Natural and anthropogenic sources and sinks

There are three main sources and sinks for atmospheric CO2 in near-surface environments: the land biosphere (including freshwaters), the oceans, and anthropogenic emissions from the burning of fossil fuels and other industrial activities. In the natural state the land biosphere and the ocean reservoirs exchange CO2 with the atmosphere in an essentially balanced two-way transfer. These reservoirs are also sinks for anthropogenic CO2. Volcanic emissions (see Section 3.4.1) are not considered here since they are thought to be quantitatively unimportant on short timescales.

The land biosphere

In their pristine state the land areas of the Earth are estimated to exchange about 120GtC (gigatonnes expressed as carbon; 1 Gt = 109 tonnes = 1015 grams) per annum with the atmosphere. This is a balanced two-way flux, with 120 GtC moving from land to air and the same amount going in the opposite direction every year. However, this is a yearly averaged figure and in temperate and polar regions the fluxes are seasonally unequal. For such areas, in spring and summer, when plants are actively extracting CO2 from the atmosphere in the process of photosynthesis (see Section 5.5), there is a net flux from air to ground. By con trast, in autumn and winter, when the processes of respiration and decomposition of plants remains dominate over photosynthesis (see Section 5.5), the net flux is into the air. Averaged over the whole yearly cycle there is no net flux in either direction. In the tropics, where there is less seasonality in biological processes, the up and down fluxes are in approximate balance throughout the year. However, it should be noted that in the tropics, as at higher latitudes, the fluxes show considerable spatial variability (patchiness).

The seasonal asymmetry in the up and down CO2 fluxes at middle and high latitudes provides the explanation for the seasonal cycle of atmospheric CO2 shown in Fig. 7.1. The decreasing values found in spring and summer result from net plant uptake of CO2 from the air during photosynthesis and the rising limb is due to net release of CO2 during the rest of the year when respiration and decomposition are dominant. The amplitude of this seasonal pattern varies with latitude, being least at the poles (see CO2 record for the South Pole in Fig. 7.1) and equator due to lack of biological activity and seasonality respectively. At mid-and sub-polar latitudes the amplitude (peak to peak) is 10-15 ppm, i.e. considerably greater than the average yearly increase (1-2 ppm). The amplitude tends to be greater in the northern compared with the southern hemisphere because of the greater land area in the former compared with the latter. With the uptake and release of CO2 during photosynthesis and respiration/decomposition there is a concomitant release and absorption of atmospheric oxygen, as indicated in equations 5.19 and 5.20 (see Section 5.5). It has recently become possible to measure these changes in atmospheric oxygen and the data are shown in the inset to Fig.7.1. The oxygen record is much shorter than that for CO2 due to the very considerable analytical difficulties of measuring the small percentage changes in oxygen compared with CO2, due to the former being about 550 times more abundant. The seasonality discussed above for CO2 is observed for oxygen at both Cape Grim in Tasmania (southern hemisphere) and Barrow in Alaska (northern hemisphere), but with the opposite sign (i.e. when atmospheric CO2 is falling due to plant uptake during photosynthesis, oxygen is rising, and vice versa). It is also clear that the seasonality for atmospheric oxygen is displaced by 6 months between the Barrow and Cape Grim measurement sites, due to their location in the northern and southern hemispheres, respectively.

From the above discussion it is apparent that, while human activities in burning fossil fuel are the primary control on the year-to-year increase in atmospheric CO2, it is biologically induced exchanges that determine the observed seasonal pattern. Thus, it is clear that the land biota can strongly affect the levels of atmospheric CO2. This raises the question of whether human activities, for example through change in land use (e.g. clearing of virgin forest), or through enhanced photosynthesis arising from the increasing concentration of atmospheric CO2, can have produced significant net transfers of carbon into, or out of, the atmosphere.

Turning first to changes in land use, it is clear that when areas formerly storing large amounts of carbon fixed in plant material, for example forests, are converted to urban, industrial or even agricultural use, a large percentage of the fixed carbon is released to the atmosphere as CO2 quite rapidly. This occurs when the forest is cleared and in part burned, but also by bacterially aided decomposition of dead plant matter, including the soil litter (see Section 4.6.5). None of the new uses for the land store carbon as effectively as the original forest. Even cultivated land, which might appear to be a good store of carbon, contains approximately 20 times less fixed carbon per hectare than a typical mature forest.

Humans have been converting virgin forest and other well-vegetated areas into carbon-poor states for many hundreds of years. This process must therefore be a substantial source of CO2 to the atmosphere, both in the past and today. It has, however, been difficult to quantify the size of this source. Several attempts have been made to assess how its magnitude has varied over the last century (arguably the period of most rapid change in land use the world has ever experienced). The results from three studies, published in 1983, 1990 and 1993, are shown in Fig. 7.3. There are large discrepancies between the three results and it appears that the earliest attempt overestimated the source compared with the more recent studies. The best estimate of the flux for the 1980s is 1.7 GtC yr-1, with a range from 0.6 to 2.5. The large range confirms the considerable difficulty in trying to quantify CO2 emissions due to changes in land use.

Year

Fig. 7.3 Estimates of CO2 flux to the atmosphere from land-use changes made in 1983, 1990 and 1993. After Houghton (2000). With kind permission of Cambridge University Press.

Year

Fig. 7.3 Estimates of CO2 flux to the atmosphere from land-use changes made in 1983, 1990 and 1993. After Houghton (2000). With kind permission of Cambridge University Press.

It is possible that increasing atmospheric concentrations of CO2 from fossil-fuel burning and land-use change might cause enhanced growth of plants. This is an important issue, as plant growth could reduce some of the CO2 emission effects caused by land clearance. Certainly, crops grown in greenhouses under elevated CO2 regimes produce higher yields. However, extrapolation of such findings to the real environment is problematical. Although CO2 is fundamental to the process of photosynthesis, in most field situations it is not thought to be the limiting factor for plant growth, availability of water and nutrients such as nitrogen (N) and phosphorus (P) being more important (see Section 5.5.1). It would, however, be wrong to dismiss the possible effect of CO2 concentration on plant growth, since there may be situations in which the higher CO2 levels pertaining now, and even more so in the future, may be enough to enhance growth. One suggestion is that elevated CO2 leads to more efficient use of water by plants, which can then grow in areas previously too dry to sustain them.

The subject of enhanced CO2 concentration affecting plant growth is being actively researched at present. Studies range from the use of pot-grown plants in controlled (greenhouse) environments, to small-scale field enclosure studies, right through to large-scale field trials, part of the IGBP effort (Section 7.1). In these large-scale experiments, substantial areas (500 m2) of field crops are exposed to elevated CO2 concentrations and/or changes in other variables important for growth and the responses monitored over short and long time periods, which can be up to several seasonal cycles. The results of these FACE ('free-air CO2 enrichment') studies are of considerable interest since, unlike smaller and more confined attempts, they enable the effects of changes in CO2 and other variables to be studied at as close to real environmental conditions as possible. At the time of writing (2003), over 50 of these experiments have been conducted. In summary, the results indicate that doubled CO2 can lead to increased plant yield (biomass) by 10-20%, but that factors including changes in temperature, soil moisture and nutrient status, as well as plant species biodiversity, can also affect biomass positively and negatively. Since all these and other factors are operative in the natural environment, prediction of the net effect of such changes is clearly difficult.

In addition to the possibility of enhanced terrestrial take up of CO2 due to increasing atmospheric levels of the gas, there are other changes that may increase the amount of carbon stored on land. One of these arises from increased deposition from the atmosphere of plant nutrients, such as nitrogen coming from high-temperature combustion sources (automobiles, power stations), in which nitrogen gas from the air is converted into oxides of nitrogen (eqns. 3.22-3.24) which are emitted to the atmosphere. After processing in the atmosphere the nitrogen is deposited on soil where it can fertilize and so potentially enhance plant growth, and thus storage of carbon. A further effect is that of reforestation and regrowth on areas of land previously cleared for agricultural and other purposes. None of these potential enhanced or new sinks for carbon on land is at all straightforward to quantify. The best estimate we have is for the situation in the 1980s when fertilization, whether by elevated CO2 or nutrients such as nitrogen oxides, together with reforestation and regrowth, were assessed to amount to 1.9 GtC per annum, with a very large range of uncertainty. This once again stresses the great difficulties in trying to estimate changes in carbon uptake and release by the land biosphere resulting directly or indirectly from human actions.

The oceans

As with the land biosphere, the oceans also exchange large amounts of CO2 with the atmosphere each year. In the unpolluted environment, the air-to-sea and sea-to-air fluxes are globally balanced, with about 90 GtC moving in both directions every year. These up and down fluxes are driven by changes in the temperature of the surface water of the oceans, which alter its ability to dissolve CO2, as well as by biological consumption and production of the gas resulting from photosynthesis and respiration/decomposition processes in near-surface waters. All of these processes can vary both seasonally and spatially by significant degrees. In general, the tropical oceans are net sources of CO2 to the atmosphere, whereas at higher and particularly polar latitudes the oceans are a net sink.

Averaged globally and over the yearly cycle, the unpolluted oceans are in approximate steady state with respect to CO2 uptake/release. This does not mean that over long time periods there is no change in these rates. Indeed, it is thought that the much lower atmospheric CO2 level which ice core records indicate existed in the past (down to 200 ppm during the most recent glaciations—Fig. 7.10) was due, at least in part, to increased ocean uptake of CO2 in the cooler waters that existed then, as compared with the present.

The above discussion refers to the ocean/atmosphere system in its pristine state. We know, however, that fossil fuel burning and other human-induced changes have led to substantial additional input of CO2 into the atmosphere. How much of this extra CO2 enters the oceans?

Several factors must be taken into account. Firstly, there is the chemistry of seawater itself. Compared with distilled water or even a solution of sodium chloride (NaCl) of equivalent ionic strength (see Box 5.1) to the oceans, seawater has a significantly greater ability to take up excess CO2. This comes about from the existence in seawater of alkalinity (see Sections 5.3.1, 6.4.4 & Box 5.2) in the form of carbonate ions (CO2-), which can react with CO2 molecules to form bicarbonate ions (HCO-):

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