Reflections on cloud effects How much impact does the abundance of cloud condensation nuclei (CCN) aerosols above the oceans have on global temperatures? Rosenfeld et al. analyzed how CCN affect the properties of marine stratocumulus clouds, which reflect much of the solar radiation received by Earth back to space (see the Perspective by Sato and Suzuki). The CCN abundance explained most of the variability in the radiative cooling. Thus, the magnitude of radiative forcing provided by these clouds is much more sensitive to the presence of CCN than current models indicate, which suggests the existence of other compensating warming effects. Science, this issue p. eaav0566; see also p. 580 Marine stratocumulus clouds are more sensitive to cloud condensation nuclei than was thought. INTRODUCTION Human-made emissions of particulate air pollution can offset part of the warming induced by emissions of greenhouse gases, by enhancing low-level clouds that reflect more solar radiation back to space. The aerosol particles have this effect because cloud droplets must condense on preexisting tiny particles in the same way as dew forms on cold objects; more aerosol particles from human-made emissions lead to larger numbers of smaller cloud droplets. One major pathway for low-level cloud enhancement is through the suppression of rain by reducing cloud droplet sizes. This leaves more water in the cloud for a longer time, thus increasing the cloud cover and water content and thereby reflecting more solar heat to space. This effect is strongest over the oceans, where moisture for sustaining low-level clouds over vast areas is abundant. Predicting global warming requires a quantitative understanding of how cloud cover and water content are affected by human-made aerosols. RATIONALE Quantifying the aerosol cloud–mediated radiative effects has been a major challenge and has driven the uncertainty in climate predictions. It has been difficult to measure cloud-active aerosols from satellites and to isolate their effects on clouds from meteorological data. The development of novel methodologies to retrieve cloud droplet concentrations and vertical winds from satellites represents a breakthrough that made this quantification possible. The methodologies were applied to the world’s oceans between the equator and 40°S. Aerosol and meteorological variables explained 95% of the variability in the cloud radiative effects. RESULTS The measured aerosol cloud–mediated cooling effect was much larger than the present estimates, especially via the effect of aerosols on the suppression of precipitation, which makes the clouds retain more water, persist longer, and have a larger fractional coverage. This goes against most previous observations and simulations, which reported that vertically integrated cloud water may even decrease with additional aerosols, especially in precipitating clouds. The major reason for this apparent discrepancy is because deeper clouds have more water and produce rainfall more easily, thus scavenging the aerosols more efficiently. The outcome is that clouds with fewer aerosols have more water, but it has nothing to do with aerosol effects on clouds. This fallacy is overcome when assessing the effects for clouds with a given fixed geometrical thickness. The large aerosol sensitivity of the water content and coverage of shallow marine clouds dispels another belief that the effects of added aerosols are mostly buffered by adjustment of the cloud properties, which counteracts the initial aerosol effect. For example, adding aerosols suppresses rain, so the clouds respond by deepening just enough to restore the rain amount that was suppressed. But the time scale required for the completion of this adjustment process is substantially longer than the life cycle of the cloud systems, which is mostly under 12 hours. Therefore, most of the marine shallow clouds are not buffered for the aerosol effects, which are inducing cooling to a much greater extent than previously believed. CONCLUSION Aerosols explain three-fourths of the variability in the cooling effects of low-level marine clouds for a given geometrical thickness. Doubling the cloud droplet concentration nearly doubles the cooling. This reveals a much greater sensitivity to aerosols than previously reported, meaning too much cooling if incorporated into present climate models. This argument has been used to dismiss such large sensitivities. To avoid that, the aerosol effects in some of the models were tuned down. Accepting the large sensitivity revealed in this study implies that aerosols have another large positive forcing, possibly through the deep clouds, which is not accounted for in current models. This reveals additional uncertainty that must be accounted for and requires a major revision in calculating Earth’s energy budget and climate predictions. Paradoxically, this advancement in our knowledge increases the uncertainty in aerosol cloud–mediated radiative forcing. But it paves the way to eventual substantial reduction of this uncertainty. Coverage and droplet concentrations (Nd) of shallow marine clouds over the northeast Pacific. Smoke particles emitted from ship smokestacks form cloud droplets and elevate Nd. The smoke-free clouds (Nd < ~30 cm−3) precipitate and break up. The fraction of cloud cover increases with more Nd that suppresses precipitation. The solid cloud cover is maintained by smoke that was spread from old ship tracks, crossed by newer ones. A lack of reliable estimates of cloud condensation nuclei (CCN) aerosols over oceans has severely limited our ability to quantify their effects on cloud properties and extent of cooling by reflecting solar radiation—a key uncertainty in anthropogenic climate forcing. We introduce a methodology for ascribing cloud properties to CCN and isolating the aerosol effects from meteorological effects. Its application showed that for a given meteorology, CCN explains three-fourths of the variability in the radiative cooling effect of clouds, mainly through affecting shallow cloud cover and water path. This reveals a much greater sensitivity of cloud radiative forcing to CCN than previously reported, which means too much cooling if incorporated into present climate models. This suggests the existence of compensating aerosol warming effects yet to be discovered, possibly through deep clouds.

中文翻译：

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气溶胶驱动的液滴浓度主导了海洋低层云的覆盖范围和水

对云效应的思考 海洋上方大量的云凝结核 (CCN) 气溶胶对全球温度有多大影响？罗森菲尔德等人。分析了 CCN 如何影响海洋层积云的特性，海洋层积云将地球接收到的大部分太阳辐射反射回太空（见佐藤和铃木的观点）。CCN 丰度解释了辐射冷却的大部分变化。因此，这些云提供的辐射强迫的大小对 CCN 的存在比当前模型显示的要敏感得多，这表明存在其他补偿性变暖效应。科学，这个问题 p。eaav0566; 另见第。580 海洋层积云对云凝结核的敏感性比想象的要高。引言 人为排放的颗粒物空气污染可以通过增强将更多太阳辐射反射回太空的低层云层来抵消部分由温室气体排放引起的变暖。气溶胶粒子具有这种效果，因为云滴必须以与冷物体上的露水相同的方式凝结在预先存在的微小粒子上；更多来自人为排放的气溶胶颗粒会导致更多更小的云滴。低层云增强的一个主要途径是通过减少云滴大小来抑制降雨。这会在更长的时间内在云中留下更多的水，从而增加云覆盖和水含量，从而将更多的太阳热量反射到太空。这种影响在海洋上最为强烈，因为那里有充足的水分来维持广大地区的低层云。预测全球变暖需要对云量和水含量如何受到人造气溶胶的影响进行定量了解。基本原理 量化气溶胶云介导的辐射效应一直是一项重大挑战，并导致气候预测的不确定性。很难从卫星测量云活性气溶胶并将其对云的影响从气象数据中分离出来。从卫星获取云滴浓度和垂直风的新方法的开发代表了一项突破，使这种量化成为可能。这些方法被应用于赤道和 40°S 之间的世界海洋。气溶胶和气象变量解释了云辐射效应中 95% 的可变性。结果实测的气溶胶云介导的冷却效应比目前的估计大得多，特别是通过气溶胶抑制降水的影响，这使得云保留更多的水分，持续时间更长，并具有更大的覆盖率。这与大多数先前的观察和模拟背道而驰，这些观察和模拟报告称，垂直整合的云水甚至可能随着额外的气溶胶而减少，尤其是在降水云中。这种明显差异的主要原因是更深的云层有更多的水，更容易产生降雨，从而更有效地清除气溶胶。结果是气溶胶少的云有更多的水，但这与气溶胶对云的影响无关。在评估具有给定的固定几何厚度的云的影响时，可以克服这种谬误。水含量和浅海云覆盖的较大气溶胶敏感性消除了另一种信念，即添加气溶胶的影响主要通过调整云特性来缓冲，这抵消了初始气溶胶效应。例如，添加气溶胶会抑制降雨，因此云会通过加深足以恢复被抑制的降雨量来做出反应。但完成这一调整过程所需的时间尺度远大于云系统的生命周期，大多在12小时以下。因此，大多数海洋浅云并没有缓冲气溶胶效应，导致冷却的程度比以前认为的要大得多。结论 对于给定的几何厚度，气溶胶解释了低层海洋云冷却效果变化的四分之三。将云滴浓度加倍，冷却效果几乎加倍。这表明对气溶胶的敏感性比以前报告的要大得多，这意味着如果将其纳入当前的气候模型，则意味着冷却过多。这个论点已被用来消除如此大的敏感性。为了避免这种情况，一些模型中的气溶胶效应被调低。接受本研究中揭示的大敏感性意味着气溶胶具有另一个大的正强迫，可能是通过深云，这在当前模型中没有考虑在内。这揭示了必须考虑的额外不确定性，并且需要在计算地球能量收支和气候预测时进行重大修订。矛盾的是，我们知识的这种进步增加了气溶胶云介导的辐射强迫的不确定性。但它为最终大幅减少这种不确定性铺平了道路。东北太平洋上空浅海云的覆盖率和液滴浓度 (Nd)。从船舶烟囱排放的烟雾颗粒形成云滴并提升 Nd。无烟云 (Nd < ~30 cm−3) 沉淀并破裂。云量的比例随着抑制降水的 Nd 增加而增加。坚固的云层是由旧船轨道散布的烟雾维持的，新船轨道穿过。缺乏对海洋上空云凝结核 (CCN) 气溶胶的可靠估计，严重限制了我们通过反射太阳辐射来量化其对云特性和冷却程度影响的能力——这是人为气候强迫的一个关键不确定性。我们介绍了一种将云特性归因于 CCN 并将气溶胶效应与气象效应隔离的方法。它的应用表明，对于给定的气象，CCN 解释了云辐射冷却效应变化的四分之三，主要是通过影响浅云覆盖和水路。这揭示了云辐射强迫对 CCN 的敏感性比以前报告的要大得多，这意味着如果将其纳入当前的气候模型，则意味着冷却过多。