From Emissions to Concentration

The concentration of a non-carbon dioxide greenhouse gas is modeled using a mass balance equation, in which the removal process in the stratosphere is proportional to the atmospheric concentration and inversely proportional to the atmospheric lifetime of a greenhouse gas. The same methodology is adopted in most current IPCC SCMs (Harvey et al. 1997). For nitrous dioxide (N2O) and the halocarbons (including CFCs), a constant lifetime is used.

For methane (CH4), matters are more complicated. Its chemical removal rate, and therefore its atmospheric lifetime, depend on the concentration of CH4 itself. The latter is affected by the concentrations of other gases like NOx, CO and VOCs. Therefore the lifetime of CH4 is non-linearly dependent on the atmospheric composition. The lifetime of

Meta-IMAGE 2.1

Meta-IMAGE 2.1

CH4 is time- and scenario-dependent and either the atmospheric chemistry has to be taken into account or the lifetime must be made time-dependent using previous results from (three-dimensional) chemical models. The current atmospheric lifetime is about nine years (Harvey et al. 1997). In addition to removal by chemical reactions in the atmosphere, CH4 is also absorbed by soils, with a specific time constant of 150 years.

Regarding carbon dioxide (CO2), there are considerable uncertainties in our knowledge of the present sources of, and sinks for, the anthropogenically produced CO2. In fact, the only well-understood source is fossil fuel combustion while, in contrast, the source associated with land-use changes is less understood. The amount of carbon remaining in the atmosphere is the only well-known component of the budget. With respect to the oceanic and terrestrial sinks, the errors are likely to be in the order of ±25 per cent and ±100 per cent, respectively, mainly resulting from the lack of adequate data and from the deficient knowledge of the key physiological processes within the global carbon cycle (for example, Schimel et al. 1995). The uncertainties can be expressed explicitly in a basic mass conservation equation reflecting the global carbon balance (all components in gigatons of carbon content per year = GtC/yr):

where dCCO2/dt is the change in atmospheric CO2, Efos is the CO2 emission from fossil fuel burning and cement production, Eland the CO2 emission from land-use changes, Soc the CO2 uptake by the oceans and Efor the CO2 uptake through forest regrowth. To balance the carbon budget, a remaining term, I, which represents the missing sources and sinks, is introduced. I might therefore be considered as an apparent net imbalance between the sources and sinks. Analysis of the net imbalance in the global carbon cycle has become a major issue since the first IPCC scientific assessment report (IPCC 1990). Table 45.1 presents the global carbon balance over 1980-89 in terms of anthropogeni-cally induced perturbations to the natural carbon cycle, as given by the IPCC (Schimel et al. 1995). Schimel et al. stated that the remaining imbalance of 1.3±1.5 GtC/yr might be attributable to terrestrial sink mechanisms; those are the terrestrial feedbacks of CO2 and N fertilization and the temperature feedbacks on net primary production and soil respiration.

Table 45.1 Components of the global carbon dioxide mass balance, 1980-89, in terms of anthropogenically induced perturbations to the natural carbon cycle

Component (GtC/yr) 1980-89

Component (GtC/yr) 1980-89

Table 45.1 Components of the global carbon dioxide mass balance, 1980-89, in terms of anthropogenically induced perturbations to the natural carbon cycle

Emissions from fossil fuel burning and cement production (Efos)

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

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