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Optical remote sensing observations have suggested that the top layer of Europa's icy surface is heavily affected by external weathering agents. To model and understand these effects, it is necessary to characterize the environment as fully as possible. In this paper, we focus on one agent in the environment (energetic electrons). We show Juno electron data from its 2022 Europa flyby and other time periods. While the Juno sensor used here (Jupiter Energetic Particle Detector Instrument) was not designed to obtain high quality electron data in an intense radiation environment, it is possible to extract information such as how Europa blocks energetic particles from accessing some of the surrounding space. The decrease in charged particle flux in Europa's wake provides an upper limit on the precipitation fluxes of the same particles. We also report that electron pitch angle distributions near Europa for the single energy channel considered here are time variable and not isotropic. Plain Language Summary: We present energetic electron data obtained when the Juno spacecraft crossed the plasma wake of the moon Europa in 2022. A major goal of this work is to estimate the amount of blockage of MeV electrons near the moon to inform radiation estimates for future Europa orbiters and landers. While the Jupiter Energetic Particle Detector Instrument does not directly detect particles between 1 and 20 MeV, using knowledge gained since the instrument began obtaining data we were able to estimate that about 80% of the particles in question are blocked by Europa in its wake at low altitude. Research on the weathering of Europa's surface by electrons is also discussed. Key Points: Jupiter Energetic Particle Detector Instrument electron data relevant to Europa are presentedWe estimate Europa's blockage causes an ∼80% reduction of ∼1–20 MeV electrons in the immediate plasma wakePitch angle distributions are computed from 70 keV electron data and show time variability [ABSTRACT FROM AUTHOR]

The estimation of cloud radiative forcing due to aerosol‐cloud interactions, RFaci (also known as the first indirect effect), relies on approximating the cloud albedo susceptibility to changes in droplet concentration, β. β depends on the cloud albedo and droplet concentration, both of which can be observed by satellites. Satellite observations are often spatially aggregated to coarser resolutions, typically 1 × 1° scenes. However, on such spatial scales, the cloud albedo tends to be heterogeneous, whereas the β approximation assumes homogeneity. Here, we demonstrate that the common practice of aggregating satellite data and neglecting cloud albedo heterogeneity results in an average overestimation of 10% in previous estimates of the RFaci. Additionally, we establish a relationship between the magnitude of the bias in β and Stratocumulus morphologies, providing a physical context for cloud heterogeneity and the associated bias. Lastly, we propose a correction method that can be applied to cloud albedo gridded data. Plain Language Summary: This paper explores the effect of cloud albedo morphology, which is a reflection of cloud heterogeneity, on radiative forcing due to aerosol‐cloud interactions (RFaci). The RFaci is estimated from satellite observations based on the assumption that clouds are homogeneous within a given scene. However, when satellite data is spatially aggregated to reduce the amount of data to a user‐friendly gridded format—a common practice—this assumption is no longer valid. Consequently, an overestimation of the RFaci occurs, particularly in heterogeneous scenes, where the overestimation can reach up to 50%. This means that the RFaci is lower than previously estimated. Our results also suggest that cloud albedo enhancement due to an increase in droplet concentrations would be most effective in homogeneous scenes. Therefore, marine cloud brightening strategies should take cloud albedo homogeneity into account to achieve the most effective albedo enhancement. Key Points: The common practice of spatial aggregation of satellite data into 1 × 1° scenes leads to an average 10% overestimation of the RFaciThe overestimation is due to neglecting cloud albedo heterogeneity, and is associated to different types of Stratocumulus morphologiesA correction is proposed, which calls for the incorporation of cloud reflectance statistics in Level 3 data [ABSTRACT FROM AUTHOR]

The relative importance of radiative feedbacks and emissions scenarios in controlling surface warming patterns is challenging to quantify across model generations. We analyze three variants of the Community Earth System Model (CESM) with differing equilibrium climate sensitivities under identical CMIP5 historical and high‐emissions scenarios. CESM1, our base model, exhibits Arctic‐amplified warming with the least warming in the Southern Hemisphere middle latitudes. A variant of CESM1 with enhanced extratropical shortwave cloud feedbacks shows slightly increased late‐21st century warming at all latitudes. In the next‐generation model, CESM2, global‐mean warming is also slightly greater, but the warming is zonally redistributed in a pattern mirroring cloud and surface albedo feedbacks. However, if the nominally equivalent CMIP6 scenario is applied to CESM2, the redistributed warming pattern is preserved, but global‐mean warming is significantly greater. These results demonstrate how model structural differences and scenario differences combine to produce differences in climate projections across model generations. Plain Language Summary: Earth System Models are used to project climate change through the 21st century. Such projections are constrained by a range of assumptions about emissions of greenhouse gases as well as other climate‐relevant human activities like deforestation and emissions of aerosols. These assumptions are carefully evaluated and represented in an emissions scenario. In a coordinated set of experiments, a given emissions scenario is applied to a number of different Earth System Models to test differences in climate projections that arise from physical (or "structural") differences among the models. However, over time, models are updated and improved, and emissions scenarios are updated as well. This continuous updating makes it difficult to track whether a new set of climate projections is different from its predecessor due to model changes or due to scenario changes. In this paper, we disentangle these two factors, clarifying the role of each. For the Earth System Model that we use, the emissions scenario largely controls the amount of global‐mean warming projected for the 21st century, but the model or structural differences matter greatly for the regional patterns of warming. Key Points: Higher equilibrium climate sensitivity, from more positive cloud feedbacks, only slightly increases 21st century transient warmingCloud and surface albedo feedbacks redistribute warming from the middle and high northern latitudes to lower latitudesThese warming pattern differences result from feedback differences, not CMIP5 to CMIP6 forcing scenario differences [ABSTRACT FROM AUTHOR]

Evidence from Arrokoth and comets strongly suggests a very low density for this and similar small Kuiper belt objects. Plausible compositions imply high porosities, in excess of 70%, and low compaction crush strengths. If so, impact craters on Arrokoth (especially Sky, its largest) formed largely by compaction of pore space and material displacement. This is consistent with geological evidence from New Horizons imaging. High porosity reduces cratering efficiency in the gravity regime whereas compaction moves it toward crush strength scaling and increased efficiency. Compaction also guarantees that most impactor kinetic energy is taken up as waste heat near the impact point, with momentum transferred to the rest of the body by elastic waves only. Monte Carlo simulations of Sky‐forming conditions indicate that the momentum imparted likely separated Arrokoth's two lobes, but displacement was limited by dissipation at the neck between them. Unusual strength properties are not required to preserve Arrokoth's bilobate configuration. Plain Language Summary: It has become apparent over the last few years that small asteroids and comets are very underdense compared with the materials they are made of. This means that their total porosities are likely quite high, in excess of 70%, both as tiny voids within particles (so‐called microscopic porosity) and spaces between particles (macroscopic porosity). But none are likely as porous as the distant denizens of the Kuiper belt such as Arrokoth (visited by the New Horizons spacecraft in 2019). This paper concerns impact craters on Arrokoth and similar small bodies, and the rather unusual effects expected. Imagine a fluffy (fine powder) snowball striking a much larger fluffy snowball, only that the snow is not pure ice but a mixture of porous icy, rocky, and carbon‐rich particles. Even at high velocities (>100 s of m/s) craters should mostly form by compacting pore space and pushing material away from the impact point, not the traditional blasting of ejecta back into space. Similar to crush‐up of an automobile bumper, compaction helps to protect from the potentially catastrophic effects of large impacts, such as complete disruption of the target or breakup of bilobate bodies like Arrokoth, and should be incorporated in future collisional evolution studies. Key Points: Arrokoth is likely a low density, highly porous, contact binary planetesimalImpact craters on Arrokoth, and particularly its largest, likely formed as compaction craters, with modest or little ejectaHigh porosity acted to protect Arrokoth from catastrophic disruption, ejecta recoil, and lobe dislocation and rearrangement [ABSTRACT FROM AUTHOR]

Absorbing aerosols uniquely impact radiation, aerosol transport, and meteorology. This paper quantifies black carbon core and sulphate shell size and mass using decadal measurements of multi‐spectral aerosol optical depth, single scattering albedo, and angstrom exponent from Aerosol Robotic Network stations located throughout East, Southeast, and South Asia, in connection with a MIE model. All sites are uniquely characterized into four types: urban, biomass burning, long‐range transport, and clean. Unique size and mass probability distributions of both the core and shell are calculated within each classification. Well known urban, biomass burning, and clean sites are all properly identified. Furthermore, two unique sites previously thought to not have multiple characteristics are identified, with urban and biomass burning significant in Beijing and long‐range transport significant in the otherwise clean South China Sea at Taiping Island. It is hoped that these results will allow for advances in attribution and radiative forcing studies. Plain Language Summary: Black Carbon strongly absorbs visible radiation, leading to unique impacts on atmospheric radiation, climate, the water cycle, and PM2.5. This work attributes different aerosol source characteristics, and further specifies the size distribution and concentration of aerosol black carbon cores and refractive shells. This work uses measurements of aerosol optical depth, single scatter albedo, and angstrom exponent, across multiple different wavelengths of light, in combination with statistics and a MIE model (physical model of aerosol/radiation interaction) using a Core‐Shell approximation. The results show that aerosols observed in East, Southeast, and South Asia can be uniquely classified into four types: urban, biomass burning, long‐range transport, and clean. These results are consistent in terms of aerosol size and mass at each site within each type of characterization. Furthermore, two unique sites are identified in which a second characteristic occurs some significant fraction of every year, which otherwise was not known or previously identified in the literature. These results are expected to help enhance the understanding of attribution of aerosols, as well as provide specific size and mass details of the aerosols useful to improve radiative forcing models and aerosol impacts on climate change. Key Points: Aerosols are categorized into biomass burning, urban, and long‐range types over Asia using decadal long multi‐spectral measurementsBased on multiple Aerosol Robotic Network Single Scatter Albedo measurements and a MIE model, physical characteristics of different aerosol types are deducedMost aerosols are found to be mixed, with two sites having different characteristics during different times of the year [ABSTRACT FROM AUTHOR]

Orbital Earth Radiation Budget measurement comparisons to models, are critical for climate prediction confidence. Satellite systems must reduce calibration drifts for this purpose. NASA Clouds and the Earth's Radiant Energy System (CERES) measures Earth albedo reductions that if correct, would increase solar forcing and suggest greater sunlight absorption is driving much of recent temperature increases. Such results are presented, alongside those from the Moon and Earth Radiation Budget Experiment (MERBE). MERBE uses constant lunar reflectivity for tracking and compensation of instrument telescope degradation, undetectable by CERES. MERBE finds Earth albedo constant compared to that of the Moon, because Arctic solar warming effects are balanced by solar cooling elsewhere, likely due to negative feedbacks. Contrary to NASA, this shows the Sun is not increasing warming and that CERES results are not as stable as claimed and assumed. Furthermore, MERBE can actually resolve Cloud Radiative Forcing (CRF) signals from the existing record, rather than in decades with official observations. Plain Language Summary: In contrast to NASA Clouds and the Earth's Radiant Energy System results, this paper shows that the Earth reflectivity has not changed since the year 2000, by comparing the sunlight bouncing off it to that from the Moon. Disagreeing with NASA, this provides physical evidence that global warming is not being increased by the Sun. It also immediately brings data to the standard requested by the climate community for cloud forcing/feedback signal detection. The Moon and Earth Radiation Budget Experiment data is being staged for free on the FAIR compliant website Pangaea for climate scientists. Key Points: The Sun is not causing climate change and Cloud Radiative Forcing (CRF) signals can now be detected immediatelyArctic heating is being balanced by reflectivity increases elsewhereThe calibration accuracy of data is sufficient to detect uncertain cloud feedback signals immediately, rather than in decades [ABSTRACT FROM AUTHOR]

The moon Ganymede has a strong internally generated magnetic field that separates the surface into two regions, with the polar surface magnetically connected to the Jovian environment. Consequently, the weathering of Ganymede's surface by plasma and energetic charged particles trapped in the Jovian magnetosphere is not uniform. At the same time, optical data suggest differences in the surface ice between the poles and the equator, with further refinements in the equatorial surface that have a longitudinal dependence. Here, we use Juno spacecraft data to characterize the charged particle environment along Ganymede's orbit (from about 50 eV to 1 MeV for electrons and 10 eV to 6 MeV for protons). These results put us into a better position to test the hypothesis that space weathering by electrons causes the brighter poles of Ganymede, given that electron fluxes are likely to be more clearly separated between the polar and equatorial regions. Plain Language Summary: Electron weathering of ice may be the reason Ganymede's poles are brighter. These particles are well separated by the two magnetic topologies of Ganymede, whereas ions are not as well separated. We suggest ions may be converted to energetic neutral atoms near Ganymede and reach the surface despite the magnetic barrier. We quantify the environmental inputs in this paper. Key Points: Juno orbital evolution has allowed better characterization of the region near Jupiter's magnetic equator and Ganymede's orbital distanceElectron and proton energy spectra from that region derived from Jovian Auroral Distributions Experiment (JADE) and Jupiter Energetic‐particle Detector Instrument (JEDI) data are presentedWe propose that polar versus equatorial albedo differences on Ganymede are due to changes resulting from electron precipitation [ABSTRACT FROM AUTHOR]

Tibetan Plateau (TP) snow cover is featured by sub‐seasonal changes, affecting weather and climate in surrounding and downstream areas. Previous studies emphasize the effect of background atmospheric circulation on rapid changes of TP snow cover as a whole. However, spatial discrepant changes of snow cover over the TP with complex topography and uneven snowfall remain unaddressed. Our research indicates that snow cover fraction dominates the rapid changes of surface albedo across the TP, and snow depth also significantly influences surface albedo changes through modulating snow albedo in central and eastern TP with shallow snow. However, the excessive snow amount and empirical snow cover fraction schemes introduce spatially divergent biases of surface albedo changes in simulations. Our research highlights the instant response of TP surface albedo to both snow coverage and depth in snow season, and provides a promising perspective for improving TP snow and surface albedo simulations. Plain Language Summary: Tibetan Plateau (TP) snow cover is characterized not only by obvious seasonal changes, but also by rapid sub‐seasonal changes, which has a significant impact on the weather and climate of the downstream areas by affecting the surface albedo over the TP. Previous studies have generally concluded that the background atmospheric circulation dominates rapid changes of overall TP snow cover. However, rapid changes of snow cover exhibit spatial discrepancies over the TP with complex topography and uneven snowfall. Our study finds that snow coverage and depth both contribute to rapid changes of surface albedo and energy budget over the TP, which further lead to rapid changes of snow cover. However, current numerical models tend to overestimate the snow and fail to simulate spatial different changes of snow coverage and depth, leading to spatially divergent simulation biases in surface albedo, surface energy balance and local atmospheric circulation over the TP. Therefore, there is an urgent need to improve the snow parameterization in models to enhance the intra‐seasonal simulation of snow cover and surface albedo over the TP. Key Points: Observed surface albedo over the Tibetan Plateau (TP) has significant weekly scale fluctuations during winterSnow cover fraction and snow depth jointly dominate observed rapid changes of surface albedo over the TPExcessive snow amount and empirical snow cover fraction schemes lead to spatially divergent bias of surface albedo change [ABSTRACT FROM AUTHOR]

Dark (low albedo) surface ice on the Greenland Ice Sheet enhances melting and subsequent runoff, a major mass loss contributor during the ablation season. The accumulation of both biological (e.g., glacier ice algae) and abiotic (e.g., mineral dust) light‐absorbing particulates are important darkening factors, that are potentially influenced by the duration of snow‐free, bare ice (a phenological factor), and other geo‐topographical factors such as elevation, slope, aspect and the distance from the ice margin. Here, we present the first medium‐resolution (30 m) analysis of the phenological and geo‐topographical controls on the distribution of dark ice in SE and SW Greenland from statistical analysis of data derived from a harmonized satellite albedo product and ArcticDEM. The duration of bare ice primarily controls the distribution of dark surface ice, allowing for algae growth on inland ice surfaces in particular, whereas geo‐topographical factors are only secondary controls. Plain Language Summary: Meltwater that runs off the Greenland Ice Sheet is currently the largest single glaciological contributor to global sea level rise. Recent darkening of the surface ice has contributed significantly to increased melt rates. We used medium resolution (30m) satellite data to link ice surface darkness (albedo) to the topography (elevation, slope, and aspect) of the ice sheet surface, the distance from the ice margin, and the duration of bare or snow‐free surface ice. Our analysis shows that the duration of bare ice or snow‐free conditions was the most significant factor in driving the darkening of ice. We also found that the impact of topographic factors on the ice darkening varied depending on whether abiotic or biological processes were driving the darkening. This information helps us to better explain where the ice sheet is melting fastest and to predict the loss of ice mass, particularly considering the expected increase in the area of bare, snow‐free ice in our warming climate. Key Points: The duration of snow‐free, bare ice is the primary control on the distribution of dark iceLonger durations of bare ice (median = 40 days) are prerequisites for ice to become darkSurface slope and aspect influence darkening by modulating the duration of bare ice and glacier ice algae growth [ABSTRACT FROM AUTHOR]

Abstract Optical remote sensing observations have suggested that the top layer of Europa's icy surface is heavily affected by external weathering agents. To model and understand these effects, it is necessary to characterize the environment as fully as possible. In this paper, we focus on one agent in the environment (energetic electrons). We show Juno electron data from its 2022 Europa flyby and other time periods. While the Juno sensor used here (Jupiter Energetic Particle Detector Instrument) was not designed to obtain high quality electron data in an intense radiation environment, it is possible to extract information such as how Europa blocks energetic particles from accessing some of the surrounding space. The decrease in charged particle flux in Europa's wake provides an upper limit on the precipitation fluxes of the same particles. We also report that electron pitch angle distributions near Europa for the single energy channel considered here are time variable and not isotropic.

"Land radiative management" (LRM)—intentionally increasing land surface albedo to reduce regional temperatures—has been proposed as a form of geoengineering. Its effects on local precipitation and soil moisture over long timescales are not well understood. We use idealized cloud‐permitting simulations and a conceptual model to understand the response of precipitation and soil moisture to a mesoscale albedo anomaly at equilibrium. Initially, differential heating between a high‐albedo anomaly and the lower‐albedo surrounding environment drives mesoscale circulations, increasing precipitation and soil moisture in the surrounding environment. However, over time, increasing soil moisture reduces the differential heating, eliminating the mesoscale circulations. At equilibrium, the fractional increase in simulated soil moisture is up to 1.3 times the fractional increase in co‐albedo (one minus albedo). Thus, LRM may increase precipitation and soil moisture in surrounding regions, enhancing evaporative cooling and spreading the benefits of LRM over a wider region than previously recognized. Plain Language Summary: One way to reduce temperatures in a warming world is to increase the fraction of sunlight reflected by the land surface back to space (the "albedo"), for example, by painting surfaces white. However, changing albedo does not only change temperatures; it can also impact rainfall and soil moisture. Here, we show that increasing a region's albedo leads to increased rainfall and soil moisture in surrounding regions, where the albedo has not been changed, spreading the cooling effect over a wider area than previously recognized. Key Points: Land radiative management (LRM)—increasing land surface albedo to reduce temperatures—has been proposed as a form of geoengineeringWe use idealized cloud‐permitting simulations and a conceptual model to understand the hydrologic response to LRMLRM increases rainfall and soil moisture in surrounding regions, potentially extending its cooling benefits [ABSTRACT FROM AUTHOR]

The role of mesoscale cellular convection (MCC) in regulating aerosol‐induced cloud brightness remains unaddressed. Using 7 years of satellite‐based observations of cloud water adjustment to aerosol‐induced perturbations for closed MCCs across different sizes (8, 16, 32, and 64 km) over the North Atlantic Ocean, we show that MCC cell‐size plays a nontrivial role in regulating aerosol‐induced cloud brightness via cloud water adjustment. In cells that are primarily non‐precipitating, the adjustment in small‐scale MCCs can be 10 times more negative than in large‐scale MCCs, consistent with stronger evaporation via cloud top entrainment. Consequently, the response of cloud brightness is significantly stronger for large‐scale MCCs. We also find notable intra‐cell co‐variability between cloud liquid water path (LWP) and drop concentration (Nd) within MCCs that varies with cell size. Erroneously considering such co‐variability as a LWP response to Nd can lead to a significant positive bias, especially for small scale MCCs. Plain Language Summary: Low clouds over the ocean often exhibit organized patterns with characteristic cell sizes but the effect of this organization on cloud‐aerosol interactions is not yet fully understood. Here we group 7 years of satellite‐measured low cloud cells over the North Atlantic Ocean by their cell size to investigate how cell size influences the response of cloud water and cloud brightness to cloud droplet number concentration perturbations. Large‐scale cells are found to have less efficient depletion of cloud water in response to increasing drop number compared to small‐scale cells. This leads to nearly an order of magnitude stronger increase in cloud brightness with increasing drop number. Furthermore, we show that the spatial distributions of liquid water path and drop concentration vary with cell‐size. Mistakenly assuming the local correlation between cloud water path and drop concentration to be a causal response can cause a significant positive error, especially for small‐scale cells. Key Points: Mesoscale cellular convection (MCC) cell‐size significantly regulates aerosol‐induced cloud albedo via its effect on cloud water adjustmentThe aerosol‐induced cloud water adjustment in small‐scale MCCs can be 10 times more negative than in large‐scale MCCsWe find notable intra‐cell co‐variability between cloud liquid water path and drop concentration within MCCs that varies with cell size [ABSTRACT FROM AUTHOR]

The Atlantic meridional overturning circulation (AMOC) is central to the climate of the Atlantic by redistributing mass, heat, and freshwater. Hydrographic sections help monitor its strength at different latitudes, and inverse box models provide estimates of AMOC, heat, and freshwater transports. We have used all available hydrographic zonal sections at 24.5°N and 30°S over the last 30 years to conclude that single section inverse models agree with monthly outputs from an ocean general circulation model at the time of the cruise. In contrast, inverse models using multiple sections at different latitudes and times of the year for each of the last three decades are more consistent with decadal averages from the same model. Therefore, solutions of inverse models with single sections are affected by aliasing and represent the state of the ocean at the time of cruise. However, aliasing is greatly reduced when using multiple sections to assess low‐frequency variability. Plain Language Summary: Heat and nutrients in the Atlantic Ocean are redistributed through a process called Atlantic meridional overturning circulation, which is being monitored to detect changes in its strength. Hydrographic data offer the possibility to assess this variability, although it can be influenced by the ocean dynamics happening at the time of the cruise. When comparing the results from inverse box models applied to single sections with numerical model output, the best fit appears for the time of the cruise. This reflects that the monthly variation affects the solutions of the inverse model, which can be understood as representative of the duration of the cruise. Inverse models using several sections from cruises carried out in different times of the year and different years average out the local temporal phenomena that can affect the results and better represent the decadal average of the numerical models. Key Points: The Atlantic meridional overturning circulation has been monitored with hydrographic data and now basin‐wide arrays enable the detection of high‐frequency variabilityInverse solutions from single sections are affected by aliasing, as they capture the circulation structure of the time of the cruiseInverse models with multiple sections at different latitudes and times agree with decadal averages from an ocean general circulation model [ABSTRACT FROM AUTHOR]

The snow physical parameters are closely related to the sizes, shapes, and chemical composition of light‐absorbing particles (LAPs). By utilizing a computer‐controlled scanning electron microscope software called IntelliSEM‐EPASTM, we first report the measured size‐resolved concentration of soot, dust, and fly ash particles in fresh (wet) and aged (dry deposition) snow samples collected at an industrial city in China during and after a snowfall at intervals of 6–8 days. Due to wet scavenging by seasonal snow, soot and dust particles in snow are absorbed by 69.7% and 30.3% at wavelengths of 550 nm, lowering snow albedo by 0.0089 and 0.0039, respectively. Soot particle size increases slightly during dry deposition, whereas size‐resolved mineral dust does not undergo a significant shift in particle size. These results indicate the essentiality to involve the effects of accurate size and composition of in‐snow LAPs for a better assessment of snow light absorption and reflectance. Plain Language Summary: A field survey was undertaken to collect freshly fallen (1) and aged surface (15) snow samples at 1‐day intervals in the center of Changchun city, China, which is surrounded by heavy industrial emission sources. We used an advanced computer‐controlled scanning electron microscope to determine particle size and number distributions of three major light‐absorbing particle types with diameters of 0.2–10 μm in seasonal snow, namely soot, dust, and fly ash. Soot and dust particles deposited in various ice‐grain sizes via wet and dry deposition were also examined in terms of their contributions to light absorption and snow albedo reduction. We report here a first attempt to detect a combination of log‐normal soot, dust, and fly ash in seasonal snow, as well as their potential effects on the reduction of snow albedo. Key Points: Computer‐controlled scanning electron microscopy was used to identify size‐resolved light‐absorbing particles in surface snowSoot particle size increases slightly during dry deposition, whereas size‐resolved dust doesn't undergo a significant shift in particle sizeSize‐resolved soot and dust are critical factors in the determination of snow light absorption and albedo [ABSTRACT FROM AUTHOR]

Satellite measurements show that the Northern and Southern hemispheres reflect equal amounts of shortwave radiation ("albedo symmetry"), but no theory exists on if, how, and why the symmetry is established and maintained. Ambiguously, climate models are strongly biased in albedo symmetry but agree in the sign of the response to CO2. We find that mean‐state biases in albedo symmetry and hemispheric surface temperature asymmetry correlate negatively. Similarly, the response of albedo asymmetry to CO2 forcing correlates negatively with the magnitude of the asymmetry in surface warming. This is true across many and within single climate model simulations: a too warm or stronger warming hemisphere is darker or darkens more than its counterpart. In the 21 years of observations we find the same tendency and hypothesize (a) albedo symmetry is a function of the current climate state and (b) we will observe an evolution toward albedo asymmetry in coming decades. Plain Language Summary: Observations indicate that the Northern and Southern Hemispheres reflect equal amounts of shortwave radiation. It is not understood whether this "albedo symmetry" is by chance or physically necessary in the current or across different climate states. Planetary albedo changes substantially under global warming and acts as a radiative feedback. Thus, it seems paramount to understand its dynamics. We are doing so by using climate models. They are not tuned for albedo symmetry and indeed show huge biases since three generations of models. We flesh out our understanding where these biases come from and further, find a robust evolution of the albedo symmetry in response to CO2 forcing across model generations. Both, the models' bias and the forced change correlate well with the surface temperature difference between the hemispheres. Finally, we find this relation also present in the observed inter‐annual deviations from albedo symmetry and surface temperatures. This implies that we might observe an evolution toward an albedo asymmetry in the next decades and symmetry depends on the climate state. Key Points: Climate models persistently do not simulate the observed planetary albedo symmetryHowever, they do agree in a reduction of Northern minus Southern hemispheric reflectance under CO2 forcingModeled bias and forced change of albedo asymmetry, as well as observed deviations from symmetry might be governed by surface temperature [ABSTRACT FROM AUTHOR]

Satellite observations of sea ice freeboard are integral to the estimation of sea ice thickness. It is commonly assumed that radar pulses from satellite‐mounted Ku‐band altimeters penetrate through the snow and reflect from the snow‐ice interface. We would therefore expect a negative correlation between snow accumulation and radar freeboard measurements, as increased snow loading weighs the ice floe down. In this study we produce daily resolution radar freeboard products from the CryoSat‐2 and Sentinel‐3 altimeters via a recently developed optimal interpolation scheme. We find statistically significant (p < 0.05) positive correlations between radar freeboard anomalies and modeled snow accumulation. This suggests that, in the period after snowfall, radar pulses are not scattering from the snow‐ice interface as commonly assumed. Our results offer satellite‐based evidence of winter Ku‐band radar scattering above the snow‐ice interface, violating a key assumption in sea ice thickness retrievals. Plain Language Summary: Arctic sea ice thickness is often estimated using radar pulses from satellite‐mounted Ku‐band altimeters, which retrieve the radar freeboard. This is a proxy for the height of the ice surface above the waterline. Ku‐band radar pulses are widely assumed to penetrate through the overlying snowpack and reflect from the top of the sea ice. This means that increased snow loading on a sea ice floe is expected to reduce its radar freeboard, as the snow weighs the sea ice down. We produce daily resolution pan‐Arctic radar freeboard data sets from CryoSat‐2 and Sentinel‐3 retrievals. Using these new products, we find that an increased snow load often increases the radar freeboard, suggesting that the radar pulses are not reflecting off the ice surface. This could explain why satellite‐based sea ice thickness estimates don't always match in situ observations. Key Points: We reveal synoptic‐scale positive correlations between snow accumulation and Ku‐band radar freeboard changeThese correlations indicate that the conventional assumption of full radar‐wave penetration of the snowpack does not always hold trueThis sensitivity in freeboard estimates to snow accumulation introduces synoptic variability into satellite estimates of sea ice thickness [ABSTRACT FROM AUTHOR]

Clouds are primary modulators of Earth's energy balance. It is thus important to understand the links connecting variabilities in cloudiness to variabilities in other state variables of the climate system, and also describe how these links would change in a changing climate. A conceptual model of global cloudiness can help elucidate these points. In this work we derive simple representations of cloudiness, that can be useful in creating a theory of global cloudiness. These representations illustrate how both spatial and temporal variability of cloudiness can be expressed in terms of basic state variables. Specifically, cloud albedo is captured by a nonlinear combination of pressure velocity and a measure of the low‐level stability, and cloud longwave effect is captured by surface temperature, pressure velocity, and standard deviation of pressure velocity. We conclude with a short discussion on the usefulness of this work in the context of global warming response studies. Plain Language Summary: Clouds are important for Earth's climate, because they affect a large portion of the planet's energy balance, and hence its mean temperature. To better understand how the interplay between cloudiness and energy balance would change in a changing climate, a better theoretical understanding of how clouds are distributed over the planet, and how this connects with the state variables of the climate system such as temperature and wind speed, is required. As theoretical understanding is currently limited, in this work we explore the possibility of very simply representing the spatiotemporal distribution of clouds over the whole planet. We believe that these simple representations advance the field in the direction of a conceptual theory of global cloudiness and its impact on the energy balance. We show that the impact of cloudiness on both solar and terrestrial radiation balance can be captured well globally with only a few predictive fields, like surface temperature or vertical wind speed, combined simply and using only three tunable parameters, and without using any supplementary information such as the particular season or location on the planet. Key Points: Model fits are performed to the spatiotemporal observed cloudiness over all oceans, using a minimal set of predictors and parametersModels capture global‐mean, spatial, and most of seasonal variability of cloud radiative effectsCloud albedo and longwave effect are captured by pressure velocity and its variance, surface temperature, and lower tropospheric stability [ABSTRACT FROM AUTHOR]

We quantified the combined effects of mineral dust nonsphericity and size on snow albedo reduction using the MOPSMAP (Modeled optical properties of ensembles of aerosol particles) package and SAMDS (Spectral Albedo Model for Dirty Snow) with the consideration of dust from Sahara, Greenland, San Juan Mountains, and Tibetan Plateau. Results indicate that the dust‐induced albedo reduction decreases by up to 30% as the effective radii of dust particles increase in 1–5 μm. Nonspherical dust enhances snow albedo reduction relative to spherical dust by up to 20%. Stronger enhancements are obtained for higher dust concentration and larger dust size. Furthermore, the dust nonsphericity‐induced enhancement of snow albedo reduction is more pronounced for more‐absorptive dust. Finally, we develop a new parameterization for quantifying the dependence of snow albedo reduction on dust nonsphericity and size, and provide a convenient way for assessing the climate impacts of dust in snow. Plain Language Summary: Mineral dust particles are a major type of light‐absorbing aerosols in the atmosphere, with the absorptivity closely dependent on their shape and size. Previous studies have confirmed that dust particles deposited on snow surface varied largely in size and shape, and caused the reduction of snow albedo worldwide. In this study, we quantified the effects of dust size and nonsphericity on the induced snow albedo reduction. Results show that small and nonspherical dust particles induce large snow albedo reduction, in comparison with large and spherical dust particles. We further develop a parameterization for the dust size and nonsphericity effects for application in climate models. Key Points: Mineral dust‐induced snow albedo reductions rapidly decrease as dust size increasesNonspherical dust causes larger snow albedo reduction than spherical dust by up to 20%A new parameterization is developed to improve the estimate of snow albedo reduction with dust size and nonsphericity [ABSTRACT FROM AUTHOR]

This study employs a fully coupled meteorology‐chemistry‐snow model to investigate the impacts of light‐absorbing particles (LAPs) on snow darkening in the Sierra Nevada. After comprehensive evaluation with spatially and temporally complete satellite retrievals, the model shows that LAPs in snow reduce snow albedo by 0.013 (0–0.045) in the Sierra Nevada during the ablation season (April‐July), producing a midday mean radiative forcing of 4.5 W m−2 which increases to 15–22 W m−2 in July. LAPs in snow accelerate snow aging processes and reduce snow cover fraction, which doubles the albedo change and radiative forcing caused by LAPs. The impurity‐induced snow darkening effects decrease snow water equivalent and snow depth by 20 and 70 mm in June in the Sierra Nevada bighorn sheep habitat. The earlier snowmelt reduces root‐zone soil water content by 20%, deteriorating the forage productivity and playing a negative role in the survival of bighorn sheep. Plain Language Summary: Snowpacks in the Sierra Nevada receive large amounts of light‐absorbing particles [LAPs, mainly dust and black carbon (BC)], which decrease snow albedo, induce additional radiative forcing, alter the balance between snowmelt water supply and water demand, and influence the survival and resource selection of the Sierra Nevada bighorn sheep. However, quantifications of LAP‐induced snow darkening effects were rare in the Sierra Nevada and showed large spatial variability. Employing a fully coupled meteorology‐chemistry‐snow model, our study shows that snow albedo is reduced by up to 0.045 during April–July, which produces a radiative forcing of up to 22 W m−2. The darkened snow absorbs more sunlight, which decreases the snow water equivalent by 20 mm and shifts the melting season earlier. As a result, the root‐zone soil water content and forage productivity are reduced in early summer, which tends to deteriorate bighorn sheep nutrition levels and is harmful to their survival. Key Points: Light‐absorbing particles decrease snow albedo by up to 0.045 during April–July, producing radiative forcing of up to 22 W m−2Impurity effects are amplified by a factor of 2–3 due to snow albedo feedbacks associated with snow aging and snow cover changeImpurity effects decrease snow water equivalent by 20 mm in June and reduce root zone soil water content by 20% in bighorn sheep habitat [ABSTRACT FROM AUTHOR]

The annual cycle is the dominant component and the most prominent climate oscillation for the temperature variation over the Tibetan Plateau (TP). Identifying the relative contribution of anthropogenic and natural forcing to the changes in the annual cycle, namely, detection and attribution, is an important aspect for mountainous climate change research. The present study documents the elevation dependence of the trend in the annual temperature cycle (ATC) amplitude over the TP and detects the influence of anthropogenic activities on it. An elevation‐dependent weakening of the ATC amplitude occurs over the TP during the period 1961–2014. This variation with altitude can mainly be ascribed to the seasonal difference in warming. The influence of anthropogenic activities is detectable, with increased aerosols being the main contributor. Under aerosol‐only forcing, the larger decrease in snow‐related albedo at higher altitudes in winter can explain the amplified negative tendency of the ATC amplitude with elevation. Plain Language Summary: The present study documents the variation with altitude of the trend in the seasonal temperature difference over the TP and detects the influence upon it of anthropogenic activities. The seasonal temperature difference weakened in most regions of the TP during 1961–2014. Also, the higher the altitude, the more notable the decrease. The trend in the temperature seasonality over the TP and its variation with altitude can mainly be ascribed to the difference in the rate of warming between winter and summer. The greater warming in winter in higher‐altitude regions over the TP causes the negative tendency of the seasonal temperature difference to amplify with elevation. The model‐simulated responses are able to capture the observed variation with altitude of the trend in the temperature seasonality only if anthropogenic forcing is involved. Moreover, the influence upon it of anthropogenic activities is statistically detectable, with the increase in anthropogenic aerosols being the main contributor. In the model‐simulated response to anthropogenic aerosol–only forcing, the larger decrease in snow‐related albedo at higher altitudes in winter can explain the amplified warming there in winter and thereby the weakening with elevation in the seasonal temperature difference. Key Points: The amplitude of the annual temperature cycle decreased over the Tibetan Plateau, and this diminished trend was amplified with elevationInfluence of anthropogenic forcing on the elevation‐dependent weakening of annual temperature cycle amplitude is detectableThe increase in anthropogenic aerosols may be the main contributor [ABSTRACT FROM AUTHOR]

The reflectance of the Earth is a fundamental climate parameter that we measured from Big Bear Solar Observatory between 1998 and 2017 by observing the earthshine using modern photometric techniques to precisely determine daily, monthly, seasonal, yearly and decadal changes in terrestrial albedo from earthshine. We find the inter‐annual fluctuations in albedo to be global, while the large variations in albedo within individual nights and seasonal wanderings tend to average out over each year. We measure a gradual, but climatologically significant ∼0.5 W/m2 decline in the global albedo over the two decades of data. We found no correlation between the changes in the terrestrial albedo and measures of solar activity. The inter‐annual pattern of earthshine fluctuations are in good agreement with those measured by CERES (data began in 2001) even though the satellite observations are sensitive to retroflected light while earthshine is sensitive to wide‐angle reflectivity. The CERES decline is about twice that of earthshine. Plain Language Summary: The net sunlight reaching the Earth's climate system depends on the solar irradiance and the Earth's reflectance (albedo). We have observed earthshine from Big Bear Solar Observatory to measure the terrestrial albedo. For earthshine we measure the sunlight reflected from Earth to the dark part of the lunar face and back to the nighttime observer, yielding an instantaneous large‐scale reflectance of the Earth. In these relative measurements, we also observe the sunlit, bright part of the lunar face. We report here reflectance data (monthly, seasonal and annual) covering two decades, 1998–2017. The albedo shows a decline corresponding to a net climate forcing of about 0.5 W/m2. We find no correlation between measures of solar cycle variations and the albedo variations. The first precise satellite measures of terrestrial albedo came with CERES. CERES global albedo data (2001‐) show a decrease in forcing that is about twice that of earthshine measurements. The evolutionary changes in albedo motivate continuing earthshine observations as a complement to absolute satellite measurements, especially since earthshine and CERES measurements are sensitive to distinctly different parts of the angular reflectivity. The recent drop in albedo is attributed to a warming of the eastern pacific, which is measured to reduce low‐lying cloud cover and, thereby, the albedo. Key Points: We report on two decades of earthshine measurements of the earth's reflectance made from Big Bear Solar Observatory yielding a large‐scale terrestrial albedoWe find a decline in albedo between 1998 and 2017, corresponding to a radiative increase of 0.5 W/m2, which is climatologically significantThe CERES data show the same behavior, which is attributed to a reversal of the Pacific Decadal Oscillation reducing the Earth's albedo [ABSTRACT FROM AUTHOR]

Earth's energy imbalance (EEI) represents the rate of global energy accumulation in response to radiative forcings and feedbacks. Ocean heat uptake (OHU) poses a vital constraint on EEI and its uncertainty. Considering recent geodetic observations, geophysical corrections, and new estimates of the ocean's expansion efficiency of heat, we translate steric sea‐level change, the difference of total sea‐level and ocean‐mass change, into an OHU of 0.86 [0.62, 1.10, 5%–95%] Wm−2 for the period 2005–2019. Adding components of non‐oceanic heat uptake, we obtain an EEI of 0.94 [0.70, 1.19] Wm−2, which is at the upper end of previous assessments, but agrees within uncertainty. Interannual geodetic OHU variability exhibits a higher correlation with top‐of‐the‐atmosphere net radiative flux than hydrographic‐only data, but has a three times larger standard deviation. The radiation fluxes and the geodetic approach suggest an increase in heat uptake since 2005, most markedly in recent years. Plain Language Summary: As the Earth is warming in response to increasing greenhouse gas concentrations, 90% of the extra heat is deposited in the world's oceans. This heat accumulation in the ocean can be used to estimate Earth's energy imbalance (EEI), which quantifies the energy uptake or deficit of our planet. Profiles of ocean temperature measured with profiling floats are often used to calculate the ocean heat uptake, but this approach does not sample the deep ocean (below 2,000 m) and misses some areas near coasts. Alternatively, the ocean's thermal expansion can be related to the heat uptake and is obtained by subtracting satellite‐observed ocean mass changes due to ice‐sheet and glacier melt, from total sea‐level changes. Using the newest data and methods over the recent two decades, we find that EEI (0.94 Wm−2) derived from the satellite data is slightly larger than that from the ocean float measurements (0.76 Wm−2). But despite the higher central value, both are equivalent within uncertainty. Key Points: We assess the sea‐level budget and its implications on Earth's energy imbalance (EEI) over 2005–2019Space geodesy suggests an EEI of 0.94 ± 0.24 Wm−2Multiple approaches suggest that heat uptake is increasing most markedly in recent years [ABSTRACT FROM AUTHOR]

Estimates of global top‐of‐atmosphere radiation on monthly, seasonal, annual, and longer time‐scales require estimates of the diurnal variability in both insolation and surface and atmospheric reflection. We compare Earth Polychromatic Imaging Camera (EPIC) and National Institute of Standards and Technology Advanced Radiometer (NISTAR) observations from the Deep Space Climate Observatory (DSCOVR) satellite with Clouds and Earth's Radiant Energy System (CERES) hourly synoptic fluxes, which are diurnally filled through geostationary observations, and find that their power spectral density functions substantially agree, showing strong relative power at subdiurnal, diurnal, seasonal, and annual time‐scales, and power growing from diurnal to seasonal time‐scales. Frequency analysis of fluxes from several coupled model intercomparison project 5 model (CMIP5) and CMIP6 models shows that they distribute too much power over periods greater than 1 day but less than one year, indicating that a closer look is needed into how models achieve longer‐term stability in reflected shortwave radiation. Model developers can consider using these datasets for time‐varying energetic constraints, since tuning parameter choices will impact modeled planetary shortwave radiation across timescales ranging from subdiurnal to decadal. Plain‐Language Summary: The balance between net incoming solar and outgoing thermal energy exerts a strong influence on the Earth's climate. The fraction of the incoming solar energy that is reflected back to space is called albedo. Even a slight change in albedo would dwarf the impacts of greenhouse gases, but direct observations indicate that on time‐scales longer than a few years, it is remarkably stable and has been for decades. However, this albedo does fluctuate greatly at shorter time‐scales, meaning that the underlying causes that ultimately shape albedo interact in a variety of ways to achieve this stability. Using novel data, we present three different observational estimates of how this reflected energy varies at these shorter time‐scales, and they all substantially concur. However, a wide variety of climate models do not capture the observed variability, suggesting that further model developments is needed to better represent the underlying contributions to the Earth's albedo and the causes for its stability to date. Key Points: We present the first intercomparison of observational estimates of the Earth's albedo at subdiurnal through interannual time‐scalesThese observational estimates, from CERES, EPIC, and NISTAR, are in consensus, and can therefore be used to confront CMIP5 modelsModel estimates of planetary albedo show that models mostly disagree with observations at time‐scales of longer than one day [ABSTRACT FROM AUTHOR]

A decrease in surface albedo over ice‐covered ocean leads to global warming and Arctic‐amplified warming. Numerical results indicate seasonal variation in Arctic amplification (AA) is a result of local forcing and feedbacks in the Arctic. A decrease in surface albedo leads to a positive feedback, which dominates the local forcing and feedback mechanism. Ocean heat storage in the subsurface acts as a heat forcing to delay the influence of surface albedo feedback. In summer (autumn), heat storage increases (discharges) and contributes to a negative (positive) heat forcing, which decreases (increases) the positive local forcing and feedback and triggers the occurrence of the minimum (maximum) AA. In addition, increased CO2 forcing largely decreases the outgoing longwave radiation at the surface and increases surface temperatures, especially at low latitudes and in the Arctic winters, which decreases the AA magnitude and seasonal variation, although the AA remains nearly the same during winter. Plain Language Summary: The surface albedo over ice‐covered ocean is decreasing, leading to global warming and amplified Arctic warming, which confirms the importance of sea‐ice loss to the Arctic amplification (AA; both formation and seasonal variation). We also study the role of increased CO2 in the seasonal variation in AA by comparing experiments with and without increased CO2, and reveal the direct contribution of CO2 forcing, which is usually neglected.(1)The seasonal variation in Arctic amplification is determined by local forcing and feedback in the Arctic because there is little seasonal variation in the global surface temperature increase and there is nearly no contribution from remote forcing.(2)A positive feedback of surface albedo dominates the radiation flux variation and enhances heat absorption. The Arctic amplification reaches a minimum (maximum) due to the formation (discharge) of heat storage in the subsurface ocean in summer (autumn).(3)Increased CO2 forcing enhances the global surface temperature variation and decreases the Arctic amplification, although it provides a strong positive feedback and nearly keeps Arctic amplification unchanged in winter. Key Points: Contributions from CO2 and surface albedo to Arctic amplification (AA) are discussed individually using numerical experimentsThe seasonal AA variation is controlled by surface albedo feedback and heat storage form/discharge in the subsurface oceanCO2's direct influence weakens AA's magnitude and seasonal variation by inducing stronger warming at low latitudes [ABSTRACT FROM AUTHOR]

Melting on the Greenland Ice Sheet leads to extensive supraglacial stream networks. These streams accumulate low‐albedo sediments disproportionately contribute to melting. Studies analyzing supraglacial sediment distribution and hydrodynamic properties are rare. Here, we examine a 130‐m supraglacial stream reach in southwest Greenland using drone imagery, bathymetry, and hydrology measurements from 2017. Sediment covered 24% of the channel and had a mass‐median diameter of 0.027 mm. We applied calculations of critical Shields stress to determine the minimum water depth needed to initiate sediment movement. In order for theoretical critical water depths to match observed depths, sediment grains would need to be 2.48 mm (near the grain size for cryoconite granules) indicating that microbial growth within sediment caused extensive flocculation. Without flocculation, sediment would flush out of floodplains and supraglacial streams would have significantly higher albedos. Supraglacial stream albedo might therefore be sensitive to changing stream chemistry, temperature, and meltwater supply. Plain Language Summary: Meltwater formed on the surface of the Greenland Ice Sheet drains through abundant networks of rivers and streams on the ice surface. These streams consolidate dark‐colored sediment that can increase the amount of solar energy absorbed by the ice. To investigate the extent and causes of these deposits, we collected samples of sediment from southwest Greenland along with drone imagery, high‐resolution GPS measurements of the stream geometry, and measurements of the water flow over the course of a week in 2017. We analyzed the sediment to determine its size and applied our findings to fluid dynamics equations. These calculations showed that sediment in glacial streams can only be present if the sediment is being consolidated into millimeter‐scale granules by bacteria growing in the sediment. Thus, the amount of solar energy absorbed by the streams on the ice sheet likely depends on the health and longevity of this bacteria and further warming in Greenland may lead to more extensive sediment deposits in glacial streams. Key Points: Deposits of low‐albedo cryoconite are extensive within supraglacial streams in the southwest region of the Greenland Ice SheetCyanobacteria within supraglacial stream cryoconite likely cause millimeter‐scale granules to formSediment would not be present in supraglacial streams without flocculation, indicating that ablation zone albedo is dependent on microbial growth [ABSTRACT FROM AUTHOR]

One of the most famous climate oscillations has a period of about 60 years. Although this oscillation might emerge from internal variability, increasing evidence points toward a solar or astronomical origin, as also argued herein. We highlight that the orbital eccentricity of Jupiter presents prominent oscillations with a period of quasi 60 years due to its gravitational coupling with Saturn. This oscillation is found to be well correlated with quite a number of climatic records and also with a 60‐year oscillation present in long meteorite fall records relative to the periods 619–1943 CE. Since meteorite falls are the most macroscopic aspect of incoming space dust and their motion is mostly regulated by Jupiter, we propose that the interplanetary dust influx also presents a 60‐year cycle and could be forcing the climate to oscillate in a similar manner by modulating the formation of the clouds and, therefore, the Earth's albedo. Plain Language Summary: The physical origin of the modulation of the cloud system and of many of the Earth's climate oscillations from the decadal to the millennial timescales is still unclear, despite its importance in climate science. One of the most prominent oscillations has a period of about 60 years and is found in a number of geophysical records such as temperature reconstructions, aurora sights, Indian rainfalls, ocean climatic records, and in many others. These oscillations might emerge from the internal variability of the climate system, but increasing evidence also points toward a solar or astronomical origin. Herein we speculate whether the oscillations of the orbits of the planetary system could modulate the interplanetary dust flux falling on the Earth, then modifying the cloud coverage. We find that the orbital eccentricity of Jupiter presents a strong 60‐year oscillation that is well correlated with several climatic records and with the 60‐year oscillation found in long meteorite fall records since the 7th century. Since meteorite falls are the most macroscopic aspect of infalling space dust, we conclude that the interplanetary dust should modulate the formation of the clouds and, thus, drive climate changes. Key Points: The physical origin of the 60‐year oscillation observed in several climatic records is still unknown and debatedWe found that a strong 60‐year oscillation exists both in Jupiter's orbital eccentricity and in the historical record of meteorite fallsWe speculate that the influx of interplanetary dust could present the same 60‐year oscillation forcing the clouds' formation [ABSTRACT FROM AUTHOR]

Supercooled liquid clouds are an important component of the albedo of the Southern Ocean (SO). While ice phase occurrence in liquid‐dominant clouds (hereafter mixed phase) at temperatures warmer than the homogeneous freezing point is rare in the SO, the processes that create mixed‐phase clouds are not understood. Using data from the CALIPSO lidar, we reconsider the thresholds of layer‐integrated depolarization ratio and layer‐integrated attenuated backscatter that are used to diagnose the phases of fully attenuating cloud layers. We argue that liquid‐only clouds have understood physical bounds to these thresholds allowing for unique identification of layers that are not consistent with the presence of single‐phase liquid tops. Compared to the original phase algorithm the application of these physically constrained thresholds results in a ~70% increase in mixed phase during the annual cycle considered. Combining the CALIPSO data with CloudSat data, mixed‐phase clouds seem to typically cooccur with precipitation implying secondary ice forming processes. Key Points: Mixed‐phase clouds are identified with CALIPSO data over the Southern OceanThe occurrence frequency of ice‐containing low clouds increases in the Southern Ocean when the occurrence of mixed‐phase clouds is consideredNew phase thresholds identify a strong association between ice precipitation and mixed‐phase layers in low clouds over the Southern Ocean [ABSTRACT FROM AUTHOR]

To better understand and quantify the impact of clouds on the Greenland Ice Sheet surface mass balance (SMB), we study the spatiotemporal variability of the cloud radiative effect (CRE). The total CRE is separated in short‐term and long‐term impacts by performing multiple simulations with the SNOWPACK model for 2001‐+2010. The annual total CRE is 16.8 ± 4.5 W m−2, reducing the SMB with −157 ± 3.8 Gt yr−1. Summer cloud radiative cooling is −6.4 ± 5.7 W m−2 in the ablation area, increasing the SMB with 121 ± 2.2 Gt yr−1. The annual integrated impact is cloud‐reduced SMB of −36 Gt yr−1. The short‐term effect dominates the opposing long‐term effects through the albedo‐melt feedback. A long‐term warming effect decreases the albedo and so preconditions the surface for enhanced (summer) melt. The impact of the CRE, determined by spatial, temporal and initial conditions, explains existing conflicted views on the role of cloud radiation and emphasizes the need for accurate cloud and albedo representations in future studies. Key Points: Total cloud radiative warming occurs year‐round, except in the ablation area during summerWhen cloud radiative cooling occurs, clouds reduce mass loss and dominate the impact on the surfaceShort‐term and long‐term cloud effects are opposite, governed by surface albedo changes [ABSTRACT FROM AUTHOR]

Arctic vegetation is known to influence Arctic surface temperatures through albedo. However, it is less clear how plant evaporative resistance and albedo independently influence surface climate at high latitudes. We use surface properties derived from two common Arctic tree types to simulate the climate response to a change in land surface albedo and evaporative resistance in factorial combinations. We find that lower evaporative resistances lead to an increase of low clouds. The reflection of light due to the difference in albedos between vegetation types is similar to the loss of incident sunlight due to increased cloud cover resulting from lower evaporative resistance from vegetation change. Our results demonstrate that realistic changes in evaporative resistance can have an equal impact on surface temperature to changes in albedo and that cloud feedbacks play a first‐order role in determining the surface climate response to changes in Arctic land cover. Plain Language Summary: In the Arctic, darker land surfaces lead to warmer temperatures because they absorb more sunlight. However, there are multiple types of plants that grow in the Arctic, which differ not only in how dark they are but also how easily they release water. We investigate how different Arctic plants absorption of sunlight and ability to release water to the atmosphere can affect temperature over Arctic land using an Earth System Model. We find that dark trees are capable of absorbing a greater fraction of the incoming sunlight than their brighter counterparts, which tends to warm the surface. In comparison, when the land surface has a harder time releasing water into the atmosphere, a smaller fraction of energy at the land surface is used to evaporate water. This warms the air above the surface, which leads to evaporation of cloud droplets and less cloud cover. As a result, more sunlight is able to reach the surface, and land surface temperatures are warmer even when the surface is relatively bright. In combination, we find that the darkness of the surface and the plants' ability to release water have an equal influence on surface temperatures over land in the Arctic. Key Points: Two Arctic plant types with different properties cause substantial changes to land surface temperature through different physical pathwaysReducing land surface evaporative resistance increases low clouds and increases shortwave cloud forcingAlbedo directly warms the land surface, while changes in evaporation warms mostly by modifying cloud cover [ABSTRACT FROM AUTHOR]

Space weathering effects on the rocky S‐class asteroids are well understood. However, on the low‐albedo C‐complex asteroids, such as spacecraft targets Bennu and Ryugu, the situation is more complicated, especially due to a lack of spectral features throughout the visible‐near infrared spectral region. Here we show, through a combination of observational data and laboratory data of carbonaceous chondrites, phyllosilicates, and mixtures, that the UV‐visible spectral region is a diagnostic regime for studying space weathering effects on C‐complex asteroids. We show that space‐weathering‐produced opaque constituents, such as graphitized carbons, darken mixtures with phyllosilicates and produce a bluing the UV‐visible spectrum, consistent with what is seen on the asteroids compared with carbonaceous chondrites. Furthermore, we demonstrate that the UV upturns in the spectra of Bennu and Ryugu are consistent with the presence of graphitized carbon on those surfaces, the result of surface processing. Plain Language Summary: We suggest that the relatively weak UV absorptions of C‐complex asteroids compared with carbonaceous chondrite meteorites could be due to the increased presence of carbonized species on the asteroid surfaces, the result of space weathering processes. In fact, carbonized materials could be the source of the UV‐visible spectra of Ryugu and Bennu, which appear to show UV upturns rather than UV absorptions. Though iron‐rich and carbon‐rich opaques have similar spectral effects in the UV‐VIS, many laboratory experiments indicate that carbonaceous materials are the products of space weathering processes and thus carbonaceous materials could dominate spectral trends. Furthermore, we show that carbons may exhibit spectral features that change with processing and thus can help us to understand exposure age of some low‐albedo targets. Key Points: New insights about the surface composition of low‐albedo asteroids are revealed using the rarely utilized ultraviolet spectral regionCarbonaceous surfaces can be uniquely studied in the ultraviolet‐visible region using diagnostic spectral signaturesSpace‐weathering‐derived graphitized carbon on the surfaces of Bennu and Ryugu could be responsible for their unique spectral shapes [ABSTRACT FROM AUTHOR]

The final analysis of the Cassini radar observations of Saturn's icy moons presented here shows that the exchange of material between the planet's dust rings and moons, which is specific to the Saturnian system, plays a key role in the current state of the airless satellite regolith. Far from Saturn, the vast debris ring from Phoebe progressively coats the leading side of Iapetus with optically dark material reducing its radar brightness. On the contrary, close to the planet, the extreme radar brightness of the innermost moons Mimas, Enceladus, and Tethys (that exceeds that of the Galilean satellites) is most likely related to Enceladus's geysers and the E‐ring which brings ultraclean water ice to their surfaces. The measured radar albedos and observed hemispheric dichotomies require at least a few decimeters thick "snow" cover and that the near surfaces of Saturn's innermost moons contain especially efficient backscattering structures whose nature remains an outstanding problem. Plain Language Summary: From 2004 to 2017, the Cassini spacecraft explored Saturn's family of moons which, Titan excepted, are devoid of atmosphere. As such, these moons are not protected against the effects of bombardment by grains of various origins which alter their surface composition and texture. Cassini radar observations can help assess these effects by giving insights into the purity of the satellites' water ice. Indeed, when the RADAR sends waves toward a surface made of a transparent medium such as pure water ice, the waves penetrate into the ground down to few meters and have therefore more opportunities to bounce on buried structures and return to the spacecraft to be recorded. Inversely, "dirty" surfaces appear radar‐dark. Cassini radar observations support the idea that the dark deposit on Iapetus's leading face originates from Phoebe's ring and is a few decimeters thick. On the other hand, closer to Saturn, the E‐ring (fed by Enceladus's geysers) acts as a snow cannon, depositing fresh water ice particles onto Enceladus and its neighbors. These satellites are radar‐brighter than Galilean moons because their surfaces are cleaner and also contain structures especially efficient at returning waves to the spacecraft. The nature of these "boomerang" structures is still unknown. Key Points: The final analysis of Cassini radar observations of Saturn's icy moons gives insights into the degree of purity of the regolithSaturn innermost moons are extremely radar‐bright partly due to the deposition of ultraclean water ice particles from Enceladus' geysersThe high radar albedos of Saturn inner moons also imply the presence of especially efficient retroreflecting structures in their subsurface [ABSTRACT FROM AUTHOR]

On top of a declining trend driven by global warming, the Arctic sea ice extent (SIE) exhibits considerable interannual variations. In this study, we analyze the interannual anomalies of September SIE in relation to the surface radiation anomalies. We find that the accumulation of radiation energy in the early months (June, July, and August) very well explains the September SIE variability (R2 = 0.81). In Particular, strong correlations are found between September SIE and June radiation anomalies, which in the shortwave is due to cloud and surface albedo changes and in the longwave due to atmospheric warming. The results show that monitoring the radiation anomalies affords a potential means to improve the prediction of the late summer sea ice. Plain Language Summary: This work investigates the relationship between radiation flux and sea ice extent anomalies in the Arctic. We for the first time investigate both the top‐of‐atmosphere and surface radiation anomalies, together with their atmospheric and surface contribution components, in relation to the interannual sea ice variations. This investigation discloses the major radiative drivers of the late summer sea ice anomalies: the cloud forcing in the shortwave (solar radiation) and atmospheric warming in the longwave (terrestrial thermal radiation). These findings provide clues for improving sea ice prediction and afford means to validate and improve its simulation in the dynamical models. Key Points: The interannual variability of September SIE is strongly influenced by the radiative anomaly in the earlier months (JJA)Besides shortwave forcing, longwave forcing due to atmospheric warming in June strongly predicts September SIEThe June radiation forcing associated with September SIE anomaly is linked to the North Atlantic Oscillation [ABSTRACT FROM AUTHOR]

Climate sensitivity is a key metric used to assess the magnitude of global warming given increased CO2 concentrations. The geological past can provide insights into climate sensitivity; however, on timescales of millions of years, factors other than CO2 can drive climate, including paleogeographic forcing and solar luminosity. Here, through an ensemble of climate model simulations covering the period 150–35 million years ago, we show that climate sensitivity to CO2 doubling varies between ∼3.5 and 5.5 °C through this time. These variations can be explained as a nonlinear response to solar luminosity, evolving surface albedo due to changes in ocean area, and changes in ocean circulation. The work shows that the modern climate sensitivity is relatively low in the context of the geological record, as a result of relatively weak feedbacks due to a relatively low CO2 baseline, and the presence of ice and relatively small ocean area in the modern continental configuration. Key Points: Climate sensitivity on geological timescales depends on continental configurationOcean area and ocean circulation nonlinearly determine climate and climate sensitivityPast climate sensitivity is not necessarily a good analogue for future climate sensitivity [ABSTRACT FROM AUTHOR]

Abstract: Previous studies quantify surface albedo feedback (SAF) in climate change, but few assess its variability on decadal time scales. Using the Coupled Model Intercomparison Project Version 5 (CMIP5) multimodel ensemble data set, we calculate time evolving SAF in multiple decades from surface albedo and temperature linear regressions. Results are meaningful when temperature change exceeds 0.5 K. Decadal‐scale SAF is strongly correlated with century‐scale SAF during the 21st century. Throughout the 21st century, multimodel ensemble mean SAF increases from 0.37 to 0.42 W m−2 K−1. These results suggest that models' mean decadal‐scale SAFs are good estimates of their century‐scale SAFs if there is at least 0.5 K temperature change. Persistent SAF into the late 21st century indicates ongoing capacity for Arctic albedo decline despite there being less sea ice. If the CMIP5 multimodel ensemble results are representative of the Earth, we cannot expect decreasing Arctic sea ice extent to suppress SAF in the 21st century. [ABSTRACT FROM AUTHOR]

Along many rivers, dams trap sediment and water released from the dams is clear. Downstream of the dam, temperature variability along the river is controlled by climate that warms or cools the water, the flow magnitude, and the spectral properties of water and the river's bed. Using field observations, a synoptic numerical model without calibration couples a full-spectrum radiation balance with turbulent heat fluxes, bed conduction, and a hydraulic model that estimates depth and velocity. We show that variations in the river's temperature are sensitive to the albedo of the sediment on the bed, especially at shallow depths and smaller discharges. However, about half the solar radiation lies in a spectral range where water is highly absorptive; in these wavelengths, absorption is independent of depth. In spring and summer with many hours of sunlight, releases of cold water will have limited influence on temperatures beyond tens of kilometers downstream of a dam. [ABSTRACT FROM AUTHOR]

We have invented a new method for detecting solar particle events using data from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO). Using a simple function of the total particle detection rates from four of CRaTER's six detectors, we can precisely identify solar energetic particle event periods in the CRaTER data archive. During solar quiet periods we map the distribution of a mare‐associated mixture of elements in the lunar regolith using this new method. The new map of the moon probably reflects an as‐yet unknown combination of lunar albedo protons, neutrons, and gamma rays, and most closely resembles Lunar Prospector maps of gamma rays characteristic of thorium and iron. This result will lead to multiple follow‐up studies of lunar albedo particles and may also contribute to the study of diurnally varying hydrogenation of the lunar regolith. Key Points: The CRaTER instrument on LRO can detect and quantify small solar particle events with a simple new analysis techniqueOur new lunar map of albedo radiation resembles gamma ray maps from Lunar ProspectorFollow‐up studies will investigate contributions from neutrons, protons, and gamma rays, and signatures of hydrogen in lunar regolith [ABSTRACT FROM AUTHOR]