How important are future marine and shipping aerosol emissions in warming Arctic summer and autumn?
Authors: Gilgen Anina , Huang Wan Ting Katty , Ickes Luisa , Neubauer David , Lohmann Ulrike 
Published: November 2017

Abstract. Future sea ice retreat in the Arctic in summer and autumn is expected to affect both natural and anthropogenic aerosol emissions: sea ice acts as a barrier between the ocean and the atmosphere, and reducing it increases dimethyl sulphide and sea salt emissions. A decrease in the area and thickness of sea ice could in addition lead to enhanced Arctic ship traffic, e.g. to shorten the paths of cargo ships. Changes in the emissions of aerosol particles can then influence cloud properties, precipitation, surface albedo, and radiation. Next to changes in aerosol particles, clouds will also be affected by increases in Arctic temperatures and humidities. In this study, we quantified how future aerosol radiative forcing, aerosol-cloud interactions, and cloud radiative effects might change in the Arctic in late summer (July/August) and early autumn (September/October). Simulations were conducted for the years 2004 and 2050 with the global aerosol-climate model ECHAM6-HAM2. In 2050, simulations with and without additional ship emissions in the Arctic were carried out to quantify the impact of these emissions on the Arctic climate. We found that aerosol number concentrations in the Arctic will generally increase in the future due to enhanced emissions of sea salt as well as dimethyl sulphide. The increase in cloud condensation nuclei will enhance cloud droplet number concentrations over the Arctic Ocean. Furthermore, both liquid and total water content will increase since the specific humidity will be enhanced due to higher temperatures and the exposure of the ocean's surface. Changes in both aerosol radiative forcings and cloud radiative effects at the top of the atmosphere will not be dominated by the aerosol particles and clouds themselves but by the decrease in surface albedo (and by the increase in surface temperature for the longwave cloud radiative effect). Due to the reduction in sea ice, the aerosol radiative forcing will become less positive and the cloud radiative effect more negative, i.e. the cooling component of both will gain importance in the future. We found that future Arctic ship emissions related to transport and oil/gas extraction (Peters et al., 2011, ACP) will not have a large impact on clouds and radiation: changes in aerosol concentrations only become significant when we increase these ship emissions by a factor of ten. The net aerosol radiative forcing shows only small, non-significant changes. Enhanced black carbon deposition on snow leads to a significant but very small warming over the central Arctic Ocean in early autumn. Furthermore, the tenfold higher ship emissions increase the optical thickness of low clouds and thus induce a small Twomey effect (cooling) in late summer. This Twomey effect has a considerably larger influence on temperature than the direct effect of particles (both aerosol particles in the atmosphere and particles deposited on snow), but it is more uncertain because of the large variability of clouds. In summary, future ship emissions might have a net cooling effect, which is small compared to other changes in future Arctic climate such as those caused by the decrease in surface albedo.