Global Carbon Cycle

Measurements of the concentration of air trapped in Antarctic ice cores indicate that over the last 200,000 years, atmospheric concentrations of C02 have fluctuated between 200 and 280ppmv, until the last century. Data for the period ad 1000 and 1800 indicate that the concentration was quite stable, averaging 280ppmv and varying over that period by only about 10 ppmv, indicating that the C02 cycle was in equilibrium in the centuries prior to the industrial revolution. Over the past 200 years, however, the concentration has increased from 280 to more than

360ppmv; this increase is attributed primarily to burning of fossil fuels (primarily at northern middle latitudes) and tropical biomass burning. Through an examination of the isotopic composition of the C02 in the ice core samples, it can likewise be shown that nearly all of this increase is a result of fossil fuel combustion. Carbon has two isotopes, one with a molecular weight of 12 and the other with a molecular weight of 13. Naturally occurring carbon dioxide that has been put into the atmosphere through photosynthesis will be comprised of ~1% of the heavier 13C; C02 that has been put into the atmosphere from fossil fuel burning will be slightly depleted in 13C. Analysis of the isotopic 13C02-to-12C02 ratio from air trapped in ice cores indicates that a smaller fraction of the C02 released to the atmosphere before the industrial revolution came from fossil fuel sources when compared to the modern-day ratios.

Currently, approximately 6GtC (1 GtC = 1012kg of carbon) are released to the atmosphere as a result of fossil fuel combustion. Between about 1850 and the early 1970s, the release of C02 increased exponentially at a relatively constant rate of 4.3% per year (Fig. 4). Since the oil crisis of 1973, the concerted efforts to reduce energy consumption have slowed the trend, but, nonetheless, an upward trend still continues, especially since the late 1980s. The cumulative production of C02 from fossil fuel is estimated to be 225 GtC, or ~30% of the current amount of C02 in the atmosphere. The result of this increase to the atmosphere is reflected in both ambient

Figure 4 Estimates of C02 released to the atmosphere from fossil fuel combustion from 1850 to the present.

NOAA CMDL Monthly Mean Carbon Dioxide

YEAR

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900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Figure 5 (ste color insert) (a) Monthly concentrations of CO, measured from gas samples at four monitoring sites operated by NOAA's Climate Monitoring and Diagnostics Laboratory from the early 1970s; (6) COi concentrations determined from ice core samples estimated to go back 1000 years. See ftp site for color image.

900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Figure 5 (ste color insert) (a) Monthly concentrations of CO, measured from gas samples at four monitoring sites operated by NOAA's Climate Monitoring and Diagnostics Laboratory from the early 1970s; (6) COi concentrations determined from ice core samples estimated to go back 1000 years. See ftp site for color image.

4 GLOBAL CARBON BUDGET 15

measurements from several monitoring sites of NOAA's Climate Monitoring and Diagnostics Laboratory (see Fig. 5a) and from ice core data (Fig. 5b). The current rate of C'02 increase is .8 ppmv per year.

4 GLOBAL CARBON BUDGET

Balancing the global carbon budget is a challenging effort, but large strides have been made in recent years. One of the largest problems is the difficulty of measuring carbon fluxes over large scales as well as accurately modeling atmospheric and oceanic transport of carbon species. The global carbon cycle is shown schematically in Figure 6. In this figure, the numbers in the reservoirs are given in GtC and the fluxes in GtC per year. This figure shows that the gross exchange flux between the ocean and the atmosphere (on the order of 90 GtC/yr), and between the terrestrial biosphere and the atmosphere (on the order of 60 GtC/yr) arc at least an order of magnitude larger than the CO, emissions from fossil fuel burning (5.5 GtC/yr) and from deforestation {net of 1.6 GtC/yr). On the other hand, the total anthropogenic input of C02 to the atmosphere (7.1 GtC/yr) is significant compared to the net exchange fluxes between the three carbon reservoirs. In particular, the net C02 flux into the terrestrial biosphere is the subject of an ongoing debate. Instead of attempting to deal with the complete carbon budget, it is easier to limit this discus

Climat Change Antropogenic Input

Atmosphere 750

. Veijrtatofi 610 \ SoHsandDelrflus 1580

Fossil Fuels and Cement Production

Surface Ocean 1020

Marine Bioîa

Intermediate and Deep Ocean 38.100

Figure 6 Schematic diagram showing the size of the carbon reservoirs and the amount of annual exchange between them.

sion to the perturbation budget for C02 (i.e., what happens to the C02 injected into the atmosphere by the combination of fossil fuel and biomass combustion and land-use change).

The single value in the global cycle that is known most accurately is the change of atmospheric C02 concentration, which has been measured continuously and averaged over a number of sites; it corresponds to 3.3±0.2GtC/yr. Fossil fuel combustion can also be estimated fairly accurately (through a knowledge of global coal and petroleum production) at a value of 5.5±0.5 GtC/yr. Estimates of C02 contributions resulting from deforestation, primarily in tropics, are quite uncertain, ranging from 0.6 to 2.5 GtC/yr. Using an average value of 1.6± 1.0 GtC/yr, the Intergovernmental Panel on Climate Change estimates the average sources of anthropogenic C02 to the atmosphere as 7.1 ±1.1 GtC/yr. Since 3.2±0.2GtC/yr accumulate in the atmosphere, the remaining 3.9GtC/yr must be reabsorbed either by the oceans or by the terrestrial biosphere. Current models calculate an oceanic uptake of 2.0±0.8, leaving an imbalance (or "missing sink") of 1.4± 1.5 GtC/yr. A considerable amount of research in recent years has been directed toward partitioning this missing sink into ocean and land components.

Research on many of the individual scientific issues is currently being conducted by numerous scientists throughout the world and the scientists involved in this research transcend a number of disciplines such as atmospheric science, ecology, microbiology, and others. These scientists are brought together to foster research related to the issues involving global change through the International Geosphere-Biosphere Program (IGBP), under the sponsorship of the International Council of Scientific Unions. IGBP has several "core" programs and one of them is the International Global Atmospheric Chemistry (IGAC), which focuses on tropospheric chemistry. Much of the discussion in the remainder of this chapter and in the other chapters in this section discuss results and research that are linked to the goals of IGBP.

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