Why is biomass energy not always sustainable?

The problem with biomass is that it is being promoted widely in recent years as a way to reduce carbon dioxide emissions to the atmosphere ... Amplified production of wood pellets for fuel is likely to put more carbon dioxide into the atmosphere and preserve fewer species on the landscape during the next several decades.

Explanation:

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Biomass energy usually provides an irreversible mitigation effect by reducing carbon dioxide at source, but it may emit more carbon per unit of energy than fossil fuels unless biomass fuels are produced in a sustainable manner.