Global emissions estimates and source uncertainties

As indicated above, the amount of N2O in the atmosphere has been increasing approximately linearly, at around 0.7ppb, or 0.26 per cent, per year for the past few decades (Plate 4.3). The IPCC Third Assessment Report (IPCC, 2001) concluded that the primary driver of this increase was enhanced microbial production of N2O in expanding and fertilized agricultural lands. In the report, as in the earlier evaluations (IPCC, 1990, 1992, 1996) and the later Fourth Assessment Report (IPCC, 2007), the contribution from agriculture was based on 'bottom-up' extrapolations to the global scale from measurements at a limited number of experimental sites. The same was true for the contributions from natural terrestrial ecosystems. (In a later section we compare this bottom-up approach to assessing agricultural emissions with a top-down calculation.)

In IPCC (1990) and IPCC (1992), the uncertainty concerning agricultural N2O emissions was so great that the range of estimates covered two orders of magnitude, 0.01-2.2 and 0.03-3.3Tg N yr_1, respectively. The widely varying conditions in agroecosystems, particularly of N supply and water availability, inevitably led to measured annual emissions covering a huge range, from a few grams of N2O per hectare to tens of kilograms per hectare (see Chapter 5). This contributed to the huge uncertainties in the early global assessments. However, many more detailed emission measurements at agricultural sites, and their relationships to N inputs, were carried out from the early 1990s onwards, and provided convincing evidence that emissions of N2O from agricultural systems make up the largest single global source. Thus in the next assessment (IPCC, 1996) the mean value for the N2O agricultural source was given as 3.5Tg N yr_1, which was above the previous upper limit, and the range had narrowed from two orders of magnitude to ± 50 per cent of the mean (Table 4.3). Since then there has been a consensus that the emissions from agriculture - direct and indirect - are of this order, i.e. several Tg N yr_1 (Table 4.3), and constitute the largest single source category (Table 4.1), while uncertainty margins have been extended again.

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