The hemispheric contrast in cloud microphysical properties constrains aerosol forcing
The hemispheric contrast in cloud microphysical properties constrains aerosol forcing
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DOI：
10.1073/pnas.1922502117
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发表时间：
2020-07
影响因子：
11.1
通讯作者：
I. McCoy;D. McCoy;R. Wood;L. Regayre;D. Watson‐Parris;D. Grosvenor;J. Mulcahy;Yongxiang Hu;F. Bender;P. Field;K. Carslaw;H. Gordon
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作者：
I. McCoy;D. McCoy;R. Wood;L. Regayre;D. Watson‐Parris;D. Grosvenor;J. Mulcahy;Yongxiang Hu;F. Bender;P. Field;K. Carslaw;H. Gordon
能够使云滴成核的气溶胶的增强增加了云滴的数量、浓度和反照率。这增加了反射到太空的太阳光的量。在工业时期气溶胶−云相互作用如何通过这一机制增加全球反照率方面的不确定性导致气候预测的重大不确定性。我们的工作提出了一种从观测上限制人为气溶胶排放引起的反照率变化的方法：原始的南大洋(工业化前的替代品)和污染的北半球之间遥感云滴数量的半球差异。将这一限制应用到气候模型中，减少了工业化以来估计的反照率变化范围，并表明目前的模型低估了前工业化时代的云滴数量浓度。工业时代气溶胶−云相互作用引起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−云相互作用的变量是液滴数浓度。全球气候模式表明，原始的南半球和被污染的北半球海洋之间今天的云滴数量浓度的半球对比可以作为人类驱动的云滴数量浓度变化的替代。遥感估计将这种液滴数浓度的变化限制在8 cm−3到24 cm−3之间。推而广之，自1850年以来来自气溶胶−云相互作用的辐射强迫被约束在−1.2W⋅m−2到−0.6W⋅m−2之间。这一约束的稳健性取决于这样一个假设，即原始的南大洋液滴数浓度是工业化前浓度的合适替代品。根据卫星数据计算的南大洋上空的液滴数密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外流的典型数值。这些浓度被发现与几个现场数据集一致。相比之下，气候模型显示了对南大洋上云滴数量密度的系统性预测不足。在南极洲附近，气溶胶的降雨量很小，气候模型低估的程度尤其大。这促使需要对原始环境中的气溶胶产生和气溶胶−云相互作用进行详细的过程研究。卫星估计的云滴数浓度的半球差异表明，工业化前的气溶胶浓度比大多数模式估计的要高。
Significance Enhancement of aerosol that can nucleate cloud droplets increases the droplet number concentration and albedo of clouds. This increases the amount of sunlight reflected to space. Uncertainty in how aerosol−cloud interactions over the industrial period have increased planetary albedo by this mechanism leads to significant uncertainty in climate projections. Our work presents a method for observationally constraining the change in albedo due to anthropogenic aerosol emissions: a hemispheric difference in remotely sensed cloud droplet number between the pristine Southern Ocean (a preindustrial proxy) and the polluted Northern Hemisphere. Application of this constraint to climate models reduces the range of estimated albedo change since industrialization and suggests current models underpredict cloud droplet number concentration in the preindustrial era. 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.