Dealing with emissions uncertainties

For N2O, the observed growth in atmospheric concentration (amounting to 3.9Tg N2O-N yr-1), combined with the best assessment of how much new reactive N has become available for N2O production (discussed in Chapter 4), point to an overall EF of 4 per cent, whereas summing the default values for individual emission pathways gives a lower value; in other words, estimated emissions are too small to account for observed atmospheric growth, unless the upper bounds of the uncertainty ranges for the default values (IPCC, 2006) are taken into account.

This situation is the inverse of that relating to sources and sinks of CO2. The estimated net global CO2 emission rate from fossil fuel burning and cement manufacture was 7.8Gt CO2-C yr-1 in 2005 (up from 6.5Gt CO2-C yr-1 in 1999), to which must be added somewhere between 0.5 and 2.7Gt C yr-1 from deforestation (IPCC, 2007), whereas the observed growth in the atmospheric concentration is much less than indicated by these figures. The difference of around 3Gt CO2-C yr-1 is attributed to the combined effect of a terrestrial carbon sink in growing vegetation and an oceanic sink (for example Hymus and Valentini, 2007). Thus for CO2 we need to improve quantification of the apparent sinks, while for N2O we need to do the same for the sources, to achieve balanced global budgets.

A priority area for future investigation of N2O EFs is indirect emissions caused by nitrogen transfer from land to waters. Chapter 8 documents the existing knowledge, and highlights the variability encountered in studies to date. For example, recent evidence is presented that emissions from riparian areas receiving water from agricultural land may be higher than those predicted by the IPCC default EF. Further work in a wider range of such environments is desirable, together with more studies of the biogeochemical processes going on in the major rivers of the world. There is potentially a big difference in the propensity for leached nitrate to denitrify to N2O in the course of its passage to the sea, between river systems such as the Mississippi-Missouri and the Yangtze, where the distance may be thousands of kilometres, and the short rivers measured in tens or a few hundreds of kilometres where many recent studies have taken place. It may well be that, because of the sheer complexity of the systems needing study, only techniques integrating emissions over substantial areas will provide the data required. A new generation of sensors capable of determining changes in atmospheric N2O concentration at the ppt (parts per trillion), rather than the ppb level, may be required.

Such devices would also have a complementary role (if ship- or air-borne) in determining fluxes over the estuaries and seas receiving river discharges and the wider ocean receiving atmospheric deposition of reactive N. According to Duce et al (2008), increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for up to about a third of the ocean's external (non-recycled) nitrogen supply and up to around 3 per cent of the annual new marine biological production, around 0.3Pg of carbon per year. This input, they calculate, could account for the production of up to around 1.6Tg of N2O per year. This obviously needs experimental verification.

One additional area that deserves priority for new work is in emissions from irrigated agriculture. Worldwide, about 70 per cent of all water abstracted from rivers and aquifers is used for agriculture, and although only 18 per cent of agricultural land is irrigated, this land provides 40 per cent of global food production (Siebert et al, 2007). In view of the forecast upward trend in population (see above), the demand for irrigation is likely to intensify, even though some areas are predicted to experience water shortages because of climate change. The diversity, and the variation in quality, of land management systems is great, and once again there is a need for targeted research to investigate the direct EF for, for example, flood irrigation systems in hot countries; in these environments, nitrification of urea fertilizer to nitrate can occur very rapidly, and the lack of sophisticated land-levelling equipment can result in variable ponding of water in parts of a field: ideal conditions for substantial denitrification to take place. Should it turn out that such environments are, indeed, hot spots of N2O emission, the introduction of technology available elsewhere, particularly water-saving systems such as drip irrigation, should both mitigate the emissions and help to conserve scarce and/or expensive water - a good example of a 'win-win' strategy. A substantial part of the efforts of the developed world to minimize N2O emissions from agriculture would be best devoted to challenges of this type in the developing world; for a long-lived, well-mixed greenhouse gas such as N2O, reducing its emission anywhere on the planet is equally valuable, and making a large emission reduction abroad is a better strategy than reducing a smaller amount at home. It is important that the need for national governments to meet their Kyoto (and post-Copenhagen) domestic commitments do not make them lose sight of this important consideration.

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