Published in:
Atmospheric Chemistry and Physics, 18 (2018) 14, Seite 10521-10555
Language:
English
DOI:
10.5194/acp-18-10521-2018
ISSN:
1680-7324
Origination:
Footnote:
Description:
Abstract. Future sea ice retreat in the Arcticin summer and autumn is expected to affect both natural and anthropogenicaerosol emissions: sea ice acts as a barrier between the ocean and theatmosphere, and reducing it increases dimethyl sulfide and sea saltemissions. Additionally, a decrease in the area and thickness of sea icecould lead to enhanced Arctic ship traffic, for example due to shorter routesof cargo ships. Changes in the emissions of aerosol particles can theninfluence cloud properties, precipitation, surface albedo, and radiation.Next to changes in aerosol emissions, clouds will also be affected byincreases in Arctic temperatures and humidities. In this study, we quantifyhow future aerosol radiative forcings and cloud radiative effects mightchange in the Arctic in late summer (July–August) and early autumn(September–October). Simulations were conducted for the years 2004 and 2050 with the globalaerosol–climate model ECHAM6-HAM2. For 2050, simulations with and withoutadditional ship emissions in the Arctic were carried out to quantify theimpact of these emissions on the Arctic climate. In the future, sea salt as well as dimethyl sulfide emissions and burdenswill increase in the Arctic. The increase in cloud condensation nuclei, whichis due to changes in aerosol particles and meteorology, will enhance clouddroplet number concentrations over the Arctic Ocean (+10 % in late summerand +29 % in early autumn; in-cloud values averaged between 75 and90∘ N). Furthermore, both liquid and total water path will increase(+10 % and +8 % in late summer; +34 % and +26 % in earlyautumn) since the specific humidity will be enhanced due to highertemperatures and the exposure of the ocean's surface. Changes in both aerosol radiative forcings and cloud radiative effects at thetop of the atmosphere will not be dominated by the aerosol particles andclouds themselves but by the decrease in surface albedo (and by the increasein surface temperature for the longwave cloud radiative effect in earlyautumn). Mainly due to the reduction in sea ice, the aerosol radiativeforcing will become less positive (decreasing from 0.53 to 0.36 W m−2in late summer and from 0.15 to 0.11 W m−2 in early autumn). Thedecrease in sea ice is also mainly responsible for changes in the net cloudradiative effect, which will become more negative in late summer (changingfrom −36 to −46 W m−2). Therefore, the cooling component of bothaerosols and clouds will gain importance in the future. We found that future Arctic ship emissions related to transport and oil andgas extraction (Peters et al., 2011) will not have a large impact on clouds andradiation: changes in aerosols only becomesignificant when we increase theseship emissions by a factor of 10. However, even with 10-fold ship emissions,the net aerosol radiative forcing shows no significant changes. Enhancedblack carbon deposition on snow leads to a locally significant but very smallincrease in radiative forcing over the central Arctic Ocean in early autumn(no significant increase for average between 75 and 90∘ N).Furthermore, the 10-fold higher ship emissions increase the optical thicknessand lifetime of clouds in late summer (net cloud radiative effect changingfrom −48 to −52 W m−2). These aerosol–cloud effects have aconsiderably larger influence on the radiative forcing than the directeffects of particles (both aerosol particles in the atmosphere and particlesdeposited on snow). In summary, future ship emissions of aerosols and theirprecursor gases might have a net cooling effect, which is small compared toother changes in future Arctic climate such as those caused by the decreasein surface albedo.