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.