Why are people concerned about the sustainability of fossil fuels?

Burning fossil fuels are our main way of generating energy for the world to use, but the process emits greenhouse gases into the sky, promoting global warming and pollution. Fossil fuels are also running out, and pretty quickly as the need for energy in the human world is just too large. Fossil fuels take so long to make, millions of years in fact. Though they are the most efficient way of getting energy, they emit greenhouse gases, and due to the extreme usage of them for energy, the amount of the fuels would decrease rapidly.

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