Carbon Sinks

A carbon sink is defined as a pool or reservoir that absorbs carbon released from another part of the carbon cycle (the net exchange between the biosphere and the atmosphere). If the net exchange is toward the atmosphere, the biosphere is the source, and the atmosphere is the sink. Carbon sources usually release more carbon than they absorb, while sinks soak up more carbon than they emit. Another definition of carbon sink is: any natural or anthropogenic system that absorbs CO2 from the atmosphere and stores it. Trees, plants, and oceans all absorb CO2 and, therefore, are carbon sinks. The concept of carbon sinks is based on the natural ability of trees, other plants, and soil to soak up CO2 and temporarily store the carbon in wood, roots, leaves, and earth.

Fossil fuel deposits are another important carbon store. Buried deep inside the earth, they are naturally separated from carbon cycling in the atmosphere until humans decide to release them into the atmosphere by burning coal, oil, or natural gas. The burning of fossil fuels results in the release of greenhouse gases (GHGs, such as water vapors, CO2, methane, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, and chlorofluorocarbons).

The downward radiation of long waves from the atmosphere, as opposed to radiation by the sun, is known as the greenhouse effect. A buildup of GHGs in the atmosphere forms a layer that keeps heat from escaping into space and reflects it back to Earth. Because of human burning of fossil fuels, the concentration of GHGs has soared to levels more than 30 percent higher than at the beginning of the Industrial Revolution. Instead of controlling and reducing GHGs, human activities continue to add more than 6 billion tons of carbon per year to the atmospheric carbon cycle, thus exacerbating the situation and increasing the rate of global warming.

While forests act as sinks, deforestation prevents the absorption of CO2. Therefore, fewer trees mean more CO2 in the atmosphere. The causes of deforestation include logging for lumber, pulpwood, and fuel wood. The clearing of new land for farming and pastures for livestock, or the building of new housing, are some of the other reasons for deforestation. About 860 acres—the size of New York City’s Central Park—are being destroyed every 15 minutes in the tropics.

Oceans are also important carbon sinks. Antarctica’s Southern Ocean is a crucial carbon sink into which 15 percent of the world’s excess CO2 flows. It is estimated that the world’s oceans have absorbed about a quarter of the 500 gigatons of carbon emitted into the atmosphere by humans since the beginning of the Industrial Revolution.

Observations have shown that the Southern Ocean’s ability to absorb CO2 has weakened by about 15 percent per decade since 1981. This is attributed to an increase in wind strength over the ocean because of higher levels of GHGs in the atmosphere and long-term ozone depletion in the stratosphere. The strengthened winds influence the processes of mixing and upwelling in the ocean, resulting in an increased release of CO2 into the atmosphere, and, thus, reducing the net absorption of CO2 into the ocean, showing that climate change, itself, is responsible for the saturation of the Southern Ocean carbon sink.