Overview


The terrestrial biosphere takes up 60 GtC (Gigaton carbon) per year via photosynthesis, while the phytoplanktons in the ocean absorb almost as much. The carbon assimilated on land is allocated to three major carbon pools: leave, root, wood. As leaves fall and trees die, they are decomposed by bacteria, fungi, insects and animals, and eventually the carbon goes back to the atmosphere on the timescale of a season to hundreds of years. As a result the net carbon exchange between the atmosphere and the biosphere is nearly zero. This is part of the so-called ‘fast carbon cycle’, compared to the ‘slow carbon cycle’ which determines fossil fuel formation on geological timescales. Modifying the fast carbon cycle is usually considered not effective at directly influencing the atmospheric CO2 which is controlled by the slow imbalance of the geological reservoirs.

However, 60 GtC/y is a large flux compared to the current fossil fuel emission rate of 8 GtC/y. If we can ‘siphon’ only a fraction of this fast carbon cycle and store it away semi-permanently via active human management, we may be able to sequester a significant amount of carbon to counter global warming. In this respect, all the biospheric carbon sequestration proposals falls in this category. Some of them are briefly discussed below in two steps: carbon carrier and storage method.

Carbon Carriers

Wood
Wood is preferable to leaves and roots because its lignin-cellulose structure gives it several advantages including resistance to decay and less nutrient loss. Estimates suggest a 10 GtC/y in the form of coarse wood is produced in the world’s forests. The practical potential may be 1-5 GtC/y. However, the wood needs to be harvested via collection of dead wood or selective cutting in a way such that the forest is least disturbed.

Crop Residue
Crop residue could be partially harvested and stored. It is estimated that there may be about 1 GtC/y from the world’s agriculture.

Biochar
A process called pyrolysis, heating without oxygen, can convert essentially any biomass into char, a reduced and very stable form of carbon.

Plankton and Fish
Dead plankton and fish fall to the bottom of the ocean, and some of them eventually get buried in the sediment. The carbonates carried in shells and bones, though inorganic, are biologically produced. A small fraction of organic matter buried this way eventually becomes oil. The productivity of the ocean is mostly limited by nutrient, so artificially increasing nutrients such as iron can increase the productivity, and possibly increasing carbon burial rate.

Storage Methods

Under Soil
Under a sufficiently thick layer of soil oxygen would be quickly depleted, thus preventing decomposition. Coal was formed by the burial of ancient plants in anaerobic conditions such as swamp and peatland. The proposed wood burial method is essentially a first step of a fossil fuel formation process.

Under Water
Oxygen is generally depleted under water compared to in the air, thus would also slow down decomposition. In particular, peats accumulate in wetland.

Above-Ground Shelters
This has the advantage of saving the carbon and energy stored in these ‘carbon/energy banks’ and they can be used more easily than burial if future bioenergy technologies such as large-scale cellulosic ethanol and advanced wood combustion become economical and environmentally sound.

Mixing Biochar in Soil
This enhances soil’s ability to retain water and nutrients, and until it decomposes, also stores carbon.

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