|Fig. 1: Cost of Renewable Energy.  (Source: K. Palma)|
The earth's population is continuously increasing. This rapid growth of populations has also lead to the migration to cities and the overall growth of these urban areas. As cities continue to grow in size, the energy demand increases along with the amount of emissions from current natural resources. This scenario proposes a series of issues for ecosystem services (goods we get from the environment) such as water, air, and food quality. As these issues begin to compound, other important factors such as economic and social issues arise. As you can see in Fig. 1, there is a very high concentration surrounding urban cities. Currently 51% of the Earth's population lives within 2% of the Earth's surface area. As the density of urban areas grow, the challenge is to create an urban environment for humans that does not impact the natural ecosystem in the surrounding areas. In order to achieve a sustainable city, one must take a multidisciplinary approach to solve this interconnected puzzle. The goal of solving this puzzle is to create an urban environment with clean ecosystem services and this will allow a city to prosper. Once this puzzle is completed, we will have a sustainable urban area.
Urban areas are typically known for carbon dioxide emissions. Heating from natural gas, cars burning fossil fuels, factories emitting for production purposes, these are all contributors to poor air quality. Although household units in urban areas typically consume less resources that lead to carbon emissions, the density (amount of units) is per capita is a lot higher. Poor air quality leads to health issues for citizens and overall negatively impacts the quality of life in urban areas. 
Water quality in urban areas is very often polluted, most of the time not even on purpose. Urban runoff is a process caused from everyday car use such as the rubber from tires, oil spills and other car fluids from the road being washed away by rainfall. As the mixture reaches soil and storm drains, a majority of it begins to sit in the soil and some of it even reaches bodies of water. It is quite often that water is sent to a waste water treatment plant, where the water is filtered and recycled. This can at times be a costly process depending on the contaminants in the water. Essentially the less polluted the water is to start, the less the city will be paying to filter it.
First and foremost, the technology on renewable energy resources has begun to greatly increase. Solar, wind, and hydro power sources have the best potential to reduce carbon emissions. These sources are currently not developed enough to support entire cities. Hydro-power is very limited because of the lack of large bodies of water to support a dam. Solar power is limited because of a lack of ability to harness energy from the sun and store it. Another large issue is the pricing of the resources. Renewable energy sources are less developed, thus there is more complicated technology that is required to harness this source of energy. As the technology is developed and continues to grow in complications, it is likely that these sources will continue to be more expensive compared to natural gas and petroleum. Governments have already set carbon taxes and other policies to reach total reduction form carbon emissions within a few decades.
Urban environments are full of carbon emissions, they come from a variety of sources and negatively impact the quality of life with the city. Renewable energy resources are the answer to achieving urban sustainability. These sources will work with the environment to harness energy and ultimately draw humans away from fossil fuels and natural gas. With these polluting sources gone, the air will be fresher, the water will have less pollutants, and we will have achieved a sustainable urban city.
© Kevin Palma. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 R. L. Maynard, "Health Effects of Urban Pollution," in Air Quality in Urban Environments, ed. by R. E. Hester and R. M. Harrison (R. Soc. Chem. Publishing, 2009).
 R. Gelman, "2009 Renewable Energy Data Book," U.S. Office of Energy Efficiency and Renewable Energy, DOE/GO-102010-3074, August 2010.