The change in planetary albedo due to aerosol−cloud interactions during the industrial era is the leading source of uncertainty in inferring Earth’s climate sensitivity to increased greenhouse gases from the historical record. The variable that controls aerosol−cloud interactions in warm clouds is droplet number concentration. Global climate models demonstrate that the present-day hemispheric contrast in cloud droplet number concentration between the pristine Southern Hemisphere and the polluted Northern Hemisphere oceans can be used as a proxy for anthropogenically driven change in cloud droplet number concentration. Remotely sensed estimates constrain this change in droplet number concentration to be between 8 cm⁻³ and 24 cm⁻³. By extension, the radiative forcing since 1850 from aerosol−cloud interactions is constrained to be −1.2 W⋅m⁻² to −0.6 W⋅m⁻². The robustness of this constraint depends upon the assumption that pristine Southern Ocean droplet number concentration is a suitable proxy for preindustrial concentrations. Droplet number concentrations calculated from satellite data over the Southern Ocean are high in austral summer. Near Antarctica, they reach values typical of Northern Hemisphere polluted outflows. These concentrations are found to agree with several in situ datasets. In contrast, climate models show systematic underpredictions of cloud droplet number concentration across the Southern Ocean. Near Antarctica, where precipitation sinks of aerosol are small, the underestimation by climate models is particularly large. This motivates the need for detailed process studies of aerosol production and aerosol−cloud interactions in pristine environments. The hemispheric difference in satellite estimated cloud droplet number concentration implies preindustrial aerosol concentrations were higher than estimated by most models.

工业时代气溶胶与云相互作用导致的行星反照率变化是从历史记录推断地球气候对温室气体增加的敏感性的主要不确定性来源。控制暖云中气溶胶与云相互作用的变量是液滴数量浓度。全球气候模型表明，当今半球原始南半球和受污染的北半球海洋之间云滴数量浓度的差异可以用作人为驱动的云滴数量浓度变化的代理。遥感估计将液滴数浓度的变化限制在8 cm ^-3和24 cm ^-3之间。推而广之，自 1850 年起气溶胶与云相互作用产生的辐射强迫被限制为 −1.2 W⋅m ⁻²到 −0.6 W⋅m ⁻² 。这一约束的稳健性取决于这样的假设：原始南大洋液滴数量浓度是工业化前浓度的合适代表。根据卫星数据计算出的南大洋上空的液滴数浓度在南半球夏季很高。在南极洲附近，它们达到了北半球污染流出量的典型值。发现这些浓度与几个现场数据集一致。相比之下，气候模型显示南大洋云滴数量浓度的系统性低估。在南极洲附近，气溶胶降水汇很小，气候模型的低估特别大。这就激发了对原始环境中气溶胶产生和气溶胶与云相互作用的详细过程研究的需求。 卫星估计的云滴数量浓度的半球差异意味着工业化前的气溶胶浓度高于大多数模型估计的浓度。