The carbon used to construct biomass is absorbed from the atmosphere as carbon dioxide (CO2) by plant life, using energy from the sun. Plants may subsequently be eaten by animals and thus converted into animal biomass. However the primary absorption is performed by plants.
If plant material is not eaten it is generally either broken down by micro‐organisms or burned:
These processes have happened for as long as there have been plants on Earth and is part of what is known as the carbon cycle .
Fossil fuels such as coal, oil and gas are also derived from biological material, however material that absorbed CO2 from the atmosphere many millions of years ago.
As fuels they offer high energy density, but making use of that energy involves burning the fuel, with the oxidation of the carbon to carbon dioxide and the hydrogen to water (vapour). Unless they are captured and stored, these combustion products are usually released to the atmosphere, returning carbon sequestered millions of years ago and thus contributing to increased atmospheric concentrations .