Observational constraints on Arctic radiative forcing due to aerosol-cloud interactions

Anthropogenic aerosols can have a significant impact on the Earth’s radiation budget through their interactions with clouds. The effective radiative forcing due to aerosol-cloud interactions (ERFaci) is believed to be negative globally, albeit with significant uncertainty. Despite the region experiencing rapid climate change, climate models have so far failed to constrain the sign of ERFaci in the Arctic, while observation-based estimates of ERFaci in the polar regions are challenging due to a relative lack of ground-based observatories and uncertainties in satellite retrievals.  

Here we provide an observation-based estimate of the top-of-atmosphere radiative forcing due to aerosol-cloud interactions in the Arctic using passive remote sensing satellite data and aerosol reanalyses. To address potential satellite retrieval errors over sea ice and at high latitudes, we use observed cloud optical thickness from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) to measure the accuracy of passive retrievals of cloud properties from the Moderate Resolution Imaging Spectrometer (MODIS). We find strong agreement on cloud detection for optically thicker clouds, which represent the large majority of the clouds used in this study. 

The effective radiative forcing from adjustments in cloud fraction and cloud water path are calculated using droplet number concentration as a mediating variable. The ERFaci in liquid clouds over the Arctic ocean and sea ice is found to be negative on average, with a stronger forcing over the ocean. Negative forcings from instantaneous changes in cloud droplet number concentration and subsequent cloud fraction adjustments are partially offset by a positive forcing caused by apparent decreases in cloud water path. The data suggests that the overall ERFaci in the Arctic is more likely negative compared to previous estimates from model outputs, but smaller in magnitude compared to lower latitudes due to reduced insolation and higher surface albedo. As anthropogenic aerosol emissions in the Arctic are expected to increase in the coming decades, a stronger ERFaci could follow and partially offset other positive forcings. These results for the present-day forcing could constrain model outputs of future Arctic radiative forcing, reducing the contribution of aerosol-cloud processes to the overall uncertainty.