3 Description of the methods to estimate the carbon footprint of Denmark
3.6 Inclusion of increased radiative forcing from aviation
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annual increase in N‐fertiliser is calculated based on time series of N‐fertiliser production obtained from IFA (2013). The global average annual increase in N‐fertiliser production from 2000 to 2009 was 2.33 million tonne N. The associated N2O‐emission is calculated based on IPCC (2006) for a weighted average of maize, rice and what which are the most important crops regarding intensification globally.
Table 3.10: Overview of the transactions of the intensification activities.
Intensification From Intensification of arable land Intensification of grassland
Product output
Reference flow ha yr 1 1
Fertiliser inputs
N‐fertiliser, as N t 0.094 0
Emissions
N2O t 0.0021 0
In Table 3.11 the GHG‐emissions from iLUC for occupation of one hectare in one year (1 ha yr) in different regions of the world are shown.
Table 3.11: GHG‐emissions from iLUC related to the occupation of one hectare arable land in one year (1 ha yr) in different regions of the world.
Country/region Indicator for potential productivity (NPP0 measured as t C ha‐1)
Relative indicator for potential productivity (pw ha yr)
GHG‐emissions from iLUC per occupation of 1 ha yr
World average arable land 6.1 1.0 1.7
Denmark (DK) 7.0 1.1 1.9
Brazil (BR), Cerrado region 9.0 1.5 2.5
Europe (EU27), Central 7.0 1.1 1.9
Malaysia (MY) 11.0 1.8 3.0
India (IN), Southern India 7.0 1.1 1.9
Indonesia (ID) 13.0 2.1 3.5
Ukraine (UA) 5.0 0.8 1.4
3.6 Inclusion of increased radiative forcing from aviation
There are specific effects of emissions in high altitude, which lead to a higher contribution of aviation to the problem of climate change than just the emission of CO2 from burning fuels. For subsonic aviation, these non‐CO2 effects include (Lee et al. 2010):
Emissions of NOx result in the formation of tropospheric ozone (O3) with a positive radiative forcing (warming).
Emissions of NOx result in the destruction of ambient methane (CH4), with a negative radiative forcing (cooling), which is accompanied by a parallel, decadal loss of tropospheric O3.
Emissions of sulphate (SO4) particles result in a negative radiative forcing (cooling).
Emissions of soot particles result in a positive radiative forcing (warming).
The formation of persistent linear contrails in the wake of an aircraft result in both positive and negative radiative forcing effects but overall, cause a positive one (warming).
The formation of contrail‐cirrus clouds from spreading contrails similarly to line shaped‐contrails results in both positive and negative radiative forcing effects but overall, is considered to cause a positive one (warming).
A sub‐component of aviation‐induced cirrus (AIC) is a mechanism whereby soot particles seed cirrus clouds. This effect may result in either positive or negative radiative forcing effects (warming/cooling) but is rather uncertain over the sign and proven existence of the effect.
The first assessment of the climatic impact of the emissions from aviation was done by the IPCC (Penner et al. 2000), using radiative forcing as a metric, see Figure 3.11. This assessment found that in 1992 the total radiative forcing caused by aviation emissions (excluding cirrus formation) was 2.5 times that of CO2 emissions alone. This so‐called radiative forcing index (RFI) has been used as a multiplication factor to calculate the overall carbon footprint of aviation emissions, however this approach is plainly wrong (Forster et al. 2007), given that the RFI is based on radiative forcing only, whereas the GWPs used in the Kyoto Protocol and in carbon footprinting take into account the lifespan of substances in the atmosphere.
Currently there are no clear recommendations by the IPCC on how to deal with this subject, and as shown by the review carried out by Jungbluth (2012), in LCA, carbon footprinting, emission trading, etc.,
practitioners have either omitted aviation’s non‐CO2 effects altogether, or they have applied a RFI ranging from 2 to 2.7‐2.8, sometimes to the total CO2 emissions, sometimes only to those occurring in the
stratosphere.
Figure 3.11: Radiative forcing from aircraft in 1992 (Penner et al. 1992).
The lack of recommendations by the IPCC in this area is related to the fact that the scientific understanding of these environmental processes is not yet good enough (Forster et al. 2007). Many of the non‐CO2 emissions from aviation correspond to short‐lived substances, not currently covered by the Kyoto protocol.
As opposed to emissions of e.g. CO2, the climatic impact of short‐lived substances depends on where and when the emissions are introduced into the atmosphere. For aircraft emissions it is not only important to know the location in the globe where the emission takes place, but also the altitude (Fuglestvedt et al.
2010). For these reasons, Penner et al. (2000) considered that the GWP metric was not suited at all to assess this kind of emissions and preferred to use radiative forcing instead.
In spite of the methodological limitations, GWP values for different time horizons have been provided for the different emitted substances (see Fuglestvedt et al. 2010), and these have been applied in peer‐
reviewed research (Borken‐Kleefeld et al. 2013, Lee et al. 2010). In this study we have decided to apply the GWP100 values to aviation emissions as proposed by Lee et al. (2010), including those for NOx, water vapour, contrails and AIC. No GWP100 values have been applied to sulphate and black carbon, due to the fact that in practice their contribution to CO2‐eq. emissions is very low in a 100‐year perspective.
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Table 3.12: GWP100 values considered for non‐CO2 aviation emissions.
GHG GWP100
(kg CO2‐eq/kg GHG)
Comments
N‐NOx 71 Very high uncertainty. Values range from ‐2.1 to 71.
H2O 0.14 High uncertainty. This factor is applied to the total amount of water emitted in the exhaust gases.
Contrails 0.21 High uncertainty. This factor is applied to the total amount of CO2 emitted in the exhaust gases.
Aviation‐induced cirrus (AIC) 0.63 Very high uncertainty. This factor is applied to the total amount of CO2 emitted in the exhaust gases.
It must be highlighted that NOx produces a radiative forcing not only when emitted at high altitudes, but also at ground level, where it produces a cooling effect. Thus for consistency a (negative) GWP100 value for
NOx emitted at ground level could be considered, and applied to emissions sources other than aircraft.
According to Wild et al. (2001) global NOx emissions (on a N basis) have a GWP100 of ‐11 kg CO2‐eq/kg. In practice though this contribution in terms of CO2‐eq. is negligible15 when compared to that from
conventional long‐lived GHG‐emissions over a 100‐year time horizon, and for this reason it has not been considered in the study.
15 As an example, an average European car (ecoinvent dataset ‘operation, passenger car, RER’) has a tailpipe emission of 0.19 kg
CO2/km, whereas the NOx emission is of 0.00053 kg/km. Assuming a N fraction of 39% in the NOx (i.e. 50% NO and 50% NO2) and applying a GWP100 of ‐11 kg CO2/kg N‐NOX, the NOx correspond to 0.0025 kg CO2‐eq, that is a contribution three orders of magnitude smaller than the CO2 actually emitted.