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The environmental impacts of fossil fuels aside, the central issue here is geological. Whether in an individual coal pit, oil, or gas field, a nation or the planet as a whole, fossil fuel reserves and production tend to follow a bell curve over time. That means production rates increase on the upslope of the bell curve, and decrease on the down slope. Typically, the peak of this curve occurs when reserves are 50 percent depleted, which is followed by an irreversible slow decline in production following the law of diminishing returns.