The subscript "0" denotes the unperturbed (1750) concentration; f (M,N) = 0.47 ln [1+2.01 x 10-5 (MN)075 + 5.31 x 10-15M (MN)152]; C is CO2 in ppm, M is CH4 in ppb, N is N2O in ppb, X is CFC in ppb; Co = 278 ppm, Mo = 700 ppb, No = 270 ppb, Xo = 0.
et al. 1982). Most of the N2O monitoring studies from crop fields have been carried out in Asia, United States, Canada and some parts of Europe. A summary of some of the available N2O emission data from crops collected worldwide is presented in Table 8.4 (for rice) and Table 8.5 (all other crops together) separately, as rice cultivation is done under a totally different set of crop management practices. Fodder and forage crops have been kept out of scope of this summary. Emission data clearly indicate that N loss via N2O from rice fields is generally less than 1% of added fertilizer N except a few cases where it reaches up to 5.6% (Xu et al. 2004). In other crops, N2O-N loss is more pronounced than rice, but remains generally below 1% or sometimes in between 1 and 2%. N2O flux differs from experiment to experiment mainly due to variation in water regime, N dose and fertilizer type and various other crop management and inherent soil factors. In agriculture, on an average, 1.25% of the N input (as fertilizer, manure or through biological N fixation) is emitted from the field as N2O (Bouwman 1996), though there is a large variation ranging from 0.25 to 2.25% in most cases (IPCC 1996a). EPA (1995) uses a factor (mean value of N2O flux) of 1.17% for N2O-N loss from added N in agriculture.
8.11 Radiative Forcing of N2O Emitted from Crop Fields
The Intergovernmental Panel on Climate Change (IPCC) defines climate forcing as "an externally imposed perturbation in the radiative energy budget of the earth climate system through changes in solar radiation, changes in the earth's albedo or changes in atmospheric gases and aerosol particles". Radiative forcing is the difference between the incoming radiation energy and the outgoing radiation energy in a given climate system. A positive forcing i.e. more incoming energy tends to warm the system, while a negative forcing or more outgoing energy tends to cool it. Greenhouse gases, responsible for greenhouse effect and consequently global warming, are standardized for their warming potetials by an index known as global warming potential (GWP). It is a relative scale which compares the concerned gas to the same mass of carbon dioxide (whose GWP is 1, by definition) over a specific time interval. The GWP has been defined as the ratio of the time-integrated radiative forcing from the instantaneous release of 1 kg of a trace substance relative to that of 1 kg of a reference gas, i.e. CO2 (IPCC l990).
Notably, GWP depends on the time span over which the potential is calculated. A gas, which is quickly removed from atmosphere may initially have a large effect, but not for longer time periods. Thus, N2O has a GWP potential of 296 over 100 years, but 175 over 500 years (IPCC 2001) which is not precisely known and hence the values are considered only approximate. Higher the global warming potential (GWP) of a greenhouse gas, more will be its positive radiative forcing than the same amount of other greenhouse gases with lower GWPs. The perturbation to radiative climate forcing, which has the largest magnitude and the least scientific uncertainty, is the forcing related to changes in long-lived and well mixed greenhouse gases, e.g. carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and halogens (mainly CFCs).
National Oceanic and Atmospheric Agency (NOAA), USA, has been estimating atmospheric concentrations of global greenhouse gases and their radiative forcing since 1979. The change in the annual mean of total radiative forcing by all long-lived greenhouse gases since pre-industrial era (1750) is used to define the NOAA Annual Greenhouse Gas Index (AGGI), introduced in 2004 (Hofmann et al. 2006a) and updated in 2006 (Hofmann et al. 2006b). AGGI is the ratio of total radiative forcing due to long-lived greenhouse gases for any year for which adequate global measurements exist to that which was present in 1990, the baseline year for Kyoto Protocol. This index is a measure of the interannual changes in conditions that affect CO2 emission and uptake, CH4 and N2O sources and sinks and the decline in the atmospheric abundance of ozone-depleting chemicals related to Montreal Protocol. For 2006, the AGGI was 1.23 (an increase in total radiative forcing of 23% since 1990). The increase in CO2 forcing alone since 1990 was about 32%. The decrease in methane emissions and CFCs has possibly tampered the increase in net radiative forcing. The AGGI is updated each spring when air samples from all over the globe for the previous year have been obtained and analyzed.
To determine the total radiative forcing of the greenhouse gases for a current year, NOAA uses IPCC (IPCC 2001) recommended expressions to convert greenhouse gas changes, relative to 1750, chosen as the pre-industrial base year, to get instantaneous radiative forcing by using the direct forcing only (Table 8.6). These empirical expressions used for radiative forcing are derived from atmospheric radiative transfer models and generally have an uncertainty of about 10%. Model-dependent feedbacks, for example, due to water vapor and O3 depletion, are not included. Other spatially heterogeneous, short-lived, climate forcing agents having uncertain global magnitudes, such as aerosols and tropospheric O3 are also not included here in order to maintain accuracy. Figure 8.2, prepared from NOAA's calculations (http://www.esrl.noaa.gov/gmd/aggi/), shows the radiative forcing for the major gases and a set of 10 minor long-lived halogen gases including CFC-113, CCl4, CH3CO3, HCFCs 22, 141b and 142b, HFC134a, SF6 and halons 1211 and 1301. CO2 dominates the total forcing with CH4 and the CFCs becoming relatively smaller contributors to the total forcing over time. The five major greenhouse gases account for about 97% of the direct radiative forcing by long-lived greenhouse gas increases since 1750. The remaining 3% is contributed by the 10 minor halogen
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