Water Released by Oxidation of Fossil Fuels and Vegetation

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The combustion of fossils fuels and burning of tropical forests release water into the atmosphere together with carbon dioxide according to the general chemical equation

Conversely, water and carbon dioxide are removed from the atmosphere as inorganic bicarbonate and carbonate ions in the ocean, and as new organic matter (reduced carbon) in forest regrowth (C02 assimilation) and soil carbon storage. Water exchanges associated with the carbon cycle (C) make a relatively minor contribution to sea level rise, as shown below (Table 5.5).

Table 5.5

Water Release Associated with C02 Emissions

Average annual C02 budget Water released, in SLR (GTC/yr) equivalent (mm/yr)

C02 sources

Fossil fuel combustion and cement production Tropical deforestation Total sources C02 sinks

Atmosphere Ocean uptake Northern Hemisphere forest regrowth Additional sinks (C02 fertilization, nitrogen fertilization, soil storage) Total sinks

Equivalent sea level rise

Data from Houghton et al., 1996.

5.2.3.1 Sources of CO2 and H20

The three dominant fossil fuel types emit C02 in the following proportions: 0.16 (natural gas), 0.43 (liquid), and 0.41 (solid fuels) (after Marland and Boden, 1993), corresponding to emissions of 0.81, 2.28, and 2.14 GT C/yr, respectively (1 GT = 1015 g). These values can be used to estimate the volumes of water produced. (See Gornitz et al., 1997, for details of calculation.)

The mass of C02 released by burning natural gas (methane) is 2.97 GT C02/yr, which is equivalent to 2.43 km3/yr HzO or 0.0068 mm/yr SLR. Combustion of petroleum yields 8.36 X GT C02/yr, which is equivalent to 3.63 km3/ yr H20 or 0.01 mm/yr SLR. Burning coal generates 7.8 GT C02/yr, equivalent to 1.3 km3/yr H20 or 0.004 mm/yr SLR. The total water released by fossil fuel combustion is therefore equivalent to a SLR of 0.021 mm/yr (Table 5.5).

The net atmospheric carbon release from global land-use change is around 1.1 GT C/yr, representing the difference between tropical deforestation (1.6 GT C/yr) and temperate-boreal forest regrowth (0.5 GT C/yr, Table 5.5). This mass of carbon is equivalent to 1.65 km3 H20 (0.9-2.4 km3 H20), or 0.005 (0.003-0.007 mm/yr SLR. Equal numbers of C02 and HzO molecules are produced by the burning and/or bacterial decay of dry biomass.) The net annual water released by vegetation clearance each year, assuming a dry-to-

wet biomass ratio of 0.25 (Rohrig, 1991), comes to 8.25 km3 HzO (4.5-12.0),4 equivalent to 0.023 (0.013-0.033 mm/yr) SLR (Table 5.5).

5.2.3.2 Sinks of C02 and H20

The world's oceans absorb around 2.0 GT C/yr of C02 released by fossil fuel emissions and deforestation (Table 5.5). The oceanic uptake of C02 to form hydrogen ions and carbonate ions consumes the equivalent of 3.0 km3/ yr H20, which corresponds to about -0.008 mm/yr SLR.

Other terrestrial sinks for atmospheric carbon dioxide include enhanced plant assimilation under elevated C02 and enhanced plant growth due to nitrogen from fertilizers and fossil fuel burning. The uptake of 1.3 GT C (Table 5.5) corresponds to 1.95 km3 of dry biomass. Using a dry-to-wet biomass ratio of 0.25 (Rohrig, 1991) gives the water taken up by new vegetation as 9.75 km3 or -0.026 mm/yr SLR equivalent. The net contribution of water released by all of these oxidative processes is then around 0.01 mm/yr (in terms of SLR equivalent) (Table 5.4).

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