Black carbon in snow (BCS) has a significant impact on global climate and is an important component of Earth system modeling. Here, we provide a comprehensive evaluation of BCS simulations in the Coupled Model Intercomparison Project Phase 6 (CMIP6) and its radiative forcing on a global scale. Overall, the multi-model mean generally captures the characteristics of BCS spatial patterns, with maximum concentrations in East Asia and the Tibetan Plateau (~120 ng·g⁻¹), and the lowest in Antarctica (~0.05 ng·g⁻¹). The BCS concentrations in all CMIP6 multi-model mean and individual models generally exhibit a temporally increasing trend globally, with particularly large increases after the 1940s. In terms of seasonal cycles, individual models are generally consistent in most regions. Globally, BCS concentrations are highest around January and lowest in September. The albedo reduction in the Tibetan Plateau and East Asia simulated by the CMIP6 multi-model mean reached ~0.06 in 2014 and may influence climate more than expected.

Light-absorbing particles (LAPs) deposited on snow can significantly reduce surface albedo and contribute to positive radiative forcing. This study firstly estimated and attributed the spatio-temporal variability in the radiative forcing (RF) of LAPs in snow over the northern hemisphere during the snow-covered period 2003–2018 by employing Moderate Resolution Imaging Spectroradiometer (MODIS) data, coupled with snow and atmospheric radiative transfer modelling. In general, the RF for the northern hemisphere shows a large spatial variability over the whole snow-covered areas and periods, with the highest value (12.7 W m⁻²) in northeastern China (NEC) and the lowest (1.9 W m⁻²) in Greenland (GRL). The concentration of LAPs in snow is the dominant contributor to spatial variability in RF in spring (~73%) while the joint spatial contributions of snow water equivalent (SWE) and solar irradiance (SI) are the most important (>50%) in winter. The average northern hemisphere RF gradually increases from 2.1 W m⁻² in December to 4.1 W m⁻² in May and the high-value area shifts gradually northwards from mid-altitude to high-latitude over the same period, which is primarily due to the seasonal variability of SI (~58%). More interestingly, our data reveal a significant decrease in RF over high-latitude Eurasia (HEUA) of −0.04 W m⁻² a⁻¹ and northeastern China (NEC) of −0.14 W m⁻² a⁻¹ from 2003 to 2018. By employing a sensitivity test, we find the concurrent decline in the concentration of LAPs in snow accounted for the primary responsibility for the decrease in RF over these two areas, which is further confirmed by in situ observations.

The tropopause aerosol layer (TAL) represents the increase of aerosols in tropopause. It was first discovered over Asia but was found in this study to also occur over South America and Africa owing to the combined effects of monsoon dynamics and pollutant emissions. Over Asia, the TAL has the highest altitude and widest spread due to strong deep convection and the upper troposphere anticyclonic system there. TAL intensity is highest in South America due to heavy pollutant emissions. Anthropogenic pollution from India and western China produces two Asian TAL centers, whereas widespread wildfires result in single centers over South America and Africa. TAL radiative forcing induced by carbonaceous aerosols at the top of the atmosphere has warming effects over Asia (+0.21 W m−2), whereas cooling effects occur over South America (−0.47 W m−2) and Africa (−0.12 W m−2) owing to the divergent strengths of black-carbon absorption and organic-carbon scattering.