Biomass is the total weight or quantity of one or multiple species of living and recently living organisms in a specific area. It is composed of organic molecules containing carbon and hydrogen, usually also oxygen, and often nitrogen as well as small quantities of other elements, including heavy metals.
The term biomass can also refer to a fuel that consists of organic matter, including virgin wood, wood processing waste, energy crops, agricultural residues, food waste, and some industrial waste. Such fuel removes carbon from the atmosphere while it is growing, and returns it during combustion.
Current estimates of the earth's total biomass are around 1,100 gigatons (i.e., 1,100 billion metric tons), or about 545 to 550 gigatons of carbon, which constitutes roughly 50 percent of the dry mass of most organisms. Biomass is frequently measured in terms of carbon content because carbon is the fundamental element in organic molecules. This standardization simplifies the complex mixture of elements in living matter into a single metric for comparison.
Approximately 450 gigatons of this carbon is from plants, followed by 70 gigatons from bacteria, 12 gigatons from fungi, seven gigatons from archaea, and four gigatons from protista. An estimated 2 to 2.6 gigatons is attributed to animals, both terrestrial and marine, and 0.2 gigatons to viruses. Most microbial biomass, estimated at between 30 to 37 gigatons, is found in the terrestrial deep subsurface biosphere and ocean sediments. Humans represent just a mere 0.06 gigatons, or 0.01 percent of the earth's total biomass.