Natural aerosols and their interactions with clouds remain an important uncertainty within climate models, especially at the poles. Here, we study the behavior of sea salt aerosols (SSaer) in the Arctic and Antarctic within 12 climate models from CMIP6. We investigate the driving factors that control SSaer abundances and show large differences based on the choice of the source function, and the representation of aerosol processes in the atmosphere. Close to the poles, the CMIP6 models do not match observed seasonal cycles of surface concentrations, likely due to the absence of wintertime SSaer sources such as blowing snow. Further away from the poles, simulated concentrations have the correct seasonality, but have a positive mean bias of up to one order of magnitude. SSaer optical depth is derived from the MODIS data and compared to modeled values, revealing good agreement, except for winter months. Better agreement for AOD than surface concentration may indicate a need for improving the vertical distribution, the size distribution and/or hygroscopicity of modeled polar SSaer. Source functions used in CMIP6 emit very different numbers of small SSaer, potentially exacerbating cloud-aerosol interaction uncertainties in these remote regions. For future climate scenarios SSP126 and SSP585, we show that SSaer concentrations increase at both poles at the end of the 21st century, with more than two times mid-20th century values in the Arctic. The pre-industrial climate CMIP6 experiments suggest there is a large uncertainty in the polar radiative budget due to SSaer.

The magnitude of aerosol radiative effects remains the single largest uncertainty in current estimates of anthropogenic radiative forcing. One of the key quantities needed for accurate estimates of anthropogenic radiative forcing is an accurate estimate of the radiative effects from natural aerosol. The dominant source of natural aerosols over Earth’s forested regions is biogenic volatile organic compounds (BVOC) which, following oxidation in the atmosphere, can participate in new particle formation or condense onto aerosols to form secondary organic aerosol (SOA). Consequently, BVOC emissions could introduce a regionally relevant cooling feedback in a warming climate. The main objective of this study is to provide a quantitative estimate of the regional aerosol direct radiative effect caused by the temperature-dependent biogenic emissions over the boreal forests in present day conditions and in a warmer future. The study is done using a combination of climate modeling and satellite data. The aerosol-chemistry climate model used is ECHAM-HAMMOZ, which describes the relevant atmospheric aerosol processes. The BVOC emissions are computed online using the MEGAN model, which enables the simulation of the effects of temperature changes on atmospheric aerosol load. Key remote sensing data used are the AATSR based aerosol optical depth (AOD) and land surface temperature (LST) products available from the Aerosol-CCI and GlobTemperature projects, together with ancillary data, such as column concentrations of CO and water vapour from AIRS, and NO2 column densities from OMI. Our analysis shows that there could be a small temperature dependence in AOD over the boreal forests but it cannot be reliably detected from the simulations or observations. The only subregion with a clear temperature dependence in AOD was found over western Russia. Anthropogenic emissions affect this subregion more than the other regions analyzed thus, it is likely that in addition to BVOC emissions hygroscopic sulfate aerosols affect the temperature dependence of AOD. In a warmer future the clear-sky radiative forcing caused by biogenic aerosols will increase, following the increase of BVOC emissions, but if anthropogenic emissions will decrease at the same time the total clear-sky forcing will also decrease.