The carbon cycle and the greenhouse effect

Three of the principal greenhouse gases—CO2, methane (CH4) and the CFCs—contain carbon, one of the most common elements in the environment, and one which plays a major role in the greenhouse effect. It is present in all organic substances, and is a constituent of a great variety of compounds, ranging from relatively simple gases to very complex derivatives of petroleum hydrocarbons. The carbon in the environment is mobile, readily changing its affiliation with other elements in response to biological, chemical and physical processes. This mobility is controlled through a natural biogeochemical cycle which works to maintain a balance between the release of carbon compounds from their sources and their absorption in sinks. The natural carbon cycle is normally considered to be self-regulating, but with a time scale of the order of thousands of years. Over shorter periods, the cycle appears to be unbalanced, but that may be a reflection of an incomplete understanding of the processes involved or perhaps an indication of the presence of sinks or reservoirs still to be discovered (Moore and Bolin 1986). The carbon in the system moves

THE GREENHOUSE EFFECT AND GLOBAL WARMING 147 Figure 7.2 Schematic representation of the storage and flow of carbon in the earth/atmosphere system

Greenhouse Effects Schematic

Source: Compiled from data in Gribbin (1978), McCarthy et al. (1986)

Carbon Sinks Green House Effect

Figure 7.3 CO emissions: percentage share by region

Source: Based on data in World Resources Institute (1992)

Figure 7.4 Rising levels of atmospheric CO2 at Mauna Loa, Hawaii. The smooth curve represents annual average values; the zig-zag curve indicates seasonal fluctuations. The peaks in the zig-zag curve represent the winter values and the troughs represent the summer values

Figure 7.4 Rising levels of atmospheric CO2 at Mauna Loa, Hawaii. The smooth curve represents annual average values; the zig-zag curve indicates seasonal fluctuations. The peaks in the zig-zag curve represent the winter values and the troughs represent the summer values

Mauna Loa Co2 1992

Source: After Bolin et al. (1986)

between several major reservoirs (see Figure 7.2). The atmosphere, for example, contains more than 750 billion tonnes of carbon at any given time, while 2,000 billion tonnes are stored on land, and close to 40,000 billion tonnes are contained in the oceans (Gribbin 1978). Living terrestrial organic matter is estimated to contain between 450 and 600 billion tonnes, somewhat less than that stored in the atmosphere (Moore and Bolin 1986). World fossil fuel reserves also constitute an important carbon reservoir of some 5,000 billion tonnes (McCarthy et al. 1986). They contain carbon which has not been active in the cycle for millions of years, but is now being reintroduced as a result of the growing demand for energy in modern society being met by the mining and burning of fossil fuels. It is being reactivated in the form of CO , which is being released into the atmospheric reservoir in quantities sufficient to disrupt the natural flow of carbon in the environment. The greatest natural flow (or flux) is between the atmosphere and terrestrial biota and between the atmosphere and the oceans (Watson et al. 1990). Although these fluxes vary from time to time, they have no long-term impact on the greenhouse effect because they are an integral part of the earth/ atmosphere system. In contrast, inputs to the atmosphere from fossil fuel consumption, although smaller than the natural flows, involve carbon which has not participated in the system for millions of years. When it is reintroduced, the system cannot cope immediately, and becomes unbalanced. The natural sinks are unable to absorb the new CO as rapidly as it is being produced. The exces2s remains in the atmosphere, to intensify the greenhouse effect, and thus contribute to global warming.

The burning of fossil fuels adds more than 5 billion tonnes of CO2 to the atmosphere every year (Keepin et al. 1986), with more than 90 per cent originating in North and Central America, Asia, Europe and the republics of the former USSR (see Figure 7.3). Fossil fuel use remains the primary source of anthropogenic CO2 but augmenting that is the destruction of natural

Figure 7.5 Net regional release of carbon to the atmosphere as a result of deforestation during the 1980s (teragrams of carbon)


Figure 7.5 Net regional release of carbon to the atmosphere as a result of deforestation during the 1980s (teragrams of carbon)

Carbon Cycle And Natural Vegetation

101 Thailand Laos 60 95 85

Source: After Mintzer (1992)

vegetation which causes the level of atmospheric CO2 to increase by reducing the amount recycled during photosynthesis. Photosynthesis is a process, shared by all green plants, by which solar energy is converted into chemical energy. It involves gaseous exchange. During the process, CO2 taken in through the plant leaves is broken down into carbon and oxygen. The carbon is retained by the plant while the oxygen is released into the atmosphere. The role of vegetation in controlling CO2 through photosynthesis is clearly indicated by variations in the levels of the gas during the growing season. Measurements at Mauna Loa Observatory in Hawaii show patterns in which CO2 concentrations are lower during the northern summer and higher during the northern winter (see Figure 7.4). These variations reflect the effects of photosynthesis in the northern hemisphere, which contains the bulk of the world's vegetation (Bolin 1986). Plants absorb CO2 during their summer growing phase, but not during their winter dormant period, and the difference is sufficient to cause semi-annual fluctuations in global CO2 levels.

The clearing of vegetation raises CO2 levels indirectly through reduced photosynthesis, but CO2 is also added directly to the atmosphere by burning, by the decay of biomass and by the increased oxidation of carbon from the newly exposed soil. Such processes are estimated to be responsible for 5-20 per cent of current anthropogenic CO2 emissions (Waterstone 1993). This is usually considered a modern phenomenon, particularly prevalent in the tropical rainforests of South America and SouthEast Asia (Gribbin 1981) (see Figure 7.5), but Wilson (1978) has suggested that the pioneer agricultural settlement of North America, Australasia and South Africa in the second half of the nineteenth century made an important contribution to rising CO2 levels. This is supported to some extent by the observation that between 1850 and 1950 some 120 billion tonnes of carbon were released into the atmosphere as a result of deforestation and the destruction of other vegetation by fire (Stuiver 1978). The burning of fossil fuels produced only half that much CO2 over the same time period. Current estimates indicate that the atmospheric CO2 increase resulting from reduced photosynthesis and the clearing of vegetation is equivalent to about 1 billion tonnes per year (Moore and Bolin 1986), down slightly from the earlier value. However, the annual contribution from the burning of fossil fuels is almost ten times what it was in the years between 1850 and 1950.

Although the total annual input of CO2 to the atmosphere is of the order of 6 billion tonnes, the atmospheric CO2 level increases by only about 2.5 billion tonnes per year. The difference is distributed to the oceans, to terrestrial biota and to other sinks as yet unknown (Moore and Bolin 1986). Although the oceans are commonly considered to absorb 2.5 billion tonnes of CO2 per year, recent studies suggest that the actual total may be only half that amount (Taylor 1992). The destination of the remainder has important implications for the study of the greenhouse effect, and continues to be investigated. The oceans absorb the CO2 in a variety of ways— some as a result of photosynthesis in phytoplankton, some through nutritional processes which allow marine organisms to grow calcium carbonate shells or skeletons, and some by direct diffusion at the air/ocean interface (McCarthey et al. 1986). The mixing of the ocean waters causes the redistribution of the absorbed CO2. In polar latitudes, for example, the added carbon sinks along with the cold surface waters in that region, whereas in warmer latitudes carbon-rich waters well up towards the surface allowing the CO2 to escape again. The turnover of the deep ocean waters is relatively slow, however, and carbon carried there in the sinking water or in the skeletons of dead marine organisms remains in storage for hundreds of years. More rapid mixing takes place through surface ocean currents such as the Gulf Stream, but in general the sea responds only slowly to changes in atmospheric CO2 levels. This may explain the apparent inability of the oceans to absorb more than 40-50 per cent of the CO2 added to the atmosphere by human activities, although it has the capacity to absorb all of the additional carbon (Moore and Bolin 1986).

The oceans constitute the largest active reservoir of carbon in the earth/atmosphere system, and their ability to absorb CO2 is not in doubt. However, the specific mechanisms involved are now recognized as extremely complex, requiring more research into the interactions between the atmosphere, ocean and biosphere if they are to be better understood (Crane and Liss 1985).

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    What is the carbon cycle in the environment?
    7 years ago
  • reija hautala
    How air pollution may have effect on photosynthesis and balance of nature?
    7 years ago
  • poppy
    What environmental problems affect the carbon cycle?
    4 years ago
  • Girma
    What environmental problems effect the carbon cycle?
    4 years ago
  • Paladin
    Why is there a zig zag pattern in the carbon dioxide cycle every year?
    4 years ago

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