Carbon

At about 280 ppm the preindustrial atmospheric CO2 amounted to almost exactly 600 billion tonnes (Gt) C. The ocean contained roughly 37000Gt of dissolved inorganic C, about 95 per cent of it as bicarbonate ion (HCO~3). The fate of marine C is controlled by interactions of physical, chemical and biotic processes. Amounts of dissolved inorganic C depend on the release of CO2 to the atmosphere and on its equilibrating uptake; on precipitation and dissolution of marine carbonates; and on photosynthetic assimilation and organic decay. The equilibrium absorptive capacity of the ocean is a function of temperature and acidity (pH). Moreover, it becomes available only after the whole water column equilibrates with the added CO2, a process limited by the ocean's layered structure. Intermediate and deep waters, beyond the reach of solar radiation with temperatures at 2-4°C, contain nearly 98 per cent of the ocean's C.

Photosynthesis in the ocean assimilates annually about 50Gt C, turning circumpolar oceans into major C sinks during the summer months. Respiration by zooplankton, and by other oceanic herbivores, returns at least 90 per cent of the assimilated carbon to the near-surface waters from which the gas can either escape to the atmosphere or be re-used by phytoplankton. Carbon in the remaining dead biomass settles to the deeper ocean. In reversing the photosynthetic process, this settling, oxidation and decay increases the ocean's total carbon content while slightly lowering its alkalinity (increasing acidity).

Pre-agricultural terrestrial photosynthesis took up annually about twice as much C as did marine phytoplankton. Both C storage and net C exchange of ecosystems are dependent on photosynthetically active radiation. Plant C uptake is marked by pronounced daily and seasonal cycles, the latter fluctuations producing unmistakable undulations of the biospheric 'breath'. Rapid recycling of Plant C was almost equally split between autotrophic and heterotrophic respiration. Decomposition of organic litter (dominated by bacteria, fungi and soil invertebrates) and herbivory remain the two most important forms of heterotrophic respiration.

Soils contain more than twice as much C as the atmosphere, and their C storage density goes up with higher precipitation and lower temperature. Accumulation of a tiny fraction of assimilated C in sediments is the principal terrestrial bridge between the element's rapid cycle - whereby decomposition of biomass returns the assimilated C into the atmosphere in just a matter of days or months so that it can be re-used by photosynthesis - and slow cycles which sequester the element in breakdown-resistant humus or in buried sediments. Long-term exposure of the sequestered organic C compounds (for up to 108 years) to higher temperatures and pressures results in the formation of fossil fuels whose global resources are most likely in excess of 6 trillion tonnes C. Only a small part of these fossil fuels will be eventually recovered by our industrial civilization.

Figure 21.1 Global carbon cycle

Extraction of these fossil fuels has been emitting increasing amounts of C into the atmosphere. The annual global rate rose from less than 0.5Gt C in 1900 to 1.5Gt C in 1950 and by the year 2000 it surpassed 6Gt C. Additional net release of 1-2Gt C comes from ecosystems converted to other uses (mainly from tropical deforestation). The atmospheric CO2 trend is reliably known for nearly half a million years thanks to the analyses of air bubbles from Antarctic and Greenland ice cores. During this period CO2 levels have stayed between 180 and 300 ppm. During the 5000 years preceding 1850 they had fluctuated just between 250 and 290 ppm.

When the first systematic measurements began in 1958, CO2 concentration averaged 320 ppm. In the year 2000 the mean at Mauna Loa surpassed 370 ppm. A nearly 40 per cent increase in 150 years is of concern because CO2 is a major greenhouse gas whose main absorption band coincides with the Earth's peak thermal emission. Greenhouse effects maintain the Earth's average surface temperature at around 15°C, or about 33°C warmer than would be the case otherwise. Thermal energy reradiated to Earth by the atmosphere, 325WIm2 (Watts per square meter), is the most important source of heat for oceans and continents, and current atmospheric levels of CO2 contribute about 75WIm2.

Anthropogenic CO2 has already increased the heat flux by 1.5WIm2, and higher levels of other infrared (IR) absorbing gases have brought the total forcing to about 2.5 WIm2. Rising emissions would eventually lead to the doubling of preindustrial greenhouse gas levels and to average tropospheric temperatures 1-5°C higher than today's mean. This warming would be more pronounced on the land and during the night, with winter increases about two to three times the global mean in higher latitudes than in the tropics, and greater in the Arctic than in the Antarctic.

Major worrisome changes arising from a relatively rapid global warming would include intensification of the global water cycle accompanied by unequally distributed shifts in precipitation and aridity; higher circumpolar run-offs, later snowfalls and earlier snow-melts; more common, and more intensive, extreme weather events such as cyclones and heat waves; thermal expansion of sea water and gradual melting of mountain glaciers leading to appreciable (up to 1m) sea level rise; changes of photosynthetic productivity and shifts of ecosystemic boundaries; and poleward extension of tropical diseases (Watson et al. 1996). International efforts to reduce the rate of CO2 emissions have been, so far, unsuccessful: cuts proposed by the Kyoto Treaty (amounting to emissions about 7 per cent below the 1990 rate) were rejected by the USA, the world's largest greenhouse gas producer, and they do not even apply to China, now the second largest contributor of CO .

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