipating the risks, making cities resilient able to withstand shocks such as these is absolutely a fundamental part of sustainability and a fundamental signal of unsustainability when those preparations are not made. Pollution of the air and the water, the kind that afflicts unfortunately dozens and dozens of China's cities today after 30 years of rapid industrial growth, but insufficient attention to the pollution consequences cause many cities to be hazards for their populations, the air that people breathe, the water they drink, can take years of life away. Cities are unsustainable when they haven't prepared properly for their water supplies. Just about every city in the world, has a huge job to do in anticipating water needs and ensuring that water and sewerage, waste treatment are properly managed. And there are many, many choices as to how to do that, and I will want to show you a bit about that in New York City because New York has managed very cleverly to provide a city of more than eight million people and a urban agglomeration far larger than that with safe water at relatively low cost by thinking ahead. Cities are unsustainable when they are unproductive because people are sitting in traffic jams for hours everyday breathing polluted air, losing productivity, poor health, social inequalities that make it impossible for a large parts of the population to participate in a productive way in the economy of the city. And cities are unsustainable when populations are suffering from a massive and growing disability of health. When an obesity epidemic coming from unsafe food combined with sedentary behavior of city life, with an absence of places for walking, bicycling, exercise for a healthy lifestyle mean that people are ill, they are often disabled, absent from work, and of course suffering a serious setback in their sense of well-being. So these are choices that cities have to make. And a lot is known about what it is that determines whether the ecological impact is high or low, whether the city has a large greenhouse gas emission factor per person or a low one. Whether the city is functioning in a way that is providing for the productivity of the population. Let me focus for a few minutes on one key determinant of this, and that's density. Highly dense cities, if properly prepared for, tend to be both highly productive and lower emitting of greenhouse gases than lower density settlements. This may seem surprising. High density seems a lot of people are jammed together. But in high density, it's also possible to have more efficient transportation, to have more opportunities to walk, to reach places close by. And so, places of high population density tend to be places with lower ecological impacts, notably lower carbon emissions per person of the population. Let me give an example of my own city, New York City. It is, when you look at the overall urban area, the urban agglomeration, the highest density in the United States of any urban area with urban density of about 33,000 people per square mile. Compare that with Los Angeles, for example, at about 12,000 people per square mile. So, obviously, a big difference a factor of roughly three. And it's not surprising that Los Angeles is an automobile city, whereas New York is not. Density makes a huge difference. Other cities in the United States, Atlanta, for example, a real automobile city, is just one fifteenth of the density of New York at about 2,000 people per square mile. Of course where one draws the circles around these urban agglomerations makes a difference in these comparisons. I wouldn't want to overstate the precision of the comparison, but I would emphasize that differences of density make enormous differences in how people move about the urban area, and therefore how they also affect the environment. If you look at New York, at the number of commutes between two points by walking, for example, it's estimated that in New York about 36 percent of all commutes or transit are by public transit or by walking. People either are taking buses, the metro service or walking. Compare that with an automobile type city, say, Atlanta or LA. In LA, instead of it being 36 percent, the estimate is just eight percent. People are going to by car to most places. In Atlanta, about five percent are by public transit and by walking. When you look specifically, as this pie chart shows how New Yorkers get to work, quite interesting. More than 40 percent are getting to work by subway or by rail, another 12 percent by bus. Only 28 percent of the population is getting to work by automobile, 23 percent driving alone, and another five percent by carpool. This is quite extraordinary. In fact, one could say that it is unique or nearly the case in among American cities. It's a joy for me, living in Manhattan, in this highly dense area, I have the great pleasure of being able to walk to work, walk to the United Nations, walk to the university, and to satisfy amenities of all the shops right in the area never having to get in a car. And my great joy is not owning a car. When I lived in suburban Boston, we would get in a car everywhere. In New York City because of the much higher density. It's a great pleasure, believe me, being able to to walk. Not everybody has that particular pleasure, but not having a car is an experience of a higher proportion of New Yorkers than any other urbanites in the United States. And the result of all of this is that New York is exceedingly favorable compared to the rest of the United States in carbon emissions. In other words, New York's impact on human induced climate change is something that New York can be proud of in relative terms. In the United States, on average, Americans are emitting about 20 tonnes of CO2 per person per year. But in New York City, it's roughly about a third of that, roughly six tonnes of CO2 per capita. You see on this chart that of America's cities, New York City is at the very low end of CO2 emissions. People are taking energy efficient subway or the energy efficient buses or are walking, these days increasingly also cycling. And as a result of this, the emissions in the transport sector are very, very low. Because of high density emissions in the building sector, are also rather low with houses packed one next to the other in row houses or townhouses, as they're called in New York, as opposed to self standing houses, there's less surface area for the heat to dissipate. It's easier to keep the housing warm in the wintertime and better insulated just by virtue of the tight packing of the townhouses and row houses that characterize New York City. Another reason for the very, very low emissions per capita. Why urbanization, as it's taking place right now, gives us the possibility if cities are smart in the kinds of energy systems and transport systems that they build and in the kind of zoning that encourages high density settlements with proximity of people to the shops and to the amenities that they're looking for in the urban areas offers a real chance for a lower ecological footprint and for carbon emission levels per capita that are far more consistent with the global needs if we are to head off the worst of climate change. To get the best of what cities can offer in terms of low ecological footprint combined with high productivity, with an ease of movement, with low congestion, with the low level of time wasted in traffic jams and other disamenities, we have to look at how cities invest in the infrastructure and the choices they make. That is our next topic.
