The carbon cycle: a simple explanation

Carbon dioxide in the atmosphere is a 'greenhouse gas' - it traps some of the radiation that would otherwise be lost to space, and causes the Earth's atmosphere to be warmer than it would otherwise be. Man-made emissions of carbon dioxide have caused the amount in the atmosphere to increase by about 30% since pre-industrial times, and this is a major cause of global warming. Therefore it is important for us to understand how the carbon cycle works in order for us to be able to predict how it may behave in the future.
Carbon is continuously cycled between reservoirs in the ocean, on the land, and in the atmosphere, where it occurs primarily as carbon dioxide. On land, carbon occurs primarily in living biota and decaying organic matter. In the ocean, the main form of carbon is dissolved carbon dioxide and small creatures, such as plankton. The largest reservoir is the deep ocean, which contains close to 40,000 Gt C, compared to around 2,000 Gt C on land, 750 Gt C in the atmosphere and 1,000 Gt C in the upper ocean. The atmosphere, biota, soils, and the upper ocean are strongly linked. The exchange of carbon between this fast-responding system and the deep ocean takes much longer (several hundred years).

The ocean takes up carbon dioxide when it is cold, at higher latitudes, and releases it near the tropics. Photosynthesis takes carbon dioxide from the atmosphere and transfers it to vegetation, while respiration releases carbon dioxide back into the atmosphere. These processes are shown schematically in the figure below. Although natural transfers of carbon dioxide are approximately 20 times greater than those due to human activity, they are in near balance, with the magnitude of carbon sources closely matching those of the sinks. The additional carbon resulting from human activity is the cause of atmospheric carbon dioxide rises over the last 150 years.

Changes in climate have a significant effect on the carbon cycle. Increases in atmospheric carbon dioxide concentration increase plant photosynthesis and the amount of carbon stored in vegetation. However, increases in temperature also lead to increases in plant and soil respiration rates, which tend to reduce the size of the terrestrial carbon store. In some regions, the changes in climate (such as decreased rainfall) can also reduce plant photosynthesis and reduce the ability of vegetation to sequester carbon.

Carbon dioxide from the atmosphere dissolves in the surface waters. On entering the ocean, carbon dioxide undergoes rapid chemical reactions with the water and only a small fraction remains as carbon dioxide. The carbon dioxide and the associated chemical forms are collectively known as dissolved inorganic carbon or DIC. This chemical partitioning of DIC ('buffering') affects the air–sea transfer of carbon dioxide, as only the unreacted carbon dioxide fraction in the sea water takes part in ocean–atmosphere interaction.

The dissolved inorganic carbon (DIC) is transported by ocean currents. Near the poles, cold dense waters sink towards the bottom of the ocean and subsequently spread through the ocean basins. These waters return to the surface hundreds of years later. As more carbon dioxide can dissolve in cold water than in warm, these cold dense waters sinking at high latitudes are rich in carbon and act to move large quantities of carbon from the surface to deep waters. This mechanism is known as the 'solubility pump'.

As well as being transported around the ocean, dissolved inorganic carbon is also used by ocean biology. In the surface waters, drifting microscopic oceanic plants known as phytoplankton grow. As with land based plants, phytoplankton take in carbon dioxide during growth and convert it to complex organic forms. The phytoplankton are eaten by drifting oceanic animals known as zooplankton, which themselves are preyed upon by other zooplankton, fish or even whales. During these biological processes, some of the carbon taken in during growth of the phytoplankton is broken down from the organic forms of the biology back to inorganic forms (DIC). If between the carbon uptake by phytoplankton and the subsequent return of the carbon to DIC, the biological material has been transported to depth, for example by the sinking of large biologically formed particles, there is a net transfer of carbon from the surface to depth. This process is termed the 'biological pump'. The carbon can also sink as skeletal structures of the biology which is known as the 'carbonate pump'.

The terrestrial biosphere's role in the carbon cycle


Carbon dioxide from the atmosphere is utilised by plants by photosynthesis. The carbon they absorb is allocated within the plant to make up its roots, wood and leaves. Some of this carbon is then lost - either when the leaves drop, or when the plant dies - and becomes soil carbon. Microbes within the soil breakdown this carbon and release it back to the atmosphere as respiration, in the form of carbon dioxide. This is the terrestrial carbon cycle on a small scale (i.e. on the scale of individual plants).

On a larger scale (i.e. across geographical regions), the distribution of vegetation is important in the carbon cycle. Different plant types store different amounts of carbon, but they grow at different speeds and favour different conditions. For example trees can store more carbon than grass (per unit area of land covered), but they take a lot longer to grow. So if a previously barren area of land becomes fertile for some reason then grasses will grow first, but trees may take over later. The local climatic conditions, and how they change over time, determine which type of plant dominates in any given location.

Human activity also changes the land use, and hence the carbon stored by the biosphere - cutting down trees removes a potentially large absorber of carbon dioxide and if the wood is burnt, or left to decay, then the carbon is released back to the atmosphere. Disturbance of vegetation also affects the soil - deforestation can also lead to large amounts of carbon being lost from the soil. This has an impact on the fertility of the ground and may affect future vegetation growth in the area. Such changes in land use (predominantly in the tropical forests) accounted for the most significant part of anthropogenic carbon dioxide release during the 19th Century. It was not until about 1950 that fossil fuel emissions became significantly larger than the source from land use change. Present day emissions due to anthropogenic land use change still amount to around 1 GtC per year.



source:
http://www.metoffice.gov.uk/research/hadleycentre
/models/carbon_cycle/intro_global.html


Carbon Trading and Taxation


To stop climate change new policy solutions are required which engage individuals and organisations with the need to make dramatic cuts in emissions and at the same instance enable technology to play its part. There are two main framework approaches which can be adopted to reducing emissions:

•Capping and trading
•Carbon taxation

And each approach has many different possible variations. None of these policy opt ions would operate in isolation. I t is likely that policies which promote efficiency and renewable energy, support new technologies, give advice and information to business / consumers and so on would still be part of the overall policy package.

Carbon taxation-led policy
A carbon taxation- led response to reducing emissions would have certain advantages. Firstly, it would take advantage of administration systems which already exist. Secondly it would be an economically efficient policy. I t would also avoid much of the complexity of an expanded EUETS scheme, and taxation levels could be adjusted to deliver required levels of carbon savings. Carbon taxation can be combined with tax relief schemes for those who adopt carbon saving measures (as in the current Climate Change Agreements) and would be part of an overall taxation strategy which would be designed to meet social and economic as well as environmental goals.

Domestic Tradable Quotas (DTQs)
For Domestic Tradable Quotas to be functional it needs to establish the maximum level of greenhouse gases which can be emitted from energy use in a given year. This carbon budget would be reduced year on year in order to meet emissions reduction targets. The carbon budget would be divided into carbon units. A proportion of these units would be allocated to all adult citizens by government, on a free and equal per capita basis. Firms and other organisations would be required to purchase units on a national carbon market. When individuals and organisations purchased fuel and electricity they would be required to surrender unit s corresponding to the carbon content of their purchase. Individuals with surplus units could offer them for sale to those who wished to buy extra.

Personal Carbon Allowances / Rations + carbon caps for organisations

Personal carbon allowances (PCAs) would be a UK-wide allowance system covering the carbon emissions generated from the fossil fuel energy used by individuals within the home and for personal transport, including carbon equivalent emissions from air travel. It would account for around half of current UK carbon emissions from energy. The primary aim of the scheme would be to deliver guaranteed levels of carbon savings in successive years in an equitable way. I t is similar in most respects to the individual element of DTQs. The key differences are firstly that PCAs would cover public surface transport and personal air travel, as well as the motoring and household energy sources covered by DTQs. Secondly, children would also be awarded PCAs (but receive a smaller allowance than for adults) .

The conclusions which can be drawn from modelling research are:
•Scientists can provide much more certain answers about the effect of long- term reduct ions scenarios than short - term emissions reductions. Science cannot tell us very much about what is required, say, within the next five years to avoid particular outcomes.
•Scientists can’t quant ify the dangers of freezing emissions per annum at any level– to get a certain outcome emissions need to reduce towards zero.
•The not ion of a ‘sustainable emission rate’ is indefensible. The only safe sustainable emissions rate is zero.
•What is required is a ‘containment ’ scenario. The peak of atmospheric concentrations (e.g. whether or not 550ppm is exceeded) matters far less than the total carbon emit ted into the atmosphere.


Source: Taxing and Trading; 2005, Editors: Sarah Keay-Br ight and Tina Fawcett, St Anne’s College, Oxford University