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The Carbon Cycle

The Carbon Cycle

THE CYCLE

Summary

All life on Earth is based upon carbon. The carbon cycle describes a set of 1. The natural world processes governing the movement of carbon elements or compounds Principles of environmental systems through living organisms and the environment and how carbon is stored Ecological systems and released in various ways. The cycle can be described in terms of stores of carbon and the processes that govern exchanges between them. Water and the hydrologic cycle The carbon cycle underpins all li f e on earth and is important in The carbon cycle understanding the causes and effects of . Climate and atmospheric systems Relevance to business Biodiversity

The carbon cycle is vital to all life on earth and it is important to understand how it works and what happens when it is modified. Human activities impact upon the carbon cycle with a variety of far-reaching implications for human populations and economies. An understanding of the underlying principles of carbon management is increasingly required when considering action to combat climate change.

Carbon

Carbon is the fourth most abundant element on Ear t h and present in all living organisms. The arrangement of electrons on the carbon atom means that it easily forms covalent bonds (sharing electrons between neighbouring atoms) with other carbon atoms and those of other elements, particularly hydrogen, nitrogen, and phosphorus. Carbon can bond to itself, forming molecules in long chains, branches and rings. The complexity of these molecules helps form the basis of life. On average, living organisms include about 18 per cent carbon – around one hundred t im e s gr eat er than its concentration in the Earth. The total quantity of carbon on Earth is fixed and it is cycled and recycled through a series of geological and biological (collectively biogeochemical) exchanges known as the carbon cycle. This cycle is exceptionally complex and still only partially understood, but is becoming clear that human activities are impacting upon and altering the balance of the free carbon budget.

Carbon stores

Carbon resides in a number of ‘stores’ (Table 1) and exchanges or flows occur between them. They can be considered as sinks when they sequestrate carbon or sources when they provide it to another store.

Carbonate rocks: carbon is present as CACO3 (e.g. ) formed by sedimentary geological processes from the shells of dead marine animals that fixed (from seawater) with to produce calcium

(CACO3).

Kerogens and fossil stores: carbon is stored as geochemical and organic compounds including , oil, oil bearing shales () and nat ur al gas formed by geological processes from organic deposits.

Terrestrial : carbon is present in (living plants and animals) and dead organic matter, including humus and .

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Atmosphere: carbon exists in the as (CO2).

Oceans: carbon arises primarily from atmospheric CO2, entering water by -2 diffusion as dissolved CO2, or converted into carbonate (CO 3 ) or bicarbonate - (HCO3 ). These stores exchange carbon with each other through processes which are not yet fully understood and ar e difficult to measure. The timescales of these exchanges vary significantly. Carbon in rocks, kerogens and fossil fuels is fixed almost permanently (in human timescales) unless fossil fuels are burnt or carbonates processed. Carbon in the terrestrial biosphere, atmosphere and is more freely available and cycles more actively.

Table 1. Major stores of carbon

Carbon store Size/gigatonnes carbon carbonates <60,000,000

Kerogens 15,000,000

Ocean 38,400

Coal 3,510

Soil 1,500

Atmosphere 720

Living biomass 600

Peat 250

Oil 230

Natural gas 140

Source: Smithson et al (2002)

Processes and flows

All carbon stores are interlinked and ultimately interdependent. A number of processes and flows govern their exchange. is the process by which plants convert solar energy into food to enable growth. Photosynthesis is conducted primarily by photoautotrophs like plants and algae; organisms capable of synthesizing organic molecules from carbon dioxide using sunlight as their energy source. Carbon dioxide and hydrogen from water are combined to form carbohydrates, mainly t he sugar glucose. This forms the basis for food chains supporting most life on earth and is the route for bringing carbon into the biosphere from the atmosphere. In the oceans, marine biota use dissolved CO 2 for photosynthesis. As the atmospheric concentration of CO2 rises the biochemical ability of plants to fix carbon deceases thereby lowering their capacity to act as a .

As part of primary production, plants absorb CO2 from the atmosphere during photosynthesis and release CO2 back into the atmosphere during . Respiration is the biological equivalent of : it breaks hydrocarbons and carbohydrates into carbon dioxide and water thereby releasing energy held in the sugars for use in metabolism. The fluxes of carbon produced by photosynthesis and respiration are around 1,000 times greater than exchanges from the geological cycle. Combustion occurs through natural that oxidize organic matter, releasing CO2, and through anthropogenic activity, mainly t he burning of fossil fuels to release energy. Burning coal, oil and gas releases carbon previously stored in the into the atmosphere.

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Carbon is also returned to the atmosphere through decaying fungi and bacteria.

These produce CO2 if oxygen is present and (CH4) if it is not. Both these gases are significant in the global radiation balance (see R 1. 5 Climate and atmospheric systems).

Figure 1. Carbon flows

Source: NASA Earth Science Enterprise. GT = Gigagtonnes (1 billion tonnes) http://earthobservatory.nasa.gov/Library/CarbonCycle/carbon_cycle4.html

Carbon can transfer from geological sinks through volcanic activity, normally in dramatic but sporadic events and through the weathering of carbon-rich rocks in slow but continuous processes. The biological fixation of carbon, where photosynthesis exceeds respiration, leads to the accumulation of organic matter and supplies the geological processes of coal and oil formation. Through these processes carbon is fixed in the geological store, resulting in a net removal of carbon from the atmosphere. However, these processes take millions of years and hence the current concern over the accelerated changes to the carbon budget resulting from human activities. The atmosphere stores about 750 gigatonnes of carbon, but this level is rising due to human activities. The annual receipt of carbon from combustion is thought to be about 5.5 gigatonnes and changes such as are thought to contribute 1.5 gigatonnes comprising an annual contribution of c. 7 gigatonnes. Around 2 gigatonnes of carbon diffuse into the oceans as dissolved CO2 or as carbonates. Measurements suggest that at least 3 gigatonnes remain in the atmosphere as a net contribution, but nearly 2 gigatonnes cannot be readily accounted for. It is unclear what is happening to the ‘missing’ 2 gigatonnes annually but is most likely being carried within the biosphere perhaps though increased plant and microbial activity. There is much uncertainty in this area, it is unknown whether this extra carbon can or will be absorbed in the future or how it is being stored. This lack of knowledge adds greatly to the uncertainty of planning for and attempting to mitigate climate change. Over the past century, the average carbon dioxide content of the atmosphere has risen from 290 to 340 ppm (parts per million) or higher, a rise of c.17 per cent. About 65 per cent of the additional carbon dioxide in the atmosphere

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comes from burning fossil fuel (and production); the other 35 per cent is derived from deforestation and the conversion of natural into agricultural systems that results on the loss of organic content from soil. Natural ecosystems can store from 20-100 times more carbon than agricultural systems because of their more natural, complex and diverse structures. Thus, activities such as deforestation have both short and long-term impacts on carbon budgets. Forest clearance, particularly if burning is used, can result in a short term release of carbon dioxide that was fixed in the biomass. If the land is transferred to agricultural production the net capacity to fix carbon in the future is also diminished. In system terms this is an example of a positive loop where change results in reinforcing the direction and strength of change (see R 1: 1 Principles of environmental systems). Increasing levels of carbon dioxide in the oceans are also of concern to scientists. This can lead to increased acidity and it is thought that may impact upon the productivity of and thus have ‘upst r eam ’ effects for carbon fixing and primary production in the oceans and thereby potentially serious implications for marine ecosystems. For example, increased acidity together with increased average temperatures are thought to be par t ly responsible for the recent dramatic worldwide decline in corals.

Climate change

The concentration of carbon dioxide in the atmosphere is recorded in a number of locations but the longest continuously recorded data come from the Mauna Loa station in Hawaii (Figure 2). The Mauna Loa record shows a 19.4 per cent increase in the mean annual concentration, from 316ppmv (parts per million by volume, dry air) in 1959 to 377 ppmv (parts per million per year) in 2004. Carbon dioxide is important in the global radiation balance because it acts as a .

Figure 2. C02 concentrations monitored at Mauna Loa observatory 1958 – 2004

380

370

360

350

340

330

CO2 concentration PPM concentration CO2 320

310 1958 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002 Year

Source: Keeling & Whorf (2005) http://cdiac.ornl.gov/ftp/trends/co2/maunaloa.co2

There are other greenhouse gases both natural and human-made but carbon dioxide is produced in the largest quantities. Greenhouse gases allow solar radiation to pass through them to the earth’s surface. Some is reflected back as infrared radiation but greenhouse gases absorb this and retain heat within the

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atmosphere. While this phenomena is essential to maintain hospitable temperatures on earth, the rise in concentration of greenhouse gases such as carbon dioxide is changing the overall atmospheric equilibrium and leading to overall warming and impacts upon the climate. Climate change is difficult to predict but most scientists agree that warming will result in -level rises, more climatic variability leading to more frequent and severe weather events and impacts such as , flooding and decreased food supply security. Low-lying land is likely to be inundated by sea-level rises (see R 5:1 Climate Change).

Carbon sequestration and carbon sinks

The concept of a carbon sinks has emerged as one of the ways of managing the global carbon balance and mitigating climate change. The concept of stores and sinks is somewhat variable and the terms are used interchangeably by some commentators. Sinks generally act to retain or sequestrate carbon and sources store carbon and emit it under certain circumstances. Stores can act as both sinks and sources. Carbon sinks are based upon the ability of plants, in particular trees, and soil to tie-up and temporarily store carbon. This process is scientifically valid but its use for carbon management as carbon sink ‘credits’ ignores other related issues. The allows the use of carbon sinks to offset carbon emissions from fossil fuels. However, there is a fundamental imbalance in this equation because carbon released from burning fossil fuels moves from a permanent store into a dynamic (and accessible) r ole in the carbon cycle. The within such a carbon sink (i.e. a forest) is only temporarily sequestered and is still available to the active carbon cycle in the biosphere. When the trees die and decompose they will release their carbon and increase atmospheric concentrations. There is three times the quantity of carbon in terrestrial biomass and than in the atmosphere and perhaps ten times the quantity in fossil fuels compared to the atmosphere. As the concentration of carbon dioxide in the atmosphere is important for the global radiation balance and climate change, the net transfer of carbon from these stores into the atmosphere has the potential to significantly change the carbon balance of the atmosphere. Once carbon is active in the atmospheric cycle it is difficult to lock it up again in less than geological time spans. Temporary is performed by forests but t her e ar e limitations to the effectiveness of these to mitigate climate change for a number of other reasons. Increased temperatures and quantities of dead organic matter increase the rate of activity and hence respiration of soil microbes. Boreal (northern temperate) forests are relatively secure carbon sinks but global warming may cause a shift of their geographical location northwards. While at first this may appear positive for carbon sequestration, there are likely to be other knock-on effects. If they move north into areas that were previously tundra there will be other effects such as reduction in ground (the reflectiveness of the earth). Tundra reflects a large quantity of insolation (solar radiation) whereas forests absorb much incoming radiation - thereby retaining the Sun’s energy in the form of heat.

Conclusion

The issues of carbon capture and release are complex and imperfectly understood. However, it is clear that the continued transfer of carbon from long- term fossil stores to the atmosphere (and to a lesser extent oceans) is significantly changing the planet’s carbon balance. The impact of these changes is uncertain and hence controversial. The full extent of the impact upon environmental processes and systems may not be evident until well into t he

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future. Meanwhile, there is every reason for prudence. Business has a key role to play in helping to minimize further disruption to the natural carbon cycle.

References

Keeling, C D and Whorf T P. 2005. Atmospheric CO2 records from sites in the SIO air sampling network. In: Trends: A Compendium of Data on . Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, Tenn., USA. http://cdiac.ornl.gov/trends/co2/sio-mlo.html Smithson, P, Addison, K and K Atkinson. 2002. Fundamentals of the Physical Environment. Routledge, London, UK.

Sources of further information

Safe climate for business, calculator. http://www.safeclimate.net/business/understanding/carboncycle.php

NASA Earth Observatory. http://earthobservatory.nasa.gov/Library/CarbonCycle/carbon_cycle4.html

The opinions expressed in this reading are not necessarily those of WWF. © 2008/9 All rights reserved WWF, World Wide Fund for Nature.

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