Between 40 and 60% of all anthropogenic emissions of carbon dioxide (CO2) appear to remain in the atmosphere. The remainder of the excess (about 4 Gigatonnes of carbon GtC per year) is sequestered within the global carbon cycle – where and by what processes is still not clear. A variety of studies into ocean carbon fluxes cautiously support earlier estimates for a net ocean sink in the order of 2 GtC/year, although estimates vary between 1.4 and 3 GtC/year. "Biological pumping" to deep oceans, and incorporation in forests and soils (CO2 fertilization effect) provide for other CO2 sinks. Estimates for net contributions of the Earth's terrestrial biosphere to the atmospheric CO2 concentration (deforestation minus incremental sinks) vary between a small sink of 0.1 GtC/year to a source of up to 2.5 GtC/year. The total sources and sinks of terrestrial carbon could, in fact, be currently almost balanced, resulting in very little net terrestrial contribution to atmospheric CO2 change. With time, this net terrestrial contribution may become a significant sink. (Worldwide forest biomass contains about two-thirds of the amount of carbon currently contained in the atmosphere.) However, the geographical and seasonal distributions of ocean carbon sources and sinks are complex and can vary significantly with year-to-year climate variability.
This strategy features in the framework of Agenda 21 as formulated at UNCED (Rio de Janeiro, 1992), now coordinated by the United Nations Commission on Sustainable Development and implemented through national and local authorities.
Sink enhancement includes a number of possibilities of making CO2 dioxide emissions unavailable to the atmosphere, such as filtering out greenhouse gases at the site of production, converting them into solid material and disposing of them on the sea floor. Economic studies of such schemes are not encouraging. One USA study suggests that to remove 90% of the CO2 from power stations would double the capital costs of power stations and increase the cost of electricity by a factor 1.5-2.0; in addition, 10-20% of electricity output would be needed to power the cleaning up process.
The most popular form of sink enhancement is growing more trees and limiting deforestation. There are compelling reasons in favour of this strategy, most of which have little to do with climate change (they concern the Earth's water balance, soil erosion and the need for recreational areas. It has been calculated than an area of forest larger than France would need to be planted every year to compensate for the current rate of fossil fuel burning.
In 1997, the Government of Bolivia, the World Nature Conservancy Project and the American Electric Power Corporation sponsored a carbon sequestration project in the ecologically-rich Noel Kempff National Park, providing more than $11 million to expand the park and retire adjacent concessions. The project is expected to reduce CO2 emissions by an amount equal to the lifetime emissions of 500,000 cars. In 1997, the Government of Suriname with the Inter-American Development Bank, the World Resources Institute, Conservation International and others worked together to protect 4 million acres of primary tropical rainforest in Suriname in a similar sequestration project.
Africa's vast forest reserves serve as a sink for carbon dioxide and mitigate greenhouse gas emissions, and play a key role in alleviating and balancing the emissions of the industrialized world. However, this crucial function is threatened by accelerating deforestation.