Abstract
The influence of aerosol particles on cloud reflectivity remains one of
the largest sources of uncertainty in our understanding anthropogenic
climate change. Commercial shipping constitutes a large and concentrated
aerosol perturbation in a meteorological regime where clouds have a
disproportionally large effect on climate. Yet, to date, studies have
been unable to detect climatologically-relevant cloud radiative effects
from shipping, despite models indicating that the cloud response should
produce a sizable negative radiative forcing (perturbation to Earth’s
energy balance). We attribute a significant increase in cloud
reflectivity to enhanced cloud droplet number concentrations within a
major shipping corridor in the southeast Atlantic. Prevailing winds
constrain emissions around the corridor, which cuts through a
climatically-important region of expansive low-cloud cover. We use
universal kriging, a classic geostatistical method, to estimate what
cloud properties would have been in the absence of shipping. In the
morning, cloud brightening is consistent with changes in microphysics
alone, whereas in the afternoon, increases in cloud brightness from
microphysical changes are offset by decreases in the total amount of
cloud water. We find a radiative forcing in the southeast Atlantic
shipping corridor two orders of magnitude greater than previous
observational estimates. Approximately five years of data are required
to identify a clear signal. Extrapolating our results globally, we
calculate an effective radiative forcing due to aerosol-cloud
interactions in low clouds of -0.62 W/m2 (-1.23 to -0.08 W/m2). The
unique setup in the southeast Atlantic could be an ideal test for the
representation of aerosol-cloud interactions in climate models.