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What Does Carbon Negative Mean?
Richard S. Stein
University of Massachusetts, Amherst
There appears to be some misunderstanding of the meaning of
“carbon negative”. Most consider it to refer to a process which
reduces the CO2 concentration in the atmosphere. This is in contrast
to what happens when fossil fuels like coal and oil are burned which
produce CO2 by their reaction with the oxygen in the air, resulting in
an increase in its atmospheric concentration. This is “carbon
positive”. Processes resulting in no change in atmospheric CO2 are
properly called “carbon neutral”. This would be true for photovoltaic,
wind, and geothermal processes.
There is some controversy whether biofuels are carbon neutral.
Carbon dioxide is consumed in the growth of biomaterials, where,
through the catalytic reactions occurring through the absorption of
light, it combines with water to produce energy containing organic
molecules such as sugars, starch, and cellulose. When these are
burned, they react with oxygen to produce CO2 and liberate energy.
Ideally, the energy evolved is exactly equal to that absorbed in the
photosynthesis, and the amount of CO2 evolved equals that which
had been absorbed, In this sense, the change is “carbon neutral”.
However, one must consider the entire process. Energy is consumed
in the cultivation, harvesting, and collecting of the biomass, so if this
is considered, less energy is obtained on burning than the total
amount of energy used to grow and deliver the biomass.
Coal and oil are formed from biomass through geological processes
involving the influence of heat and pressure occurring over many
thousands of years, whereas the burning of the derived fossil fuel
usually occurs in short time periods. In this case, the CO2 which is
liberated during this short time of burning is that which had been
absorbed over many, many years. The process would only be
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carbon neutral if the material burned was that which was derived from
biomass which grew during the time period of burning.
If one obtains power from burning biomass directly, it is essential, in
order to be carbon neutral, that this restriction be observed. The
amount of biomass being burned should be limited to that which grew
during the period of burning. Harvesting of biomass following this
restriction is referred to as “sustainable harvesting”. Its collection
must be made from a larger area of growth to meet this requirement.
It is more expensive to do this, so doing so under profit motivation
may not suffice. Regulations are needed to assure that it be done.
As indicated, burning under conditions where more CO2 is produced
is “carbon positive”. Itʼs environmental impact can only be reduced if
this CO2 is prevented from entering the atmosphere. Ways to do this
1. Pumping it into the ocean.
2. Pumping it into underground cavities.
4. Sequestering through chemical reactions.
CO2 has limited solubility in water with which it reacts to form
carbonic acid. This increases the acidity of the water which can be
detrimental to marine life and cause bleaching and dissolving of coral
and and crustacean shells. It is thought that oceans are approaching
their capacity for capturing CO2 in this way. An alternative is to
liquefy the CO2 and pump it to deep regions of the ocean where the
pressure is sufficiently great for it to be stable
Pumping to underground cavities is being employed, but numbers of
suitable locations are limited and it is costly to deliver it to remote
ones. A possibility is to use it to replace depleted underground oil
deposits and even to use it to force oil to the surface. There is
concern about stability, since geological phenomena are not well
enough understood to be confident that it will not leak back to the
atmosphere or be catastrophically released as a result of
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It has been suggested that it could be adsorbed in materials like
carbon nanotubes, but at present, the cost of doing so would be
There are chemicals that will react with CO2 to form stable solids.
For example, lime, CaO, will do so to form calcium carbonate,
CaCO3. However, most lime is made from limestone in a process
which releases CO2. Cement making processes have been
proposed which trap the CO2 in the cement. However, economics is
again a problem, and one has yet to find a cost effective procedure.
A practice which is encouraged is to leave agricultural waste on the
field where it serves to enhance the organic content of the soil,
improving its agricultural value. However, these organic components
have been shown to decompose in a few years, releasing their CO2.
The CO2 capture has a short term benefit and could be regarded as
“carbon negative” during this time period, but in the long run, it is not.
The use of biochar has been proposed as a means of achieving
carbon negativity. The biomass, grown with the absorption of CO2, is
pyrolyzed (heated in the absence of or with limited air), Combustable
vapors are evolved which serve to continue the heating. The biomass
gets converted to biochar, a form of charcoal having useful value. It
serves as an agricultural additive, and when mixed with fertilizer and
mulch, promotes agricultural growth. It is believed to adsorb the
nutrients on its surface and porous interior and serve as a matrix for
the growth of fungi and bacteria, enhancing the growth of the
The biochar has been shown to remain in the soil for very long
periods of time, sone being found there derived from this practice
employed by natives in the Amazon thousands of years ago. Hence,
it serves top capture the carbon from the CO2 in a stable form, thus
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serving to reduce the CO2 concentration in the atmosphere. This is
really “carbon negative”.
In addition to this benefit, it has been shown that less fertilizer is
required when combined with biochar in that the fertilizer is
concentrated in the biochar matrix where it is more effective. Its being
being bound to the biochar prevents the loss of excess to streams
and rivers where it may pollute.
To achieve carbon negativity, one should combine biochar use with
sustainable harvesting so there is a net removal of CO2 from the
atmosphere with long term sequestering in the soil. While less heat
is produced during biochar production than would be of the biomass
were just burned, its agricultural and soil enhancement benefits more
than compensate. Its use can be cost effective for the farmer and of
value to society.