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Over the past decade, the number of studies that investigate aerosol–cloud interactions has increased considerably. Although tremendous progress has been made to improve the understanding of basic physical mechanisms of aerosol–cloud interactions and reduce their uncertainties in climate forcing, there is still poor understanding of 1) some of the mechanisms that interact with each other over multiple spatial and temporal scales, 2) the feedbacks between microphysical and dynamical processes and between local-scale processes and large-scale circulations, and 3) the significance of cloud–aerosol interactions on weather systems as well as regional and global climate. This review focuses on recent theoretical studies and important mechanisms on aerosol–cloud interactions and discusses the significances of aerosol impacts on radiative forcing and precipitation extremes associated with different cloud systems. The authors summarize the main obstacles preventing the science from making a leap—for example, the lack of concurrent profile measurements of cloud dynamics, microphysics, and aerosols over a wide region on the observation side and the large variability of cloud microphysics parameterizations resulting in a large spread of modeling results on the modeling side. Therefore, large efforts are needed to escalate understanding. Future directions should focus on obtaining concurrent measurements of aerosol properties and cloud microphysical and dynamic properties over a range of temporal and spatial scales collected over typical climate regimes and closure studies, as well as improving understanding and parameterizations of cloud microphysics such as ice nucleation, mixed-phase properties, and hydrometeor size and fall speed.

Many studies have demonstrated psychological differences between ethically motivated and health-motivated vegetarians. Adopting an ethical-health dichotomy in studying dietary motivation, however, may overlook meaningful variance between vegetarians motivated by different types of ethical concerns—namely, those related to animals and the environment. Through two preregistered studies, I compared dietary goal orientations, disgust toward meat, and dietary adherence between vegetarians motivated by animals, health, and the environment. In Study 1 (N = 361), I found that environmentally motivated vegetarians resembled animal-motivated vegetarians in construing their diets as a means of achieving more prosocial and moral, but less personal, goals than did health-motivated vegetarians. In Study 2 (N = 562), however, I found that environmentally motivated vegetarians resembled health-motivated vegetarians in reporting less disgust toward meat and lower dietary adherence than did animal-motivated vegetarians. Stronger feelings of disgust toward meat explained why animal-motivated vegetarians exhibited the most stringent dietary adherence. Distinguishing between various types of ethical motivation may reveal more nuanced insights into eating behavior.

PurposeThis paper aims to present an overview of the definitions of “plant-based and “vegetarian diets” adopted by different organizations worldwide, proposing new standard definitions and discussing the notion of vegetarianism as a restrictive dietary pattern.MethodsAn extensive literature review on the different definitions of vegetarian and plant-based diets was conducted. Definitions of different international vegetarian and vegan organizations were also taken into account. Objective definitions for vegetarian and plant-based diets, as well as for their subcategories, were proposed. Other aspects related to how vegetarian diets are viewed and defined were also discussed.ResultsWe proposed that a vegetarian diet should be defined as “a dietary pattern that excludes meat, meat-derived foods, and, to different extents, other animal products”. This definition would include, among others, ovolactovegetarian and vegan diets. The proposed definition for a plant-based diet was “a dietary pattern in which foods of animal origin are totally or mostly excluded”. Other types of diets, such as flexitarian and pescetarian diets, could be considered plant-based. A vegetarian diet should not be considered restrictive. Instead, terms such as alternative or non-conventional could be used to define it and to distinguish it from the conventional diet adopted by most of the Western population.ConclusionThis paper was able to elaborate objective definitions of vegetarian and plant-based diets. Standardizing nomenclatures may reduce misinterpretation and confusion in this field of study.

Advances in satellite observations and model development are needed to disentangle the complex interactions of aerosols and clouds and their effects on climate. Aerosols counteract part of the warming effects of greenhouse gases, mostly by increasing the amount of sunlight reflected back to space. However, the ways in which aerosols affect climate through their interaction with clouds are complex and incompletely captured by climate models. As a result, the radiative forcing (that is, the perturbation to Earth's energy budget) caused by human activities is highly uncertain, making it difficult to predict the extent of global warming ( 1 , 2 ). Recent advances have led to a more detailed understanding of aerosol-cloud interactions and their effects on climate, but further progress is hampered by limited observational capabilities and coarse-resolution climate models.

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. 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.

Cloud adiabatic fraction (fad) is crucial for understanding cloud microphysics and entrainment mixing. This study presents the first global satellite‐based assessment of land‐ocean contrasts in fad for low‐level clouds. Results show that land clouds have a lower fad (0.22) than oceanic clouds (0.35), consistent across all seasons. This contrast arises from the combined effects of cloud macrophysical and microphysical differences. Continental clouds tend to be thicker, enhancing evaporative dilution due to a longer vertical development path. Moreover, under similar cloud thickness and temperature conditions, continental clouds exhibit smaller droplet sizes, which evaporate more efficiently, especially in shallow clouds. A factor‐substitution analysis quantifies this interplay: cloud thickness provides the primary macrophysical control, droplet size delivers microphysical amplification, and colder land cloud bases provide a partial thermodynamic offset. This process‐oriented understanding advances the mechanistic interpretation of cloud–environment coupling and constrains cloud parameterizations in climate models.

Direct evidence demonstrates that urban and industrial air pollution can completely shut off precipitation from clouds that have temperatures at their tops of about -10°C over large areas. Satellite data reveal plumes of reduced cloud particle size and suppressed precipitation originating from major urban areas and from industrial facilities such as power plants. Measurements obtained by the Tropical Rainfall Measuring Mission satellite reveal that both cloud droplet coalescence and ice precipitation formation are inhibited in polluted clouds.

Aerosols alter cloud density and the radiative balance of the atmosphere. This leads to changes in cloud microphysics and atmospheric stability, which can either suppress or foster the development of clouds and precipitation. The net effect is largely unknown, but depends on meteorological conditions and aerosol properties. Here, we examine the long-term impact of aerosols on the vertical development of clouds and rainfall frequencies, using a 10-year dataset of aerosol, cloud and meteorological variables collected in the Southern Great Plains in the United States. We show that cloud-top height and thickness increase with aerosol concentration measured near the ground in mixed-phase clouds—which contain both liquid water and ice—that have a warm, low base. We attribute the effect, which is most significant in summer, to an aerosol-induced invigoration of upward winds. In contrast, we find no change in cloud-top height and precipitation with aerosol concentration in clouds with no ice or cool bases. We further show that precipitation frequency and rain rate are altered by aerosols. Rain increases with aerosol concentration in deep clouds that have a high liquid-water content, but declines in clouds that have a low liquid-water content. Simulations using a cloud-resolving model confirm these observations. Our findings provide unprecedented insights of the long-term net impacts of aerosols on clouds and precipitation. Atmospheric aerosols can both suppress and foster the development of clouds and precipitation, depending on meteorological conditions. Ten years of observations, together with model simulations, suggest that aerosols stimulate the vertical development of warm-base mixed-phase clouds.

Deep convective clouds (DCCs) play a key role in atmospheric circulation and the hydrological and energy cycle. How aerosol particles affect DCCs is poorly understood, making it difficult to understand current and future weather and climate. Our work showed that in addition to the invigoration of convection, which has been unanimously cited for explaining the observed results, the microphysical effects induced by aerosols are a fundamental reason for the observed increases in cloud fraction, cloud top height, and cloud thickness in the polluted environment, even when invigoration is absent. The finding calls for an augmented focus on understanding the changes in stratiform/anvils associated with convective life cycle. Deep convective clouds (DCCs) play a crucial role in the general circulation, energy, and hydrological cycle of our climate system. Aerosol particles can influence DCCs by altering cloud properties, precipitation regimes, and radiation balance. Previous studies reported both invigoration and suppression of DCCs by aerosols, but few were concerned with the whole life cycle of DCC. By conducting multiple monthlong cloud-resolving simulations with spectral-bin cloud microphysics that capture the observed macrophysical and microphysical properties of summer convective clouds and precipitation in the tropics and midlatitudes, this study provides a comprehensive view of how aerosols affect cloud cover, cloud top height, and radiative forcing. We found that although the widely accepted theory of DCC invigoration due to aerosol’s thermodynamic effect (additional latent heat release from freezing of greater amount of cloud water) may work during the growing stage, it is microphysical effect influenced by aerosols that drives the dramatic increase in cloud cover, cloud top height, and cloud thickness at the mature and dissipation stages by inducing larger amounts of smaller but longer-lasting ice particles in the stratiform/anvils of DCCs, even when thermodynamic invigoration of convection is absent. The thermodynamic invigoration effect contributes up to ∼27% of total increase in cloud cover. The overall aerosol indirect effect is an atmospheric radiative warming (3–5 W⋅m −2 ) and a surface cooling (−5 to −8 W⋅m −2 ). The modeling findings are confirmed by the analyses of ample measurements made at three sites of distinctly different environments.

Aerosols can invigorate deep convective clouds (DCCs) directly by nucleating more cloud droplets, named as Primary Aerosol Convective Invigoration (PAI). However, the covarying Meteorology‐Aerosol Invigoration (MAI) effect on DCC has been a long‐standing issue in quantifying PAI's contribution. Here, observations show that PAI causes positive feedback from DCC to meteorology, further invigorating DCC through enhanced humidity, updraft and destabilization, thereby adding to MAI. Further, PAI is separated from MAI observationally by quantifying the sensitivity of DCC properties to aerosol changes under fixed meteorology through the artificial neural network. When fine aerosol changes from the cleanest to optimal concentration (5 μg m−3), PAI contributes 72% ± 2% of the total aerosol‐associated cloud top cooling by 12°C, 42% ± 4% of the 30% prolonged lifetime, and 50% ± 4% of the more than doubled rainfall. This result underlines the comparable magnitudes of PAI and MAI, which have not been considered until now in weather and climate prediction. Plain Language Summary Aerosol has been observed and simulated to be directly associated with the development of strong convective storms, termed as Primary Aerosol Convective Invigoration (PAI). However, the current consensus is that meteorology also affects aerosol distribution, that is, there is covariability between meteorology and aerosols, possibly producing an overestimated observed association between aerosols and storms. Disentangling the PAI effect from the covarying meteorology‐aerosol effect on the storms has been a long‐standing challenge. Here, based on an artificial neural network technology, we quantify the PAI effect which explains ∼2/3 of the observed total aerosol‐driven variation in cloud top height, and 40%–50% of lifetime and rainfall when aerosol changes from the cleanest state to ∼5 μg m−3 concentration. Furthermore, we provide the first‐of‐its‐kind observational support for the Meteorology‐Feedback Aerosol Convective Invigoration (FAI). PAI modifies the environment and feeds back to the meteorology, which would become more conducive to storm development. This study demonstrates that fine aerosols have a large impact on the vigor of tropical deep convective clouds, which in turn adjusts environmental properties and meteorology. This is a manifestation of the way by which aerosol modulates tropical convection and, through that, further influences atmospheric circulation. Key Points Total aerosol‐driven convective invigoration can be separated into primary and meteorology‐covarying aerosol effects Primary aerosol invigoration is observed to alter the environment to be more conducive for convection in a positive feedback loop Primary invigoration explains ∼2/3 of the observed total aerosol‐driven variation in cloud top height and 40%–50% of lifetime and rainfall