Carbon cycle

opposing forces of photosynthesis and respiration drive the global carbon cycle

Photosynthesis and respiration are the two opposing processes that drive the global carbon cycle. It is predominantly a gaseous cycle, with CO2 as the main vehicle of flux between the atmosphere, hydrosphere and biota. Historically, the lithosphere played only a minor role; fossil fuels lay as dormant reservoirs of carbon until human intervention in recent centuries (Figure 18.21d).

Terrestrial plants use atmospheric CO2 as their carbon source for photosynthesis, whereas aquatic plants use dissolved carbonates (i.e. carbon from the hydrosphere). The two subcycles are linked by exchanges of CO2 between the atmosphere and oceans as follows:

atmospheric CO2 ^ dissolved CO2

In addition, carbon finds its way into inland waters and oceans as bicarbonate resulting from weathering (carbonation) of calcium-rich rocks such as limestone and chalk:

Respiration by plants, animals and microorganisms releases the carbon locked in photosynthetic products back to the atmospheric and hydrospheric carbon compartments.

The concentration of CO2 in the atmosphere has increased from about CO2 in the 280 parts per million (ppm) in 1750 to atmosphere has more than 370 ppm today and it is still increased significantly rising. The pattern of increase recorded because of... at the Mauna Loa Observatory in Hawaii since 1958 is shown in Figure 18.22. (Note the cyclical decreases in CO2 associated with higher rates of photosynthesis during summer in the northern hemisphere - reflecting the fact that most of the world's landmass is north of the equator.)

We discussed this increase in atmospheric CO2, and the associated ...the combustion exaggeration in the greenhouse effect, of fossil fuels... in Sections 2.9.1 and 2.9.2, but armed with a more comprehensive appreciation of carbon budgets, we can now revisit this subject. The principal causes of the increase has been the combustion of fossil fuels and, to a much smaller extent, the kilning of limestone to produce cement (the latter produces less than 2% of that produced by fossil fuel burning). Together, during the period 1980-95, these accounted for a net increase in the atmosphere averaging 5.7 (± 0.5) Pg C year-1 (Houghton, 2000).

Land-use change has caused a further 1.9 (± 0.2) Pg of carbon to enter the ... and exploitation atmosphere each year. The exploitation of tropical forest of tropical forest causes a significant release of CO2, but the precise effect depends on whether forest is cleared for permanent agriculture, shifting agriculture or timber production. The burning that follows most forest clearance quickly converts some of the vegetation to CO2, while decay of the remaining vegetation releases CO2 over a more extended period. If forests have been cleared to provide for permanent agriculture, the carbon content of the soil is reduced by decomposition of the organic matter, by erosion and sometimes by mechanical removal of the topsoil. Clearance for shifting agriculture has similar effects, but the regeneration of ground flora and secondary forest during the fallow period sequesters a proportion of the carbon originally lost. Shifting agriculture and timber extraction involve 'temporary' clearance in which the net release of CO2 per unit area is significantly less than is the case for 'permanent' clearance for agriculture or pasture. Changes to land use in non-

tropical terrestrial communities seem to have had a negligible effect on the net release of CO2 to the atmosphere.

The total amount of carbon released each year to the atmosphere some of the extra CO2 dissolves in the oceans... or is taken up by terrestrial plants

Figure 18.22 Concentration of atmospheric carbon dioxide (CO2) at the Mauna Loa Observatory, Hawaii, showing the seasonal cycle (resulting from changes in photosynthetic rate) and the long-term increase that is due largely to the burning of fossil fuels. (Courtesy of the Climate Monitoring and Diagnostics Laboratory of the National Oceanic and Atmospheric Administration.)

Figure 18.22 Concentration of atmospheric carbon dioxide (CO2) at the Mauna Loa Observatory, Hawaii, showing the seasonal cycle (resulting from changes in photosynthetic rate) and the long-term increase that is due largely to the burning of fossil fuels. (Courtesy of the Climate Monitoring and Diagnostics Laboratory of the National Oceanic and Atmospheric Administration.)

by human activities (7.6 Pg C year-1; see Section 2.9.1) can be compared with the 100-120 Pg C year-1 released naturally by respiration of the world's biota (Houghton, 2000). Where does the extra CO2 go? The observed increase in atmospheric CO2 accounts for 3.2 (± 1.0) Pg C year-1 (i.e. 42% of the human inputs). Much of the rest, 2.1 (± 0.6) Pg C year-1, dissolves in the oceans. This leaves 2.3 Pg C year-1, which is generally put down to a residual terrestrial sink, the magnitude, location and causes of which are uncertain, but are believed to involve increased terrestrial productivity in northern mid-latitude regions (i.e. part of the increase in CO2 may serve to 'fertilize' terrestrial communities and be assimilated into extra biomass) and the recovery of forests from earlier disturbances (Houghton, 2000).

There is considerable year-to-year variation in the estimates of CO2 sources and sinks, and of the increase in the atmosphere (Figure 18.23). Indeed, this variation is what allowed standard errors to be placed on average values in the previous paragraphs. The declines in atmospheric increase in CO2 between 1981 and 1982 followed dramatic rises in oil prices, while the declines in 1992 and 1993 followed the economic collapse of the Soviet Union. In 1997-98 (not shown in Figure 18.23), a remarkable wildfire in a small part of the globe doubled the growth rate of CO2 in the atmosphere. Massive forest fires in Indonesia produced a carbon emission of about 1 Pg in just a few weeks. The burned areas included vast deposits of peat, which lost 25-85 cm of their depth during the fire, and most of the released carbon came from this source rather than the burning of wood. The fires in Indonesia were particularly serious due to a combination of circumstances - drought caused accurate prediction of future changes in carbon emissions is a pressing matter by the 1997-98 El Niño event, the thickness of peat present, and particular logging practices that allowed the vegetation and soil to dry out (Schimel & Baker, 2002). The accurate prediction of future changes in carbon emissions is a pressing matter, but it will be a difficult task because so many variables - climatic, political and sociological - impinge on the carbon balance. We return to the many dimensions of the ecological challenges facing mankind at the very end of the book (see Section 22.5.3).

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