How important are future marine and shipping aerosol emissions in a warming Arctic summer and autumn?
Authors: Gilgen Anina , Huang Wan Ting Katty , Ickes Luisa , Neubauer David , Lohmann Ulrike 
Published: July 2018

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 sulfide and sea salt
emissions. Additionally, a decrease in the area and thickness of sea ice
could lead to enhanced Arctic ship traffic, for example due to shorter routes
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 emissions, clouds will also be affected by
increases in Arctic temperatures and humidities. In this study, we quantify
how future aerosol radiative forcings 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. For 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. In the future, sea salt as well as dimethyl sulfide emissions and burdens
will increase in the Arctic. The increase in cloud condensation nuclei, which
is due to changes in aerosol particles and meteorology, will enhance cloud
droplet number concentrations over the Arctic Ocean (+10 % in late summer
and +29 % in early autumn; in-cloud values averaged between 75 and
90∘ N). Furthermore, both liquid and total water path will increase
(+10 % and +8 % in late summer; +34 % and +26 % in early
autumn) 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 in early
autumn). Mainly due to the reduction in sea ice, the aerosol radiative
forcing will become less positive (decreasing from 0.53 to 0.36 W m−2
in late summer and from 0.15 to 0.11 W m−2 in early autumn). The
decrease in sea ice is also mainly responsible for changes in the net cloud
radiative effect, which will become more negative in late summer (changing
from −36 to −46 W m−2). Therefore, the cooling component of both
aerosols and clouds will gain importance in the future. We found that future Arctic ship emissions related to transport and oil and
gas extraction (Peters et al., 2011) will not have a large impact on clouds and
radiation: changes in aerosols only become
significant when we increase these
ship emissions by a factor of 10. However, even with 10-fold ship emissions,
the net aerosol radiative forcing shows no significant changes. Enhanced
black carbon deposition on snow leads to a locally significant but very small
increase 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 thickness
and lifetime of clouds in late summer (net cloud radiative effect changing
from −48 to −52 W m−2). These aerosol–cloud effects have a
considerably larger influence on the radiative forcing than the direct
effects of particles (both aerosol particles in the atmosphere and particles
deposited on snow). In summary, future ship emissions of aerosols and their
precursor gases 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.