The global impact of the transport sectors on aerosol and climate under the Shared Socioeconomic Pathways (SSPs)
Abstract. A global aerosol-climate model is applied to quantify the impact of the transport sectors (land transport, shipping and aviation) on aerosol and climate. Global simulations are performed for present-day (2015), based on the emission inventory of the Climate Model Intercomparison Project Phase 6 (CMIP6), and for near-term (2030) and mid-term (2050) future projections, under the Shared Socioeconomic Pathways (SSPs). The results for present-day show that land transport emissions have a large impact on near-surface concentrations of black carbon and aerosol nitrate over the most populated areas of the globe, but with contrasting patterns in terms of relative contributions between developed and developing countries. In spite of the recently introduced regulations to limit the fuel sulphur content in the shipping sector, shipping emissions are still responsible for a considerable impact on aerosol sulfate near-surface concentrations, about 0.5 to 1 µg m−3 in the most travelled regions, with significant effects also on continental air pollution and in the northern polar regions. Aviation impacts on aerosol mass are found to be quite small, of the order of a few nanograms per cubic meter, while this sector considerably affects particle number concentrations, contributing up to 20–30 % of the upper tropospheric particle number concentration at the northern mid-latitudes. The transport-induced impacts on aerosol mass and number concentrations result in a present-day radiative forcing of −164, −145 and −64 mW m−2, for land transport, shipping and aviation, respectively, with a dominating contribution by aerosol-cloud interactions. These forcings represent a marked offset to the CO2 warming from the transport sectors and are therefore very relevant for climate policy. The projections under the SSPs show that the impact of the transport sectors on aerosol and climate are generally consistent with the narratives underlying these scenarios: the lowest impacts of transport on both aerosol and climate are simulated under SSP1, especially for the land transport sector, while SSP3 is generally characterized by the largest effects. Notable exceptions to this picture, however, exist, as the emissions of other anthropogenic sectors also contribute to the overall aerosol concentrations and thus modulate the relevance of the transport sectors in the different scenarios, not always consistently with their underlying storyline. On a qualitative level, the results for present-day mostly confirm the findings of our previous assessment for the year 2000, which used a predecessor version of the same model and the CMIP5 emissions data. Some important quantitative differences are found, which can mostly be ascribed to the improved representation of aerosol background concentrations in the present study.
Mattia Righi et al.
Status: open (extended)
- RC1: 'Comment on esd-2022-52', Anonymous Referee #1, 28 Feb 2023 reply
Mattia Righi et al.
Mattia Righi et al.
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The manuscript "The global impact of the transport sectors on aerosol and climate under the Shared Socioeconomic Pathways (SSPs)" present quantification of the impact of the transport sectors (land transport, shipping and aviation) on aerosol and the resulting climate effects for present day and a selection of SSPs and future years. The global chemistry-climate model EMAC is used to quantify the effects.
The manuscript is clearly written, and well structured, and I have only minor comments on the paper.
Regarding the title: "The global impact of the transport sectors on aerosol and climate under the Shared Socioeconomic Pathways (SSPs)", it can be interpreted as the work consider the global impact of transport on climate, but it is only the aerosol impact of the transport sector on climate that is considered. Consider rephrasing, also in the abstract.
Regarding the above, is it possible to add a discussion regarding the CO2 warming from the sectors in the scenarios?
In the abstract: "In spite of the recently introduced regulations to limit the fuel sulphur content in the shipping sector, shipping emissions are still responsible for a considerable impact on aerosol sulfate near-surface concentrations", I wonder how well the scenarios take into account the most recent regulations.
L39: "some" I will say "all", as all of these components will either directly or indirectly (via chemistry) affect climate.
L57: CMIP6 emission inventory (Hoesly et al., 2018; van Marle et al., 2017) Hoesly et al. is the historical anthropogenic emissions used in CMIP6 that end in 2014.
L171: Related to above: "and future projections until the year 2100" the two references cover the historical period and not the future projections.
I find the two first paragraphs in section 3 on the emissions a bit unstructured. When reading, I was expecting the matrix structure of the SSPs to appear earlier. The link and harmonization of the historical and the scenarios could be made clearer, as described in Gidden et al. 2019. 2015 emissions in the harmonized emissions are similar in all the scenarios.
There are a wide range of SSP scenarios, but for climate modelling, only a small selection is gridded and harmonized for use. For SSP1 and SSP3, there are two other forcing targets available. Would be good if the differences related to aerosol emissions in these could be slightly mentioned.
Figure 2: Add to the figure caption the domain over which the burden is calculated.
All figures: Replace the rainbow color scale with a different color scale.
5 Climate impacts. In the beginning of this section, it would be useful with a clearer definition of the radiative forcing definition. Do you calculate RF or ERF (including the adjustments). Is it both radiative forcing for aerosol cloud interaction (aci) and aerosol radiation interaction (ari)? And more specifically, how to compare the clear sky and all sky RF to separate the role of aci and ari.
Figure 9: How is the 90% CI calculated?
L533-549: This section conclude the results for present day 2015? Indicate that in the first sentence.
L580: Any studies on ammonia in the transport sector?