Ground-based observation of pollution changing thermal emission from thin liquid clouds

Previous studies effects from anthropogenic aerosols on cloud radiative properties have emphasized their potential contributions to planetary cooling. Here we focus on an opposing effect that appears to be important to the Arctic thermal radiation balance. In the Arctic, anthropogenic aerosols transported from mid-latitudes accumulate over winter because precipitation is low. Any solar albedo effects are likely small because, even when sunlight is present, such influence is limited by low solar elevations and an already bright surface. Instead, using four years of ground-based aerosol and radiation measurements obtained near Barrow, Alaska, we find that under polluted conditions, cloud emissivity is increased by between 0.05 and 0.08. This thermal indirect effect of aerosols is calculated to correspond to an estimated Arctic surface warming under cloudy skies between 3.3 and 5.2 W m^-2, or 1 and 1.6 °C. Whether such warming is plausibly amplified by cloud or surface feedbacks remains to be determined.