carbon cycles

carbon cycles Carbon is one of the fundamental building blocks of the Earth. Most life forms on Earth consist of organic carbon, while inorganic carbon may dominate the visible physical environment. What makes this element particularly interesting is its involvement in a continuous and complex exchange between the atmosphere, ocean, and land as a result of energy flux originating from the Sun. The increase in atmospheric carbon dioxide (CO₂) by 26 per cent since the Industrial Revolution, largely attributed to the burning of fossil fuels, and the links that this increase may have with climate change, have led to an emphasis on understanding the carbon cycle. To quantify this cycle requires an understanding of the functioning of the major components of the planet.

The processes controlling the flux of carbon are, by themselves, fairly simple. Three types may be identified: (1) the major energy-transforming reactions of life—assimilation and dissimilation in photosynthesis and respiration—which cycle some 10¹¹ metric tons of carbon each year; (2) simple physical exchange of carbon dioxide; and (3) dissolution and precipitation (deposition) of carbonate compounds, resulting in the formation of sedimentary rocks such as limestone and dolomite. These are essentially balanced processes, although precipitation was dominant during the past. In aquatic systems these processes are two orders of magnitude slower than assimilation and dissimilation. These processes occur within and between three major carbon pools: the ocean, the atmosphere, and the terrestrial system. Figure 1 shows the sizes and fluxes of carbon between the major carbon reservoirs on timescales of up to 10⁵ years. Each is now discussed in turn.

The main forms of carbon in the atmosphere are carbon dioxide, methane, and carbon monoxide of which carbon dioxide and methane have long residence times. Climate and the carbon cycle have undergone huge, seemingly contemporaneous changes (Fig. 2). One of the remaining questions in climate research is which system, carbon cycling or climate, has forced the change. Over the past 200 years humans appear to have perturbed the carbon cycle. Since carbon dioxide serves as a greenhouse gas, trapping long-wave radiation emitted by the Earth in the atmosphere, it is possible that temperature increases in the atmosphere over the past decades have occurred because of this increase in atmospheric carbon.

The ocean contains over 60 times more carbon than the atmosphere, making it the largest pool of the planet's mobile carbon. Within this pool, dissolved inorganic carbon is the most common form. Smaller pools, but with higher turnover, occur in the marine biosphere as dissolved organic carbon. Primary production in the surface layer is some 30–40 per cent of terrestrial vegetation. Processes operative in this large pool may be considered as two pumps: the solubility pump, which transfers CO₂ to the deep ocean, and the biological pump.

The ocean may be divided into three layers: the surface layer (75 m), which is well mixed, overlies the thermocline (to 1 km), which is a stagnant zone stabilized by increasing density and decreasing temperature; the bottom layer is the deep ocean. Cold, saline surface water sinks, spreading out at deeper levels. Such descending waters, most common in polar regions, transport dissolved surface CO₂ to the deep ocean, where it remains trapped for hundreds or thousands of years. The efficiency of this solubility pump depends on CO₂ being dissolved in surface waters; this in turn, depends on the difference in the pressure of CO₂ in the sea water and air. More gas may be dissolved in colder water. Carbon dioxide thus leaves the ocean in the tropics and enters in the polar regions, such as the North Atlantic and Antarctic ocean. Elsewhere, such transport occurs through convergence within the subtropical gyres and vertical diffusion between the thermocline and the subtropical waters. This process, however, is very slow; it takes hundreds or thousands of years for surface waters to penetrate below the mixed layer.

The biological pump occurs through photosynthesis, although most of the atmospheric carbon fixed in this way is respired in the surface ocean within days to months. An estimated 10 per cent precipitates out to be oxidized in the deep ocean. This carbon remains at depth for centuries.

Nitrogen and phosphorous act to limit primary production. Upwelling in the ocean brings nutrient-rich water to the surface, a process which promotes phytoplankton growth. A definite seasonal cycle is attached to this occurrence, the stabilizaton of the water column due to surface warming and reduced turbulence in spring, together with increased sunlight, causes an explosive growth of phytoplankton populations. Wind-driven upwelling regimes, such as that off the west coast of southern Africa, display an interesting ocean-atmosphere coupling which may be changed if the climate is perturbed as a result of anthropogenic changes in the carbon cycle.

Most of the Earth's carbon resides in sedimentary rocks, such as limestone, but since this is largely a captive source, we shall concentrate on three sub-pools: living biomass, litter, and soil carbon. A distinction between herbaceous and woody plants, owing to differences in turnover rates, is often made within the living biomass component. Boreal, temperate, and tropical forests are regarded as the most important carbon reservoirs in this pool. Recycling occurs through photosynthesis, respiration, and decomposition. Living vegetation contains roughly the same amount of carbon as is stored in the atmosphere, while dead biomass on land consists of twice as much carbon as the atmosphere. The carbon absorbed by photosynthesis of land plants amounts annually to 100 gigatonnes of carbon (GtC) (gross primary productivity). About half this amount is returned to the atmosphere by the autotrophic respiration of plants. The remainder, termed ‘net primary production’, amounting to 60GtC, is transformed into organic carbon and built into plant tissue. Some of the dead plant carbon is transformed to soil organic carbon, which is oxidized very slowly.

One of the most important current research questions centres on plant response to changes in atmospheric carbon, and is part of the so-called indirect perturbation problem. The fertilization effect, where plants grow faster under enhanced atmospheric CO₂, provided they are not limited by nutrients or water, is thought to be the dominant result. Under water stress and enhanced CO₂, stomata remain open for shorter periods, leading to reduced water loss owing to evapotranspiration. It is not clear whether increased CO₂ in the atmosphere results in increased terrestrial carbon storage or simply a faster turnover rate. Indeed, some studies suggest that plants acclimatize to the increased CO₂. The terrestrial biosphere forms an important component of flux under climate change. Increased aridity, for example, may lead to forest dieback, increased incidence of fire, and a change in the total carbon stored on the landmass of the planet.

Models of the global carbon cycle have been developed to help quantify the carbon cycle and to assess its sensitivity to change. In most models, pools of carbon are represented as boxes (as in Fig. 1) and the fluxes between them are represented as simple first-order flux dynamics. Tracers with dynamics similar to carbon have been used to calibrate the carbon exchanges. The most important tracers are the stable carbon isotope ¹³C, and the radioactive isotope ¹⁴C. Changes in the ratio of ¹⁴C/¹³C before and after nuclear bomb tests provide one means of tracing the dynamics of the cycle. Tree rings indicate past changes in this ratio, a decrease in ratio up to 1950 resulting from the flux of carbon from fossil fuels (radiocarbon-free CO₂) into the atmosphere.

The calibrated models range from box diffusion models developed in the 1970s to high-resolution numerical models based on the general circulation models of the atmosphere and ocean, which treat the planet as a three-dimensional structure. Flux rates depend on local properties such as ecosystem structure, soil composition, land use, agricultural practices, or, in the ocean, on the rate of upwelling or deep-water production.

Perhaps the most intriguing element of the carbon cycle concerns the ‘missing sink’, whereby 1.5 GtC per year during the decade of the 1980s is unaccounted for in the total budget. This missing sink, which is thought to be a northern hemisphere effect, is probably related to fertilization. Other effects such as eutrophication, may be important. Part of this problem is the question of how much of the flux of CO₂ into the atmosphere has remained in the atmosphere? The observational record from Mauna Lao since the International Geophysical Year in 1957/8 shows an increase, but what is lost in this record is the increased flux sequestered by the ocean and the terrestrial biospheric carbon pools. Changes in atmospheric carbon with time depend on the amount released from fossil fuel burning, the amount in or out of the ocean, and the net biomass term due to land use changes, biomass burning, etc. Changes in atmospheric CO₂ over time are known. Fossil fuel burning can be estimated by energy consumption figures, but terrestrial and ocean fluxes are more difficult to estimate. Four approaches exist to estimate their value: (1) carbon modelling; (2) direct measurement of ocean-atmosphere fluxes and extrapolation; (3) direct measurement of surface- atmosphere fluxes extrapolation; and (4) tracer studies. The most accurate method is thought to be modelling, with the ocean being the better quantified component.

R. Washington

Bibliography

Schlesinger, W. H. (1991) Biogeochemistry: an analysis of global change. Academic Press, London.