Carbon cycle

The combustion of carbon based fuels (which include fossil fuels like oil, coal and gas, but also biomass) releases carbon dioxide to the atmosphere, which acts as a 'greenhouse' gas and its increased concentration results in global warming and climate change. The crucial difference between fossil fuels and biomass lies in the time frame over which carbon dioxide is released. Burning fossil fuels releases the carbon that has been locked up for millions of years. Burning biomass, however, can be a part of the natural process called the carbon cycle: plants take up carbon dioxide when they grow, to construct the organic biological molecules that make up the bulk of their dry mass, and when the plants are eaten, burned or decomposed, the carbon is released again and is returned to the pool of carbon dioxide in the atmosphere.

Biomass can affect the carbon emissions in two ways - it can serve as a substitute for fossil fuels, but it can also change the amount of carbon sequestered on land, effectively acting as a 'carbon sink'. Increased afforestation or reforestation, as well as changes in cropland management practices can help in removing additional carbon dioxide from the atmosphere. Converting the harvested wood into wood products also acts as a carbon sink, as it increases the stock of carbon. However, forests and products derived from them have only a finite capacity to remove the carbon from the atmosphere, thus the main way to decrease the current carbon dioxide emissions is the substitution of fossil fuels with biomass and other renewables.

Carbon cycle illustration (courtesy of ORNL)