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What is the difference in CO2 released between a large suburban city and a small urban city?
What is "the challenge" we hope to address with this project?
Designing the City of Tomorrow's Tomorrow, Tomorrow.
When urban development is discussed in the context of the twenty-first century, the conversation centers on two issues; how can we design cities which reduce climate change? and, how can we house a global population of more than 9 billion? Solving these problems will involve changes in lifestyle and new technologies. Urban spaces are unique in that they can create lifestyle change through new technologies and through the organization of these spaces. We compare two approaches to urban development, an "Old World" approach and a "New World" approach, assessing the environmental impact of each, and propose a new direction for urban development.
Among many challenges facing us in the 21st century, population growth is perhaps the largest. By 2025, there will be an additional billion people to feed and house. By 2050, there will be another billion.
To meet the demands of another 2 billion people will require enormous economic growth, and correspondingly large emissions of CO2. If business continues as usual, the presence of these people will lead to an increasing rate of emission, decreasing our ability to keep the concentration of CO2 in the atmosphere below the 500 ppm "point of no return" unlikely. While there will be no single solution to this problem, improved urban planning can help mitigate the impact of global population growth.
High density urban development, alongside new sustainable building construction and use strategies are a powerful way to reduce emissions compared to a baseline of "business as usual". This website will be an examination of how to construct the city of the 21st century - one which will provide people with structures, both physical and metaphorical, which reduce CO2 emissions, while providing affordable and livable spaces for a population of 10 million.
Overview and Constraints
The archetypical high density urban city - New York - has a population density of 10,640 people/km^2. To house an additional 2 billion people will therefore require ~190,000 km^2 of land, if they are to live in cities similar in density to New York. The land area of New York is ~1200 km^2, requiring ~160 cities the size of New York to be built over the next 50 years. Building even one city of this size is a monumental undertaking, and the environmental damage that will be done if this isn't done sustainably will be immense. However, high density construction is better than the alternative - expanding current cities beyond their current boundaries, while using present down construction techniques, and depending on legacy infrastructure which was designed without considering the long term impact - both on people and the environment.
City Site Selection
Each of these new cities will require a careful process of site selection, not only with respect to the livability of the new city(the Sahara isn't a great place for people to live, city or no) but also with respect to the amount of CO2 liberated from sinks as construction begins. Site selection will have to balance the environmental benefits of being able to utilize local resources with the environmental costs of any particular site. Consider, for example, a city constructed where a forest once was. CO2 is sequestered in above and below ground biomass,
deadwood, litter, and soil organic carbon(source EPA). Using the United States as a model for global forest CO2 sequestration, the following figure gives the carbon density in Megagrams/ha.
As can be seen, the CO2 emissions from clearing forests for conversion to urban environments will be highly dependent on the site selected. Regions which are ideal for conversion are areas of the American west and South - mediterranean climates and deserts. A city of the type we are considering, with an area of 1200 km^2 can release as little as 18 Megatons of CO2 in those climates, or as much as 36 Megatons of CO2. Constructing the 160 needed cities of this size could release as little as 3 Gigatons of CO2, a change in CO2 concentration of ~1.5 ppm(1 ppm = 2.13 GT atmospheric CO2). On the other hand, converting higher carbon density areas to forest could produce 6 Gigatons of CO2, and a corresponding increase in atmospheric CO2 of ~3 ppm. In comparison, constructing 160 cities the size of the Los Angeles metropolitan area would produce, at minimum, approximately 30 Gigatons of CO2, raising atmospheric CO2 by ~15 ppm. Since sprawling cities represent the dominant trend in 20th century urban planning, we take this estimate to be the baseline for land clearing emissions. The decision to build highly dense urban environments in Mediterranean/chaparral/grassland as opposed to large suburban cities on old growth forest could save as much as 57 Gigatons of CO2, and result in only 1.5 ppm, as compared to 30 ppm from city construction alone.
While this calculation examines only the CO2 released from the clearing of land for new cities, and assumes that the CO2 will enter the atmosphere directly(in reality, some lumber will be sequestered in construction, paper production and the like), the positive benefit of reducing land clearing for cities and by choosing sites in certain biomes may be counteracted by the need to transport resources which are not available in the areas which are best for new city construction. For example, constructing a city in Dubai is favorable from a land use perspective - as few old growth carbon sinks are present - but may ultimately see greater CO2 emissions from transportation of food, water and other goods to a desert city. We examine this in more detail in the section on agriculture, but we do not believe this effect to be large enough to change the above result.
The majority of population growth in the 21st century will occur in the global south, countries which are traditionally economically disadvantaged. The challenge of residential development in new cities is then centered on apparently contradictory goals. On one hand, we want to build sustainably - something that is seen as expensive, and therefore out of reach, in those countries which will see the largest growth and require the most development. On the other hand, we need to build cheaply, in order to house these people at all. The solution to this apparent contradiction is to design structures in recognition of local constraints, and to build sustainably while doing so. These involves a dependence on both new technology, and on lifestyle choices. In the following analysis, we will take as a baseline, the typical american single family home, sometimes referred to as the "American Dream", and in many ways, a symbol of the lifestyle that those in developing economies ought to have access to. According to the Environmental protection agency, the typical single family home emits 19.43 tons/CO2 per year. If each home houses 4 persons, this translates to approximately 5 tons CO2/year/person. This translates to 9 Gigatons of CO2/year, or ~4.15 ppm/year, if we are to house the growing population in American style single family homes, and this does not include emissions due to construction of single family homes.
Clearly, this is not a sustainable path forward, and represents a "worst case" condition. Instead, we propose a path forward involving sustainable construction, sustainable power generation, lifestyle choices through multi-family construction and living. We now consider utilizing apartment buildings with an average per unit area of 80 square meters - about 1/3 the size of the average American single family home.
Sustainable Residential Construction
One of the first steps towards sustainable housing and reducing our global footprint is that of the construction. While re purposing currently constructed buildings is an obvious path, current estimates put global population at 11 billion people by the year 2100. The UN report for population growth to 2300 suggests similar global population distributions but a large rise in certain areas in Africa and Asia. Despite the "American dream" home that is desired in much of the world, that path is not possible with that population rise and demands for land use for housing, food, employment, etc. We focus on an apartment style that integrates home life with social and marketplace needs.
We can get recycled materials from post consumer waste which is the most common in the public view.
If we recycle steel for construction purposes there is a huge savings in reduction of CO2 emissions. With steel production there's approximately 50% reduction.
Aluminum is another material that is recyclable in favor of further excavation. Remelting aluminum can reduce 80% of the alternative.
There are other sources for recycling efforts and they involve the industrial processes themselves.
The city will likely contain concrete and an energy efficient form utilizes waste material. Fly ash from coal fired plants is usually disposed but we can use it as a base with sand and gravel substances to create concrete. It can account for 35% of the cement.
Inside the buildings themselves processes such as recycled glass floors (as the new building 52 show) and uses of recycled wood products is another approach of attack.
By decreasing the number of single family homes, energy use for heating and cooling will be reduced. Because the exterior surface area of an apartment building is much smaller than the exterior surface area of the homes it replaces, the heat loss through the walls is smaller. But it can also be decreased with better insulation and material choices, further decreasing emissions.
Sustainable Power Generation
Along with population growth, demand for electricity for residential use will increase. With the energy supply representing 26% of global CO2 emissions,
mitigating emissions from CO2 for electricity generation is paramount. We now consider residential power generation as a potential mitigation strategy. Considering a 10 unit/floor apartment building, with each apartment being 80 square meters, this gives 800 square meter of rooftop available for solar power generation. A solar panel can generate approximately 150 W/m^2 or 1.314 MWh/year/m^2. Each rooftop, then, products about 1 GWh/year. The average American home uses 11 MWhr/year according to the EPA. Without lifestyle changes, solar panels on apartment buildings can offset about 90 American homes. The CO2 emissions offset as compared to coal generation, the generation of choice in the developing world, is approximately ## tons/building, which translates into ## for the city in one year, given some assumptions not stated here.
Total Residential Impact as Compared to Worst Case
Using these various technologies construction of the city is far more carbon neutral than the alternatives. Approximately 40% of US carbon dioxide emissions are due to residences and cars. Constructing cities of this type that are designed to use local, recycled, re-purposed materials as well as the physical layout being that of reduction of driving distances are a better approach.
The average American drives for 13000 miles/year and goes to the grocery store approximately /week. If on average a store is located 4 miles from home with an average of 2 trips per week (according to the Food Marketing Institute). Yearly this amounts to 450 pounds of CO2 emitted when we look at only grocery trips. Yearly total driving emissions are 7,280 pounds so a city plan that places grocery stores/clothing stores/restaurants/work/etc. in locations near would reduce this total usage.
According to the IPCC, transportation accounts for 13% of global CO2 emissions. In this section we address transportation strategies in the new city. These strategies include mass transit, automobile and truck emissions, as well as strategies related to the structure of cities and lifestyle changes.
Green Parking Spaces
Parking lots degrade water quality, strain stormwater management systems, consume large amounts of land and resources and enable urban sprawl.
Paved surfaces increase air temperature by 2-8 degrees F during the summer. On a hot summer day, roof and pavement surface temperatures can be 50–90°F hotter than the air, while more rural spaces (shaded or moist) remain close to air temperatures.
This surface heat island effect alters the entire region. Air temperatures in cities, particularly after sunset, can be as much as 22°F warmer than the air in neighboring, less developed regions.
The tangible effect of this heat island is the rise for electricity demand - it rises 1.5–2.0% for every 1°F increase in air temperatures. Approximately 5–10% of demand for electricity is used to compensate. This results in 6 million pounds of CO2 given our proto-city size.
Other effects are in the groundwater. If the paved surfaces are 100 degrees F and approximately 4 inches thick rainwater (which is often about 70 degrees F) to 95 F. Because of the completely paved surfaces in most cities this water goes straight into runoff until rivers and lakes have higher temperatures leading to further ecological problems there. By instituting green parking spaces and green roofs with additional landscaping the majority of the city from a top down approach would have plant or groundwater access reducing this effect to a minimum.
Land cover vs Solar panels
Another suggestion is instead of green parking spaces use solar panel shaders. This would generate electricity that would cover the demand due to the heat island effect. However, it doesn't really address the problem itself, this just offers an unsustainable solution to the issue that isn't necessary. We can reduce our usage simply without the need for unnecessary construction.
Mass transit options can be assessed according to their Carbon footprint. The following figure, from the US Department of Transportation, compares CO2 emissions for various transportation options.
At 0.22 lbs/passenger mile, heavy rail(subway) and at 0.36, light rail, options approximately 3 and 4 times more carbon efficient than automobiles. For a city of 10 million, we can estimate emissions due to transportation by comparison to our model city of a similar population and density - New York. New York public transit, in 2009,
had a ridership of ~12 billion passenger miles. At 0.22 lbs CO2 per mile, this resulted in approximately 1.3 Megatons of CO2 released. This is a significant savings compared to the 5.5 Megatons of CO2 which would have been emitted by private vehicles. This analysis does not include considerations about distances travelled - larger cities have far higher passenger miles/year. A dense urban city with abundant public transit will emit far less CO2 than a spread out city with transportation primarily due to private single occupancy vehicles. Our worst case estimate for CO2 emissions due to transportation has car use being 4 times public transit use, at 48 billion passenger miles, and zero public transit options. This would result in 21 Megatons of CO2 emission per year.
Toll roads - Impacting Individual Choice
Clearly, decreasing dependence on personal vehicles can have a significant impact on CO2 emissions in new cities. This can be accomplished by building the emission costs of our transportation into our method of choice. This can be done by decreasing bus or rail costs, while increasing the cost of automobile use. Toll roads seem a simple method of increasing these costs. By charging a small fee, the use of roads will be disincentivized for short trips, while mass transit becomes an attractive option. Those who continue to utilize the roadways will also be subsidizing mass transit, further lowering costs and encouraging the use of lower carbon transportation options.
Transportation Emissions as Compared to Worst Case Baseline
The average American drives for 13000 miles/year and goes to the grocery store approximately /week. If on average a store is located 4 miles from home with an average of 2 trips per week (according to the Food Marketing Institute). Yearly this amounts to 450 pounds of CO2 emitted when we look at only grocery trips. Yearly total driving emissions are 7,280 pounds so a city plan that places grocery stores/clothing stores/restaurants/work/etc. in locations near would reduce this total usage. Using toll roads to change the desire for short trips and mass transit options reduces this total emissions output. As stated in the mass transit section for a city using mass transit the reduction is use is approximately 4 Megatons of CO2 released.
Food demands are omnipresent but creating local areas for food production can be difficult in a developed city. Suggestions of vertical farms have been presented but we've determined that they're not cost effective. One of the challenges with incorporating food production is the space and light requirements as well as the weight of dirt and water in the space. Without a properly designed space the building space can degrade quickly.
Aquaponics is one form of food production that reduces some of those consequences. Food systems can incorporate plant production and fish farming. A combination allows fish waste to provide necessary nutrients for the plant matter above. Compared to a conventional farm an aquaponic system (to produce the same amount of food) requires only 2% of the water.
Plants are grown immersed or with direct root contact to the nutrient filled water which can then be circulated back to a fish tank continuing the cycle. Plants that can grow in the system include many leafy greens, tomatoes, peppers, legumes, melons, onions, herbs, etc. Staggering plant removal allows for plants to fully germinate as desired. Freshwater fish are often used below and include tilapia, crayfish, prawns, catfish, and some cod.
The above diagram displays the aquaponic cycle.
In terms of conservation, as stated the water usage is far less than traditional methods which is increasingly attractive when we take into account a global population of 11 billion by 2100. Technologically the system requires some electricity to run the pumps but in comparison to that needed for traditional harvesting techniques it is negligible.
One commercially available system requires 32 feet of grow space leading to 320 pounds of vegetables a year. This amount of food can fulfill one individual needs but can supplement a family. A 300 gallon fish tank can itself produce 100 pounds of fish a year.
Aeroponics is a similar practice involving growing plants in an air environment so there is no growing medium. Nutrients are delivered via nutrient rich mist. The plants are largely sealed off from outside environments largely reducing the potential for pests and diseases. In the case of sickness individual plants can be removed without affecting the entire system.
The growth rate of aeroponics compared to hydroponics and traditional farming techniques surpasses yields in other systems. Plants with various concentrations of nutrient rich mists produced more output and growth than both hydroponics and traditional farming.
These systems do require a careful eye of ph and ppm levels making a backup hydroponic system desirable. With the use of a solar panel electricity demands are mitigated. The requirements for water are even further reduced than with hydroponic systems though a hydroponic backup is sometimes used.
Plant systems have complete access to carbon dioxide necessary depending on location. A system at sea level has 450ppm during the day and 780ppm at night.
The system pictured above is located at a US airport producing local food for their needs. The aeroponic system does not have to be a flat bed but can also be managed like this allowing for more growth per square footage.
Values / Conclusions
This proto-city overview is an examination of how we need to redefine what city living is and promote a new ideal. That of suburban style American dream life is not sustainable with the current growth rates of the world. In locations like China and Africa where the population will grow most dramatically completely new construction will be required. Designing compact living spaces with the promotion of little necessary travel is largely promoted by value needs. With reduction is CO2 output the expected growth in the population can be mitigated. This project is not a complete solution but a combination of relatively simple techniques we believe are necessary in modern city design. The incorporation of these techniques into a city design reduces energy usage in construction, agriculture, and transportation requirements. Using recycled materials, green spaces, public transportation, compact layout (with carefully planned use locations) and local agriculture resources Megatons of CO2 are not emitted when compared to a city/suburb that does not take these measures but that are still being built.
One thing that is clear and obvious is that our path forward as individuals and as a growing population base is forethought into environmental effects must be considered and can be considered regardless of socioeconomic status. Promoting the use of local materials for construction is not only efficient but can be culturally relevant (such as a resurgence in the use of pueblo materials). City structure that promotes easy access and public transit has a large role. Techniques of hydroponics and aeroponics are great for locations where land quality is very poor and provide sustenance for lower income families. These technologies are also equally as helpful for traditional suburban families. Regardless of socioeconomic or geographical location many of these techniques can be applied and produce significant effects.
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