Living and recently dead biological material. In the context of climate change it often refers to these materials when used as fuel or for industrial production of energy.
Biomass, in the context of energy production, is plant material that is grown to produce biofuel or to generate electricity and heat in power plants. Plant and animal waste is also called biomass when it is used to produce energy.
Biological material that has been dead for a long time, including that used in fossil fuel, is not considered biomass.
Many plant species, even some tree species, can be used in the production of biomass. Plants that grow fast and are easy to process are preferred.
Biomass is part of the carbon cycle. During growth, biomass removes CO2 from the atmosphere through phtosynthesis. When the biomass is burned as fuel or used in energy production, it releases the same amount of CO2 back into the atmosphere. This is what is referred to as carbon neutrality.
The plants used for fuel or in energy production can be continuously replaced by new ones planted and grown for the same purpose. Biomass is therefore a renewable energy.
When biomass is used and no new plants are grown, biomass do contribute to global warming. This happens when forests are cut to use the wood as biomass and no new plantations are made to replace the old forest.
Biomass is particularly effective against global warming when it is used in power plants for energy production. This is because the relative amount of methane to CO2 in emissions from power plants are lower than when biomass is simply rotting in nature. Methane is a more potent greenhouse gas than is CO2, so the lower the fraction of methane the better.