Uncertainties in climate assessment for the case of aviation NO

Abstract

Nitrogen oxides emitted from aircraft engines alter the chemistry of the atmosphere, perturbing the greenhouse gases methane (CH 4 ) and ozone (O 3 ). We quantify uncertainties in radiative forcing (RF) due to short-lived increases in O 3 , long-lived decreases in CH 4 and O 3 , and their net effect, using the ensemble of published models and a factor decomposition of each forcing. The decomposition captures major features of the ensemble, and also shows which processes drive the total uncertainty in several climate metrics. Aviation-specific factors drive most of the uncertainty for the short-lived O 3 and long-lived CH 4 RFs, but a nonaviation factor dominates for long-lived O 3 . The model ensemble shows strong anticorrelation between the short-lived and long-lived RF perturbations (R 2 ¼ 0.87). Uncertainty in the net RF is highly sensitive to this correlation. We reproduce the correlation and ensemble spread in one model, showing that processes controlling the background tropospheric abundance of nitrogen oxides are likely responsible for the modeling uncertainty in climate impacts from aviation.aviation emissions | error correlation | model sensitivity Q uantifying uncertainties in climate processes is a major priority for climate research, as exemplified by the widespread use of probability distributions and uncertainties in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (1). To understand and attribute changes in the greenhouse gases and aerosols that force climate change, we rely on chemistry-transport models (CTMs), which require further characterization of their errors (e.g., 2-5). Aviation emissions are estimated to contribute 5% of anthropogenic climate forcing, with a nearly threefold uncertainty (6). This forcing occurs mainly through aviation-induced cloudiness and emissions of CO 2 and nitrogen oxides. Climate impacts of other aviation emissions-CO, SO 2 , soot, hydrocarbons, and water vapor-are highly uncertain, but estimated to be much smaller (6). In this paper we quantify one type of climate forcing and its uncertainty, the impact of aviation emissions on the greenhouse gases ozone (O 3 ) and methane (CH 4 ).Aircraft engines emit NO and NO 2 (NOx), which are indirect greenhouse gases through their chemical reactions impacting O 3 production and the CH 4 lifetime. As an O 3 precursor, NOx emissions increase the tropospheric column of O 3 and its radiative forcing. Aviation NOx emissions have a stronger impact on O 3 than surface emissions on a per molecule basis because they occur at high altitudes (7). Previous model-based estimates of the amount of O 3 generated by aviation NOx emissions vary by up to 100% and these differences have been attributed to a range of causes: e.g., the ratios of NO∶NO 2 and OH∶HO 2 , background NOx levels (8), location and time of emissions (9-11), the amount of sunlight (12), and atmospheric mixing (13,14).Early work on climate forcing by aviation assumed that the impacts would be short-lived and limited to the northern latitudes ...

Future impact of traffic emissions on atmospheric ozone and OH based on two scenarios

“…5, top), there is clearly a large uncertainty in the magnitude of impact from AIR in the middle of this century. Although there is a considerable spread between the models (see Holmes et al (2011) for a discussion of RF uncertainties when modelling aviation NO x ), the uncertainty is mostly induced by the large spread of emissions between the various scenarios , whereas the ensemble mean O 3 RF is 61 (± 14) mW m −2 for the A1B scenario and only 19 (± 6.8) mW m −2 for B1 ACARE in 2050, just slightly higher than in 2000 (18 ± 3.6 mW m −2 ). However, it should be noted here that B1 ACARE is a mitigation scenario assuming extensive technological improvements to the aircraft and is considered very optimistic in terms of emissions (see H2011).…”

Aviation 2006 NO<sub>x</sub>-induced effects on atmospheric ozone and HO<sub>x</sub> in Community Earth System Model (CESM)

“…Wild et al (2012) examined the impact of solar-flux variations, while and Berntsen et al (2005) investigated the effects of atmospheric mixing. However, as reported in Holmes et al (2011), modelbased estimates of aviation NO x -induced changes in O 3 vary by up to 100 %, largely because of differences between models in the ratios of NO : NO 2 and OH : HO 2 , background NO x levels, location and time of emissions, the amount of sunlight, and in atmospheric mixing (Holmes et al, 2011). Recent studies by Olsen et al (2013) and Brasseur et al (2014) found considerable differences between a set of climatechemistry models (CCMs) and chemistry transport models (CTMs) in comparisons of the background atmosphere and aviation NO x -induced changes in ozone.…”

“…The long-term reduction is associated with negative forcing (Wild et al, 2001;Stevenson et al, 2004). The short-term O 3 forcing is one of the major contributors to the overall aviation forcing and dominates the net O 3 forcing (Lee et al, 2009;Holmes et al, 2011). Since our simulations were performed with fixed CH 4 mixing ratios at the boundary layer, the calculated changes in O 3 concentration are the short-term changes.…”

Climate-Compatible Air Transport System—Climate Impact Mitigation Potential for Actual and Future Aircraft

“…Both O 3 and CH 4 are greenhouse gases. Hence, the net RF from aviation NO x depends on which pathway dominates, and this depends on emission scenarios, background concentrations and the chemical rate coefficients [17]. Depending on the location of the emission, the net RF of these emissions could be positive or negative [10].…”