Roles of Aerosols in Modulating Arctic Warming since the 1980s

Aerosols can affect the Arctic climate through their interactions with radiation, clouds, snow cover and sea ice as well as associated feedbacks. Studies have shown that the majority of anthropogenic aerosol in the Arctic comes from mid-latitude and subarctic sources. In the past few decades, global and regional anthropogenic emissions have changed rapidly. Arctic lower-tropospheric sulfate and black carbon (BC) aerosols had a significant decrease since the 1980s due to reduced emissions from Europe, Russia and Arctic local sources. Increases in emissions from South Asia and East Asia led to positive trends in Arctic sulfate and BC in the upper troposphere. Our climate modeling results show that global changes in sulfate and BC aerosols during 2014-2018, compared to the early 1980s, contribute to 20% (0.3 K) of the observed Arctic surface warming by reducing radiative cooling and increasing poleward heat transport. Within the Arctic, sulfate reductions caused a top-of-the-atmosphere warming through aerosol–radiation and aerosol–cloud interactions. While the changes in Arctic atmospheric BC had little impact on local radiative forcing, decreases in BC deposition onto snow and ice surface led to a net cooling. Our results suggest that changes in aerosols over the mid-latitudes have a larger impact on Arctic temperature than other regions through enhanced poleward heat transport. More recently, the worldwide dramatic responses to the COVID-19 pandemic have led to a large reduction in human activities and thus emissions of greenhouse gases and air pollutants. Our climate model simulations have shown discernible impacts of COVID-19 lockdowns and restrictions on the Arctic climate. Aerosol emission reductions since early 2020 in AGCM simulations (with prescribed sea surface temperatures and sea ice concentrations) are shown to warm the Arctic lower troposphere and decrease low-level clouds, leading to a cooler Arctic surface than normal in boreal winter. However, the Arctic surface temperature has a more complicated response to the short-term (2020-2022) aerosol emission reductions followed by a full recovery in mid-latitudes when responses and feedbacks from ocean and sea ice components are included in the fully coupled model simulations.