Aerosol-driven droplet concentrations dominate coverage and water of oceanic low-level clouds

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.