What is a carbon sink?

Carbon is a very common element, present in plants and animals, the atmosphere, the ocean, and rocks (such as limestone and coal).

Carbon naturally moves between these reservoirs by many processes, such as carbon dioxide (CO2) dissolving in the ocean, plants absorbing CO2 in photosynthesis, animals eating carbohydrates stored in plants and releasing CO2 through respiration.

Since the industrial revolution this balance has shifted, and extra carbon is going into the atmosphere by burning coal and oil. Processes which add extra carbon to the atmosphere are known as sources, and processes which take CO2 from the atmosphere and store it are known as carbon sinks. The step of removing the carbon from the atmosphere is known as carbon sequestration and once stored, it is known as a carbon reservoir.

Natural processes aren't the only carbon sinks. Artificial methods such as storing large amounts of CO2 deep underground are also being explored, but are currently quite expensive. Investigations have also been made into whether fertilising the ocean can increase how much carbon it can absorb, but this also looks to be of limited use.

Fossil fuels are an example of carbon that was stored millions of years ago, as are carbonate rocks such as limestone. Burning fossil fuels releases carbon back into the atmosphere, as does the process of transforming limestone into cement. Figuring out how to do less of these things while keeping economies healthy is big challenge of climate change.

Part of understanding how the climate is likely to change in future is understanding how some reservoirs which are currently absorbing carbon (carbon sinks) might stop absorbing carbon in future. For example, the ocean may change how much CO2 it absorbs as the climate changes. Also some reservoirs, which are currently stable, might change and start to release CO2 and become carbon sources. An example of this is frozen tundra, which is currently too cold to produce methane (CH4), but can start producing this again when temperatures rise. Studying samples of the atmosphere of the past, which can be found in deep ice cores, can tell us about the release of carbon from old reservoirs thousands of years ago, and help predict what will happen in future. 

Diagram illustrating a simplified version of the carbon cycle. [Thomas Murphy]