At first glance, cities seem totally divorced from the natural environment. What possible relationships could the shiny glass office buildings, paved streets, and high-rise apartment complexes that characterize most cities have with forests, fields, and rivers? In this section, we examine several different ways to think about the relationships between urban life and its natural setting. We start by exploring the climate of cities, particularly the vegetation, weather, and hydrology unique to urban environments. We then turn to an examination of the reciprocal relationship that exists between increasing urbanization and global environmental problems such as climate change and vulnerability to natural disasters, and how cities are becoming increasingly resilient in the face of these challenges.

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Cities alter virtually all aspects of local weather and climate. Temperatures are higher in cities, rainfall increases, the incidence of fog and cloudiness is greater, and levels of atmospheric pollution are much higher.

The causes of these changes are no mystery. Because cities cover large areas of land with streets, buildings, parking lots, and rooftops, about 50 percent of the urban area is a hard surface. Rainfall is quickly carried into gutters and sewers, so that little standing water is available for evaporation. Because evaporation removes heat from the air, when moisture is reduced, evaporation is lessened and air temperatures are higher.

Moreover, cities generate enormous amounts of heat. This heat comes not just from the heating systems of buildings but also from automobiles, industry, and even human bodies. The hard surfaces so prevalent in urban areas not only repel water, they also retain heat very effectively. This results in a large mass of warmer air sitting over the city, called the urban heat island (Figure 10.28). The urban heat island causes yearly temperature averages in cities to be 3.5°F (2°C) higher than in the countryside; during the winter, when there is more city-produced heat, the average difference can easily reach 7°F to 10°F (4°C to 5.6°C).

Urbanization also affects precipitation. Because of higher temperatures in the urban area, snowfall will be about 5 percent less than in the surrounding countryside. However, rainfall can be 5 to 10 percent higher in cities as well as areas downwind from them. The increased rainfall results from two factors: the large number of dust particles in urban air and the higher city temperatures. Dust particles are a necessary precondition for condensation, offering a nucleus around which moisture can adhere. An abundance of dust particles, then, facilitates condensation. That is why fog and clouds are usually more frequent around cities.

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Residential areas are usually the greatest consumers of water in urban areas. Water consumption can vary, but generally each person in the United States uses from 80 to 100 gallons (303 to 379 liters) per day in a residence. Residential demand is greater in drier climates as well as in middle- and high-income neighborhoods. Higher-income groups usually have a larger number of water-using appliances, such as washing machines, dishwashers, and swimming pools.

Not only is the city a great consumer of water, but it also alters water runoff patterns in a way that increases urban flooding. Urbanization can increase both the frequency and the magnitude of flooding because cities create large impervious areas where vegetation has been replaced with pavement, and water cannot soak into the earth. Instead, precipitation is converted into immediate runoff. It is forced into gutters, sewers, and stream channels that have been straightened and stripped of vegetation, which results in more frequent high water levels than may be found in a comparable area of rural land. Furthermore, the time between rainfall and peak runoff is reduced in cities; there is more lag in the countryside, where water runs across soil and vegetation into stream channels and then into rivers. So, because of hard surfaces and artificial collection channels, runoff in cities is concentrated and immediate, leading to urban flooding.

Until a decade ago, it was commonly believed that cities were made up mostly of artificial materials: asphalt, concrete, glass, and steel. Studies, however, show that about two-thirds of a typical North American city is composed of trees and herbaceous plants (mostly weeds in vacant lots and cultivated grass in lawns). This urban vegetation, usually a mix of natural and introduced species, is a critical component of the urban ecosystem because it affects the city’s topography, hydrology, and meteorology.

More specifically, urban vegetation influences the quantity and quality of surface water and groundwater; reduces wind velocity as well as turbulence and temperature extremes; affects the pattern of snow accumulation and melting; absorbs thousands of tons of airborne particulates and atmospheric gases; and offers a habitat for mammals, birds, reptiles, and insects, all of which play some useful role in the urban ecosystem. Furthermore, urban vegetation influences the propagation of sound waves by muffling much of the city’s noise; affects the distribution of natural and artificial light; and, finally, is an extremely important component in the development of soil profiles that, in turn, control hillside stability.

Our urban settlements are still closely tied to the physical environment. Cities change these natural processes in profound ways, and we must understand these disturbances in order to make better decisions about adjustments and control.

As we examined above (see also Chapter 9), industrialization goes hand-in-hand with urbanization, so the environmental impacts of industrialization are often found in cities. But, urbanization generates its own set of environmental impacts: supplying enough energy, food, and water to large concentrations of people puts an array of stresses on the natural environment. Scholars refer to the extent of these varied impacts of urban areas on the environment as the urban footprint. For example, Las Vegas is one of the fastest-growing urban areas in the United States, yet it is located in a desert (Figure 10.29). The city’s primary water source is the Colorado River, but in more ways than one the costs of delivering that water are extremely high. It is expensive to construct the dams and infrastructure necessary to move the water into the city, and the environmental damage to the region has been very costly. The dams alter the flows of water through the Colorado Valley, harming fish and disrupting aquatic life cycles, and the energy required to divert the water to the city leads to higher sulfur dioxide emissions, which have been shown to lead to global warming. Las Vegas’s urban footprint, in other words, is large.

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The environmental effects of increased urbanization—the urban footprint—can also spread beyond the immediate stresses caused by higher concentrations of population. Urban living often encourages rising levels of consumption, as new urbanites gain access to better jobs and acquire more disposable income. This growing demand for things like different foods can impact areas far from urban centers. For example, as the United Nations report Unleashing the Potential of Urban Growth suggests, tropical forests in Tabasco, an area 400 miles away from Mexico City, have been transformed into cattle-grazing areas in response to urbanites’ demands for meat. In a second example, a major contributing factor to the deforestation of the Amazon is the increased demand for soybeans from the newly urbanizing regions of China as well as the urbanites of the United States, Japan, and Europe.

Urbanization, however, does not necessarily imply environmental degradation (see Subject to Debate). For example, the higher densities of population in cities can be seen as a form of sustainable growth. Half the population of the world—the urban half—lives on approximately 3 percent of the landmass. The concentration of people in cities therefore opens up other areas that can be protected and left relatively free from human use. Many countries in the developing and developed world are working on local and regional planning projects that, on the one hand, maintain urban boundaries and prevent sprawl, and, on the other hand, protect natural environments outside urban areas from the sorts of environmental degradations we have just described. These sorts of urban environmental conservation projects will become increasingly important in future years.

If you follow the news, you are surely aware of the natural disasters that seem to strike urban areas with a certain vengeance: tornados, floods, hurricanes, landslides, and earthquakes are just some of them. For the most part, there is nothing particular to cities as such that makes them more vulnerable to natural disasters. Yet in urban areas, with their dense concentrations of people, disasters are more destructive because so many lives and livelihoods are at risk. In addition, since many cities are located near rivers or coasts, they often lie in the direct paths of disasters such as hurricanes and tsunamis.

Making matters even worse, scientists have recently noted an increase in the number of natural disasters, attributable in part, they believe, to global climate change (Figure 10.30). Sea level rise and storm surges have particularly impacted cities because, as we just noted, so many of them are located near coastlines. Rising temperatures have also lead to extended summer heat waves that affect city residents. This combination of increasing urbanization and a growing number of natural disasters means that more and more people’s lives are being impacted adversely. It is most often poor people who are the most affected, since they tend to live in structures that are not well built and in sections of the city that are more vulnerable. In cities in the developing world, shantytowns are frequently situated on steep hillsides or poorly drained areas, making these areas far more vulnerable to landslides, flooding, and other natural disasters.

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The International Red Cross and Red Crescent Societies have termed this disparity in vulnerability to disaster “the urban risk divide” whereby “as the world’s population becomes increasingly concentrated in large cities, we are seeing the urbanization of disasters and disaster risk.” Some cities are better prepared for disasters when they strike—a component of resilience, discussed in the next section—while others, whether for lack of funding or concern on the part of government officials, are not. Compare, for instance, the human toll taken by two major earthquakes in 2010: in Chile, the death toll from a 8.8 magnitude quake was in the hundreds; while in Haiti, the death toll from a quake of slightly less magnitude was over 200,000 dead and more than 1 million homeless.

The wealthy, however, are not immune to natural disasters. Hurricane Sandy, which hit the coastline of New Jersey and New York in October 2012, disproportionately affected somewhat wealthier individuals. The storm caused $62 billion in damages and loss, with 375,000 homes destroyed, subway tunnels flooded, and business activities curtailed. (Also see Figure 3.27.) Without a doubt, global climate change and its urban impacts are a worldwide concern.

How have cities—specifically, urban residents and governing bodies—responded to the negative environmental impacts associated with their presence and growth, and the challenges raised by natural disasters (particularly global climate change)? As with many geographic questions, the answer depends on where you look. Some cities have been leaders in creatively confronting environmental challenges and reducing risk, while others have lagged behind. You might be surprised by which areas of the world make the top of the list in the area of disaster readiness (Figure 10.31). As a region, Latin American cities lead the pack, with 95 percent of the region’s cities engaging in some form of climate change adaptation planning, while U.S. cities lag behind all other places at 59 percent. Why is the United States so far behind everyone else in this area? The reasons are varied and can differ from city to city. Generally, however, the conversation about global climate change is a relatively recent one, coming at a time when many U.S. cities are facing severe budgetary shortfalls and even declaring bankruptcy. In addition, there are still many local as well as national policy makers in the United States who, for political or ideological reasons, do not believe that global climate change is a reality.

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Cities can take some measures that attempt to mitigate, or lessen, the impact of climate change on natural systems, humans, and the urban built environment. Mitigation attempts to reduce the extent of climate change by decreasing the emission of greenhouse gasses into the environment by cities. Such mitigation measures can include restrictions on urban industry regarding pollutant emissions, shifting to low-carbon and renewable fuels, and reforestation to reduce atmospheric carbon dioxide levels.

The very architecture of buildings can also be reconfigured in ways that mitigate climate change. For instance, the installation of so-called green roofs and green walls provides a surface that does not retain heat (thereby addressing the urban heat island phenomenon), that absorbs water (addressing rapid runoff) and carbon dioxide. In addition, these surfaces calm urban noise levels and provide a pleasant environment for urban humans, and refuge for urban wildlife (Figure 10.32).

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Yet in many ways, the conversation on climate change is shifting from a focus on mitigation or even sustainability—attempting to preserve conditions as close as possible to what they have been in the past—to include the notion of resilience. Resilience involves the ability to better predict future trends rather than looking backward to how things used to be. Resilience involves an acceptance of climate change as the new normal, and an emphasis on how to most appropriately respond to threats—both human (e.g., terrorism) and natural—in order to minimize risk to human lives, livelihoods, and property.

How cities respond to climate change will be a defining feature of life on Earth in the coming decades.

Subject To Debate

CAN URBANIZATION BE ENVIRONMENTALLY SUSTAINABLE?

The fact that more than half of the world’s population now lives in cities rather than rural areas has refueled the debate over the environmental impacts of increasing urbanization. On the one hand, increasing urbanization can be seen as efficient. Cities concentrate human populations at one point in space, which leaves surrounding lands available for conservation, agriculture, or other low-density uses. Cities create economies of scale because more people can benefit from the urban infrastructure—transportation, power supply routes, water servicing, and so on—allowing these services to be delivered at a lower cost per person. On the other hand, these same concentrations of population lead to greater demands on resources that must be brought in from outside of the city, such as water, energy, and food. City residents, with their relative wealth, tend to demand more and better resources, and such consumer demands, in turn, impact large swaths of countryside outside the city that are recruited to meet these demands. Are cities, then, sustainable?

First, we have to think about what sustainable actually means, and this issue itself is subject to debate! For our purposes here, sustainable urbanization is the creation of a situation whereby a society can meet the needs of contemporary urban dwellers for water, food, and shelter, while not damaging the ability of future urban dwellers to meet their needs. It was only in the late nineteenth and the twentieth centuries, with industrialization (see Chapter 9), that cities in Europe and North America grew rapidly in size, adding stress to the environment. That stress has been further exacerbated with the recent and rapid growth of cities in the developing world. And, it has been this recent and rapid growth that has caused alarm among policy makers and scholars.

On one side of the debate are those who focus on the adverse environmental impacts of large concentrations of people. Not only do cities have a direct impact on the land, in terms of the amount of space they occupy and the pollution they create; they also have indirect impacts, since urban dwellers typically have higher incomes and expectations in terms of material satisfaction than do rural dwellers, which places additional stress on the environment. More consumer demand for items such as meat, wood, and metals from city dwellers in, say, Vancouver, can have adverse environmental impacts throughout Canada and other parts of the Asia-Pacific Rim.

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Those who are optimistic about creating a more sustainable future with increasing urbanization argue that higher-density human settlements are better for the environment in the long run than less dense settlements. Even with the current growth rate of cities in terms of size and number, recent estimates based on satellite data show that urban settlements occupy only 2.8 percent of the total land on Earth. Urban spatial expansion per se does not appear to be a major environmental stressor. Urban density actually helps maintain fragile ecosystems by keeping them free from human interference. Proponents of this side of the debate argue that policies geared toward dispersing the world’s population away from big cities are misguided. The real problem, they argue, rests with unsustainable forms of production and consumption, and it is on managing these issues that the world’s attention should be focused.

Continuing the Debate

The debate over the environmental sustainability of urbanization is certain to grow in intensity as the world’s population continues its urban course. After all, many, many people are drawn to urban life. Given what you have read here, consider the following questions:

Can an increasingly urban world become sustainable? How?

Does your city have a sustainability plan? If so, how might you learn more about this plan and its feasibility?

How vulnerable is your city to environmental or man-made threats? Is your city among those that have begun to engage in resilience planning (see the Nature-Culture section in this chapter)?