The biochar process is carbon negative: it removes net carbon from the atmosphere. When a green plant grows, it takes CO2 out of the air to build biomass. All of the carbon in the plant came from CO2 taken out of the air, and returns to the air when the plant dies and decomposes. When the biomass is instead pyrolyzed—heated in the absence of oxygen—it produces charcoal, which is called biochar when it is buried in the ground. Over 40% of the total carbon from the waste biomass is retained in biochar and sequestered in the soil for thousands of years, effectively removing that carbon from the atmosphere. The carbon in one tonne of biochar is equivalent to about 3 tonnes of CO2.
Biochar is not only a carbon sink, it increases soil fertility—increasing cat-ion exchange and water retention capacity in soils, while reducing nutrient leaching and providing a "coral reef" for soil microorganisms—thereby significantly increasing productivity and crop yield, often by 80%-220%, depending on the original quality of the soil. Initial studies also suggest that biochar reduces nitrous oxide and methane emissions from soils, two greenhouse gases that are far more potent than carbon dioxide.
Biochar research was first inspired by the discovery of Terra Preta, or "black earth", soils in the Amazon Basin, where an ancient civilization buried charcoal, along with fish bones, kitchen scraps, compost, and pottery shards, transforming otherwise barren tropical soil into Terra Preta. These soils are still incredibly fertile today.