Nitrous oxides contribution to radiative forcing

Our knowledge of the sources and sinks of atmospheric nitrous oxide has been expanded greatly in the last two or three decades by the results of pure and applied research in many countries, much of it documented in the bibliographies of the preceding chapters. The major driver has been the concern about the potential consequences of anthropogenic global warming. To improve predictions about what might happen to the climate we need to quantify better the rates of emissions of the important greenhouse gases and the likely future trends in those emissions under various scenarios, and to improve ways of minimizing the emissions; all of these desires have required additional information from research.

The most recent assessment of climate change (IPCC, 2007) shows the relatively minor role played by N2O at present in the warming ('radiative forcing') process, compared with the other long-lived greenhouse gases (Figure 10.1), but the picture varies greatly between countries. Where agriculture is the dominant industry, the contribution of N2O (and methane) to total national emissions can be very much greater than that implied by Figure 10.1. Among the developed countries, this is well illustrated by the data for New Zealand: there in 2007 N2O (almost entirely from livestock-based agriculture) contributed 17 per cent, in CO2-equivalent terms, (and methane 35.2 per cent), to emissions from all sectors. Thus the trace gases together exceeded the contribution from CO2, and contributed more than half of all national emissions (New Zealand Government, 2009). However, leaving aside such special cases, there is the likelihood that in many countries, if major curbs on CO2 emissions are forthcoming over the period 2010-2050 (see below), the trace gases will contribute a significant and ever-increasing proportion of emissions. This will mean that they are likely to receive more and more attention concerning mitigation options.

The implications of such a trend will be particularly important in agriculture. The technology for controlling industrial and vehicular emissions already exists, and many major sources have already been dealt with (see

Figure 10.1 Summary of the principal components of the radiative forcing of climate change

Source: IPCC (2007)

Chapter 9). Step-changes in national N2O emissions have occurred in a number of industrialized countries when large adipic acid-manufacturing plants have been modified to include the available abatement technology. For example, the voluntary agreements in the 1990s between the German government and the two German adipic acid producers to capture and transform N2O emissions reduced total national emissions by 8 per cent, or approximately 26Mt CO2-eq (Eichhammer et al, 2001). When any remaining industrial sources have been similarly abated, emissions from agriculture will become, more and more, the principal concern.

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