Natural energy resources can be renewable or non-renewable. Renewable resources, which include solar, wind, hydroelectric, geothermal, and tidal energies, can be replenished within a human lifetime. Non-renewable resources like fossil fuels and metals are finite, as they were formed over long periods of geological time and cannot regrow within our lifetime. Fossil fuels like petroleum, natural gas, and coal were formed from the remains of ancient organisms exposed to heat and pressure underground, and provide non-renewable energy but also emit greenhouse gases.