Carbon is continually cycled within a closed system made up of the atmosphere, the oceans and the land, where carbon is stored and exchanged in various forms. This cycle has been naturally occurring for thousands of years with the exchange between “sources” and “sinks” balancing carbon concentrations across the cycle.
There are a number of natural cycles within the overall exchange including the cycle between plants and animals with the atmosphere through the natural processes of respiration and photosynthesis. Similarly the surface of the oceans continually exchange carbon with the atmosphere on a cyclical basis and carbon is continually cycled between the deep and surface oceans.
The main natural “sources” of carbon include:
- Respiration by plants and animals
- Decay and decomposition of plant and animal organic matter
- Natural fires and volcanoes
- Natural warming of ocean surfaces releasing CO2 to atmosphere
The main natural “sinks” of carbon include:
- Photosynthesis by plants
- Sedimentation of dead organic matter and transformation to form fossil fuels
- Natural cooling of ocean surfaces to form carbonic acid
- Metabolism of carbon by ocean life to form shells which eventually form carbon sediments on the ocean floor
These cycles have been in natural balance for thousands of years with no net change in the concentrations of carbon dioxide in the atmosphere (approximately 280ppm, IPCC AR4).
Humans have always contributed to the carbon cycle through minor emissions to the atmosphere (through respiration, fires, etc) but in the19th century with the widespread use of combustion to generate heat and power and the onset of industrial times, the influence of human activity became significantly more profound. The carbon that had been naturally stored in fossil fuels for billions of years was now used for combustion to generate electricity leading to a significant increase in the volume of CO2 emitted to the atmosphere.
In addition, land use change led to the removal of forests and other green areas eliminating these natural sinks, releasing carbon dioxide and go beyond the problems caused by increased emissions.
The large scale combustion of fossil fuels with no corresponding sink to balance these emissions has continued and worsened to this day leading to global atmospheric concentrations of CO2 at 385ppm in 2008 (i.e. a 35% increase on pre-industrial times, IPCC AR4).
This human influence has led to a dramatic shift in the natural carbon cycle whereby emissions to the atmosphere cannot be effectively balanced through the existing sinks. This is primarily due to ongoing deforestation and other land use issues.
Approximately 7 Giga tonnes of carbon, as carbon dioxide, are added to the atmosphere each year. Action on climate change is required immediately to both reduce emissions from fossil fuels and to increase natural sinks such as forests to redress the balance and stabilise the carbon cycle.