Our civilization grew by leaps and bounds once we started living in cities. These communities allowed individuals to specialize. Rather than a subsistence living, individuals could rely upon others for some of their various needs and they could use their time to focus upon a singular goal. For example, painters expanded from iconic images on cave walls to surrealistic images on canvas. Cities gave them the opportunity, our civilization has flourished ever since.
At first, cities relied upon adjacent lands with which to supply the needs of their populace. Neighbouring farms produced much more food than could be consumed by the farmer. They sold the excess at market. City residents bought the food with money that they earned through their specialized craft such as candle making. All benefited and new produce and capabilities made the populace ever more capable.
Today, in our global community, city residents can contemplate minutiae such as atomic physics while munching upon food grown thousands of kilometres away. This is the state of things as we now know it. However, what would happen if city residents couldn’t draw upon the produce of the globe and had to rely upon the immediate surroundings? Let’s look.
Take Rome, a modern city in Italy. With some simple checking on the Internet, we can learn a lot. It has about 2.7 million residents and officially encompasses an area of 1285 square kilometres. The Internet can also give us an energy balance for this city with a little deductive reasoning. First, consider the local land cover types. These are available from the GlobCover project (http://ionia1.esrin.esa.int/) as determined by satellite data down to fidelity of 300metres. By plotting the data we can determine the land cover as shown in the following figure.
The red pixels indicate a non-vegetative land cover, a land type that does not capture energy from the Sun. These are effectively the city footprint. The other colours, yellow and green and such represent land with cover that contains vegetation that is capturing energy from the Sun. The extent of the figure is to match the official size of the city rather than to follow the city’s actual boundary.
The satellite shows us detail that wouldn’t be easy to see from the ground. For instance, 29% of the above region has insufficient vegetation to register with the satellite. That is, the human made structures are so dense that there is no evidence of vegetation.
The land cover tells us another story. From it, we can determine an energy balance for the city of Rome. The Italian nation has an annual energy consumption of 6.84e18 Joules per annum. Given this and the population of the city then the resulting energy draw down is 3e17Joules per annum. We can also determine the energy stored in the vegetation by using the land cover type and an estimate of the Joules per gram of vegetation. The result is 5e14Joules. That is, in one year, the city of Rome uses more than 600 times the amount of energy that’s immediately available within their city.
Let’s extend the above region to the point where the local availability of energy satisfies the annual need of the people of Rome. For this we have the push the limits out about 11 times as shown in the following figure.
The above clearly shows Rome in the centre of the Italian peninsula. It also shows the wide expanse of vegetation that would need to be consumed by the populace of Rome so as to satisfy the energy demands for one year. This doesn’t take into account the energy demands of all the other people and cities in the same region.
The purpose of this exercise is to demonstrate the energy availability if people were to satisfy their needs using immediate resources. While the possibility exists, assuming a lossless energy transfer, recall that so doing would only satisfy the energy needs for one year. Then after, no vegetation would exist for subsequent years. Nor would any vegetation exist for any other purpose such as feeding people or feeding other life forms. The energy draw would effectively consume all life in the region.