Abstract
Croplands are the single largest anthropogenic source of nitrous oxide (N2O) globally, yet their estimates remain difficult to verify when using Tier 1 and 3 methods of the Intergovernmental Panel on Climate Change (IPCC). Here, we re-evaluate global cropland-N2O emissions in 1961–2014, using N-rate-dependent emission factors (EFs) upscaled from 1206 field observations in 180 global distributed sites and high-resolution N inputs disaggregated from sub-national surveys covering 15593 administrative units. Our results confirm IPCC Tier 1 default EFs for upland crops in 1990–2014, but give a ∼15% lower EF in 1961–1989 and a ∼67% larger EF for paddy rice over the full period. Associated emissions (0.82 ± 0.34 Tg N yr–1) are probably one-quarter lower than IPCC Tier 1 global inventories but close to Tier 3 estimates. The use of survey-based gridded N-input data contributes 58% of this emission reduction, the rest being explained by the use of observation-based non-linear EFs. We conclude that upscaling N2O emissions from site-level observations to global croplands provides a new benchmark for constraining IPCC Tier 1 and 3 methods. The detailed spatial distribution of emission data is expected to inform advancement towards more realistic and effective mitigation pathways.
Highlights
Croplands are the largest anthropogenic source of atmospheric nitrous oxide (N2O) [1,2]
Large emission differences between our estimate and NMIP results can be found in southeast USA and most of India (Supplementary Fig. 3 vs. Fig. 4 of Tian et al [3] or Fig. 6 of Tian et al [4]), possibly due to the different N-input data used or different sensitivity of N2O flux to N inputs and environmental conditions. These data-driven results highlight the need for accurate estimates of global cropland-N2O emissions using observation-based non-linear emission factors (EFs) and surveybased gridded N-input data
An estimated systematic reduction (∼25%) of global cropland-N2O emissions is found compared to Intergovernmental Panel on Climate Change (IPCC) Tier 1 global inventories, we acknowledge that actual cropland-N2O emissions remain poorly quantified
Summary
Croplands are the largest anthropogenic source of atmospheric nitrous oxide (N2O) [1,2]. Global cropland-N2O emissions for the most recent decade estimated by various bottom-up approaches [3,6,7] ranged from 1.5 to 5.0 Tg N yr–1. The use of EFs multiplied by activity data (i.e. N-fertilizers applied) is the most common bottomup approach, corresponding to the Tier 1 IPCC methodology. This pragmatic approach is used in research studies and for compilation of national greenhouse gas emission inventories [6,7,8]. Tier 1 methods that assume temporally or regionally constant EFs provide a first-order approximation, which needs to be complemented by more detailed approaches to reduce estimation uncertainty at finer scales [3,9]. Evidence for non-linear characteristics of EFs has recently been confirmed regionally as well [15]
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