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.

在工业时代，由于气溶胶与云之间的相互作用而导致的行星反照率的变化是从历史记录推断地球对增加的温室气体的气候敏感性的不确定性的主要来源。控制暖云中气溶胶-云相互作用的变量是液滴数浓度。全球气候模型表明，原始的南半球和受污染的北半球海洋之间今天的半球云滴数量浓度对比可以用作人为驱动的云滴数量浓度变化的替代。遥感估计将液滴数量浓度的这种变化限制在8 cm ^-3到24 cm ^-3之间。通过扩展，自1850年以来气溶胶-云相互作用产生的辐射强迫被限制为-1.2W⋅m ^-2到-0.6W⋅m ^-2。该约束的鲁棒性取决于以下假设：原始的南大洋小滴数浓度是工业化前浓度的合适替代物。在南方夏季，根据南大洋上的卫星数据计算出的液滴数浓度很高。在南极洲附近，它们达到北半球污染流出的典型值。发现这些浓度与几个原位数据集一致。相反，气候模型显示了整个南大洋中云滴数量浓度的系统性预测不足。在南极洲附近，那里的气溶胶沉淀池很小，气候模型的低估幅度特别大。这激发了对原始环境中气溶胶生产和气溶胶-云相互作用的详细过程研究的需求。