Carbon sinks

In a broad sense, any natural reservoir able to absorb CO2 is a carbon sink. The main natural carbon sinks are oceans and forests. Photosynthesis is the main carbon sequestration mechanism, but CO2 can be also absorbed in the oceans by dissolution. Carbon dioxide exchanges between the atmosphere, vegetation and ocean surfaces are very large:

- 60 Gt of carbon is exchanged each year between vegetation and the atmosphere;

- 90 Gt of carbon is exchanged between ocean surfaces and the atmosphere;

- 40-50 Gt of carbon is exchanged between ocean surfaces and marine biomass.

The quantity of carbon emitted each year in the atmosphere as a result of fossil fuel consumption, i.e. 7Gt/year, might appear small when compared with these exchanges. In fact, this additional quantity of carbon modifies a fragile balance, producing therefore a continuous shift in the CO2 concentration in the atmosphere.

A first option for sequestering CO2 consists of producing biomass to obtain a carbon sink, for instance by growing forests which will absorb carbon present in the atmosphere. It is necessary to grow biomass on large surface areas. Biomass thus produced can represent typically around 10 tons of carbon captured per hectare per year, amounting to 37 tons of CO2. The implementation of such carbon sinks has become an economically viable option following the Kyoto Protocol, as carbon sinks can be taken into account in the carbon trade mechanisms.

Nevertheless, it must be underlined that the amounts of carbon thus sequestered are limited, as it is necessary to grow biomass over very large areas of land which, generally, are not available in a populated area.

Furthermore, it is necessary to ensure a rigorous management of the planted area, in order to achieve a positive carbon balance. Forests act as carbon sinks only during their growth period. A lack of maintenance or, even worse, deforestation at a later stage leads to a negative balance.

Biomass production has not only positive effects: agriculture and cattle breeding have a negative impact on the greenhouse effect due to CO2 emissions resulting from the use of agricultural machinery, and also as a result of methane released by ruminants and NOx emissions resulting from the use of nitrate fertilisers.

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