Energy Resources - Stock and Flow

There are two main components to an energy resource, stock and flow. Stock is the amount of the resource that is built up, and in the case of oil this would be the total amount of oil we have on the planet right now. The flow is the incoming amount of the energy resource that replenishes the stock. In the case of oil, there is basically no flow, because oil is created on the order of millions of years. In the case of solar energy, the stock is the sun itself, while the flow is the energy emitted by the sun.

If an energy resource has no flow, no matter how low the consumption rate, the resource will eventually be exhausted. Therefore, the sustainability of an energy resource is solely dependent on the flow. As long as the consumption rate is less than or equal to the flow, the energy is being used in a sustainable manner.

Since oil essentially has no flow, there is no way to use oil in a sustainable manner. No matter how much we increase the efficiency of gasoline engines, we will eventually run out. The only variable is when. The oil picture is bleak because consumption rates continue to rise even though we have reached the peak of our oil production (in 2005 in fact - see Hubbert Peak Theory and Deffeyes). As demand continues to increase and supply drops, price will rise, so clearly oil will become an energy source of the past.

The real question we face is not whether to move away from oil, but when and to what. We need to find an energy source, or mix of sources, that have a flow large enough to match our consumption (or we need to change consumption to match the available flow).

Adding more complexity to the situation is the third factor in energy use, the endogenous limit, or the limit created by the earth itself. There may be a limit to the amount of carbon dioxide the earth's atmosphere can contain and still be suitable for human life. Therefore we may want to move away from oil before it naturally runs out.