Carbon Cycle

What is the Carbon Cycle?

The carbon cycle is related to all of the other nutrient cycles discussed on the Oz Estuaries indicator pages. Carbon is processed both chemically and biologically, from the atmosphere, soils and in water, to form compounds which are in turn either recycled or stored in some form.

Figure 1. A representation of the processes involved and their magnitude (Illustration courtesy NASA Earth Science Enterprise).

How is the Carbon Cycle Changing?

As discussed in the climate change indicator page, the major human impacts on the carbon cycle are: 1) Deforestation which decreases the capture and storage carbon from the atmosphere and can also release carbon dioxide if the wood is burnt or biodegraded; 2) the burning of fossil fuels which releases carbon dioxide and 3) the production of cement which releases carbon dioxide when lime is produced from heating calcium carbonate (CaCO₃ + heat Þ CaO (lime) + CO₂ ).

There are other human impacts such as the change in land use from forest or grassland (net stores of carbon) to residential or agricultural (net releasers of carbon). Increased production of livestock such as cows and pigs which produce methane, a greenhouse gas that absorbs more heat per molecule that CO₂ , is another growing area of concern.

Considerations for measurement and interpretation

Accurate measurements for sources and sinks
A source of greenhouse gas is an activity or process that releases that gas into the atmosphere. A sink is a process or physical mass in which a greenhouse gas, usually carbon dioxide, is removed or sequestered from the atmosphere and stored. Both sources and sinks can be natural, such as the growing of a forest (a sink) or a forest fires (a source), or human activities such as burning fossil fuels (source) or geosequestration (sink).

Because of the sources and sinks are not only geographically large and diverse but also highly dependant on other factors such as temperature, rainfall, population growth and technology development, it can be very difficult to measure the masses of carbon stored or being released at any one time. Accurate and complete data sets are required to make an assessment of the current status of sources and sinks but also to be able to monitor the rate of change in both.

Carbon cycle feedbacks
A feedback is when the product of a process has an impact on the continuing rate of the process that produced it. The impact can be either positive, and make the rate of the process faster, or negative, and slow it down. There are many examples of feedbacks in the carbon cycle. If the level of CO₂ in the atmosphere is increased, the average temperature of the atmosphere will go up (Climate Change), increasing temperatures will melt permafrost releasing methane (another greenhouse gas) which will further increase temperature, melting permafrost and so on.

It is unclear at the moment how all of the feedbacks in the carbon cycle will interact with another and ultimately impact the earth’s climate.

Sinks changing to sources
As discussed above a sink is when carbon is stored away from the atmosphere, preventing it from impacting climate. However, most sinks, particularly fossil fuels but also forests, grasslands and permafrost, can become sources given the right conditions. With fossil fuels, humans play the major role in removing it from it storage and burning it. For forests, grasslands and permafrost, climate change on its own maybe enough to turn these important sinks into sources of carbon dioxide and methane. It unclear at this time what level of climate change will be necessary to produce the conditions needed for the swap to occur and which vegetation types or areas are most vulnerable to conversion.

Existing information and Data

One of the biggest programs examining the carbon cycle in Australia is the National Carbon Accounting System or NCAS. NCAS tracks emissions (sources) and removals (sinks) of greenhouse gases from Australian land based systems. The system underpins National Greenhouse Gas Inventory reporting, and provides a basis for emissions projections to assess progress towards meeting Australia’s emissions target (Kyoto). NCAS also produces the National Carbon Accounting Toolbox which provides the tools for tracking greenhouse gas emissions and carbon stock changes from land use and management. It includes the Full Carbon Accounting Model (FullCAM), that is derived from Australia’s National Carbon Accounting System, and all supporting technical documentation. ( http://www.greenhouse.gov.au/ncas/ncat/index.html )

Through the Toolbox, users may identify changes in emissions resulting from: soil cultivation, fire management, fertiliser application, climate variability and reliability etc. Users may access: carbon accounting data for a range of plant species and land management systems; historic climate records; and search all technical reports relating to development of the National Carbon Accounting System. Developed in collaboration with the CSIRO, the Australian National University and supported by NASA, the Toolbox draws on the latest of scientific advancements.

Research has shown that Australia’s net primary production (photosynthesis) is much lower than the global average for a land mass of Australia’s size, 740 Tg C/yr versus a global average of 2850 Tg C/yr for a comparative size. Other differences including the dominant role of fire in Australian ecosystems and markedly difference plant species from those found in Europe and North America . Models used to project the carbon cycle need to account for these variations in Australia

To better understand the carbon cycle in Australia , flux tower experiments are being carried out in a number of locations. These experiments study the controls on carbon flux (the movement of carbon from the atmosphere to plants and soil and tin the opposite direction), whether there are any sudden or non linear changes to the carbon cycle (eg from droughts or insect attack) and provide valuable high resolution data for carbon models.

Figure 2. The change in tree area as an estimate of carbon uptake for a temperate native forest near Tumbarumba NSW Australia. Green bars are live trees; red bars are dead trees; and the blue line is rainfall. From AGO and CSIRO.

Key questions and further research needs

• More complete modelling of oceanic carbon cycles
• More complete modelling of terrestrial carbon cycles
• A better understanding the inputs, transport and fate of carbon estuarine systems
• What will be the impact of changes to carbon cycle of health of estuaries
• Will increased CO₂ levels increase primary productivity in estuaries?

Contributor

Chris Hepplewhites, Geoscience Australia