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Natural sinks


Scientists have identified areas on the earth that have the capacity to take in the greenhouse gases and clean the air around us. These areas are known as ‘natural sinks’. Some of these natural sinks are forest cover (trees, vegetation), oceans, and soil to some extent, all of which have the ability to take in carbon dioxide. In fact, soil may also provide a removal mechanism for methane. Trees and other land plants absorb carbon dioxide and serve as a storehouse, or ‘sink’, of carbon. Forest biomass accumulates carbon over decades and centuries making the carbon accumulation potential in forests large. Forests offer the possibility of taking in significant amounts of additional carbon in relatively short periods of time.


Forest carbon may be released due to biomass reductions from fire, tree decomposition, or logging. In the case of decomposition or fire, forest carbon is released into the atmosphere. A forest fire can occur naturally or due to people’s carelessness or in areas where people practice slash and burn methods of agriculture. However, the forest may again become a carbon sink as it is restored through re-growth of the forest cover.

There are four components of carbon storage in a forest ecosystem: trees, plants growing on the forest floor, fallen leaves and other decaying matter on the forest floor, and forest soil. Carbon is taken in during the process of plant growth as carbon is captured in plant cell formation and oxygen is released. Plants growing on the forest floor add to this carbon store. Over time, branches, leaves, and other materials fall to the forest floor and may store carbon until they decompose. Additionally, forest soils may take in some of the decomposing plant litter through root/soil interactions.

Reforestation should be given priority by governments all over the world. They must ensure that deforestation activities are stopped. To succeed in this they have to provide alternative sources of energy to the people who depend on fuelwood for cooking and heating.

Like the trees, the oceans also act as natural sinks taking in carbon dioxide. The oceans influence the climate by absorbing and storing carbon dioxide. Climate change is caused by the accumulation of man-made carbon dioxide and other greenhouse gases in the atmosphere. Scientists believe that the oceans currently absorb almost half of the carbon dioxide produced by the burning of fossil fuels.

Coral reefs, often referred to as the forests of the oceans, are one of the main sinks that take in a large amount of carbon dioxide. Plankton present in the oceans, found mainly in the areas where the warm currents meet the cold currents, is an aquatic plant that, like terrestrial plants, takes in carbon dioxide during the process of photosynthesis. The amount of gas dissolved in the water is in turn influenced by the amount of phytoplankton (microscopic plants, particularly algae), which consumes carbon dioxide during photosynthesis. Phytoplankton activity occurs mostly within the first 50 metres of the surface and varies widely according to season and location. Some areas of the ocean do not receive enough light or are too cold, other areas appear to lack the nutrients for the growth of phytoplankton. Rather like a pump, plankton transports gases and nutrients from the ocean surface to the deep. Its role in the carbon cycle is quite different from that of trees. Ocean life absorbs carbon dioxide during photosynthesis and, while some of the gas escapes within about a year, some of it is transported down into the deep ocean via dead plants, body parts, faeces, and other sinking materials. The carbon dioxide is then released into the water as the materials decay, and most of it becomes absorbed in the sea water by combining chemically with water molecules.

Warm water in the ocean is generally saturated with carbon dioxide whereas the cold water is unsaturated and has a great deal of capacity to hold carbon dioxide. This gas is highly soluble in cold dense water. Towards the poles, in the higher latitudes, where the ocean is cold, the carbon dioxide present in the air close to the water gets dissolved in the water. The carbon that is absorbed by the ocean through the two processes can be retained for about 1000 years.

Efforts are now on to explore the means to utilize the oceans as a reservoir for carbon dioxide. Scientists have thought of two means: first, by capturing the carbon dioxide from focal points through pipelines or into large containers and then injecting them into the ocean. The second is to fertilize the ocean by adding more nutrients so that there will be more abundant growth of phytoplankton to take in the carbon dioxide from the air. The impact of these activities on the environment has yet to be studied before it is made functional.

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