What Is Biosequestration?

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Biosequestration is the capture of carbon from the atmosphere by the biological process of photosynthesis in plants and phytoplankton. It is what plants do. We used to call it primary production until it became part of the climate change lexicon.

Biosequestration happens all the time and without it we would not be here. The fixing of carbon from the atmosphere into carbohydrates is what provides our food and forms the basis of food chains for just about every organism on Earth. It is so fundamental that we take it as a given.

The new name for an ancient process came about when we began to look for solutions to help balance greenhouse gas emissions.

Image a wheat field.

In winter the soil is bare and in summer after the crop is sown the wheat plants grow, mature and the grain harvested to be processed and eaten with jam. The rest of the plant is eaten by livestock, ploughed back into the soil or even burnt. Most of the carbon sequestered by the wheat plants is taken away from the field or is returned to the atmosphere through decomposition.

Now imagine woodland, perhaps the one that was cleared to create the wheat field. The tall trees are roughly 50% carbon and the roots spread wide a deep hold as much carbon as the trunk and the branches. The woodland holds many times more carbon than the wheat field.

The woodland captures carbon continually as the trees grow. There are interruptions through fire, disease or when individual trees die and decompose, but most woodland and forests are net sinks of carbon.

Conversion of land from woodland to arable field generates food for humans and a business opportunity for the farmer and his supply chain. It also releases carbon to the atmosphere both from the felled trees that decompose or are burnt and from the exposed soil.

Since the industrial revolution 136,000 million tCO2e of greenhouse gas emissions have come from land clearing for agriculture and livestock production. Compare this with the 270,000 million tCO2e from the combustion of fossil fuels.

Even today greenhouse gas emissions from land clearing are around 12% of the global total from human activities.

Biosequestration is the opportunity to remove some of this carbon from the atmosphere that is created from the rehabilitation of cleared and degraded land. If trees are grown on a patch of bare ground they can sequester carbon as they grow to maturity. But this is only of use to the atmosphere if the land was bare.

If vegetation had to be cleared to grow trees then the net effect on the carbon in the atmosphere is the difference in the carbon in the newly grown trees minus the carbon in the cleared vegetation. Often this is a net gain carbon to the atmosphere and would be an emission not biosequestration. Such situation has required some complex carbon accounting rules to determine if a biosequestration project results in a net reduction of greenhouse gases in the atmosphere.

If net reduction in greenhouse gas emissions is a serious target, then biosequestration of carbon into vegetation and soil on degraded and cleared land will be critical to achieving it.

So far, however, it is an opportunity yet to be taken seriously.


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