Sustainable cities

5. fuel natural sources of energy such as petroleum, coal or natural gas that form within the earth from ancient animal and plant remains
6. having too many people for the amount of food, materials, and space available in a given area, city or country
8. power produced by running or falling water, which may be used to produce electricity
10. the change in state from a solid to a liquid
12. an overflowing of water in a normally dry area
13. energy from a source that cannot be used up or run out, such as energy from wind or water
14. buses, trains and other forms of transport that are available to the public, charge set fares and run on fixed routes
15. energy from a source that can run out or that takes a long time for the earth to produce, such as oil and coal

1. changes in weather patterns and the overall temperature of the earth
2. the total amount of grenhouse gases released into the atmosphere by a person, organization, event or product
3. the ability for something to be maintained or to keep going
4. a covering of ice over a large area, especially on the polar region of a planet
7. gases, such a carbon dioxide, methane and nitrous oxide, that trap heat in the atmosphere and make the earth warmer
9. to make something new from something that has been used before
11. energy derived from the sun in the form of solar radiation