# Publications

## Cloud_cci Advanced Very High Resolution Radiometer post meridiem (AVHRR-PM) dataset version 3: 35-year climatology of global cloud and radiation properties

Earth System Science Data Copernicus Publications 12 (2020) 41-60

M Stengel, S Stapelberg, O Sus, S Finkensieper, B Wuerzler, D Philipp, R Hollmann, C Poulsen, M Christensen, G McGarragh

We present version 3 of the Cloud_cci Advanced Very High Resolution Radiometer post meridiem (AVHRR-PM) dataset, which contains a comprehensive set of cloud and radiative flux properties on a global scale covering the period of 1982 to 2016. The properties were retrieved from AVHRR measurements recorded by the afternoon (post meridiem – PM) satellites of the National Oceanic and Atmospheric Administration (NOAA) Polar Operational Environmental Satellite (POES) missions. The cloud properties in version 3 are of improved quality compared with the precursor dataset version 2, providing better global quality scores for cloud detection, cloud phase and ice water path based on validation results against A-Train sensors. Furthermore, the parameter set was extended by a suite of broadband radiative flux properties. They were calculated by combining the retrieved cloud properties with thermodynamic profiles from reanalysis and surface properties. The flux properties comprise upwelling and downwelling and shortwave and longwave broadband fluxes at the surface (bottom of atmosphere – BOA) and top of atmosphere (TOA). All fluxes were determined at the AVHRR pixel level for all-sky and clear-sky conditions, which will particularly facilitate the assessment of the cloud radiative effect at the BOA and TOA in future studies. Validation of the BOA downwelling fluxes against the Baseline Surface Radiation Network (BSRN) shows a very good agreement. This is supported by comparisons of multi-annual mean maps with NASA's Clouds and the Earth's Radiant Energy System (CERES) products for all fluxes at the BOA and TOA. The Cloud_cci AVHRR-PM version 3 (Cloud_cci AVHRR-PMv3) dataset allows for a large variety of climate applications that build on cloud properties, radiative flux properties and/or the link between them.

## An AeroCom/AeroSat study: Intercomparison of Satellite AODDatasets for Aerosol Model Evaluation

Atmospheric Chemistry and Physics Discussions European Geosciences Union (2020)

N Schutgens, A Sayer, A Heckel, C Hsu, H Jethva, G de Leeuw, P Leonard, R Levy, A Lipponen, A Lyapustin, P North, T Popp, C Poulson, V Sawyer, L Sogacheva, G Thomas, O Torres, Y Wang, S Kinne, M Schulz, P Stier

## Global response of parameterised convective cloud fields to anthropogenic aerosol forcing

Atmospheric Chemistry and Physics Copernicus GmbH 20 (2020) 4445-4460

Z Kipling, L Labbouz, P Stier

&lt;jats:p&gt;Abstract. The interactions between aerosols and convective clouds represent some of the greatest uncertainties in the climate impact of aerosols in the atmosphere. A wide variety of mechanisms have been proposed by which aerosols may invigorate, suppress or change the properties of individual convective clouds, some of which can be reproduced in high-resolution limited-area models. However, there may also be mesoscale, regional or global adjustments which modulate or dampen such impacts which cannot be captured in the limited domain of such models. The Convective Cloud Field Model (CCFM) provides a mechanism to simulate a population of convective clouds, complete with microphysics and interactions between clouds, within each grid column at resolutions used for global climate modelling, so that a representation of the microphysical aerosol response within each parameterised cloud type is possible. Using CCFM within the global aerosol–climate model ECHAM–HAM, we demonstrate how the parameterised cloud field responds to the present-day anthropogenic aerosol perturbation in different regions. In particular, we show that in regions with strongly forced deep convection and/or significant aerosol effects via large-scale processes, the changes in the convective cloud field due to microphysical effects are rather small; however in a more weakly forced regime such as the Caribbean, where large-scale aerosol effects are small, a signature of convective invigoration does become apparent. &lt;/jats:p&gt;

## Assessing California wintertime precipitation responses to various climate drivers

Journal of Geophysical Research: Atmospheres American Geophysical Union 125 (2020) e2019JD031736

RJ Allen, J-F Lamarque, D Watson-Parris, D Olivie

Understanding how drivers of climate change affect precipitation remains an important area of research. Although several robust precipitation responses have been identified under continued increases in greenhouse gases (GHGs), considerable uncertainty remains. This is particularly the case at regional scales, including the West Coast of the United States and California. Here, we exploit idealized, single forcing simulations from the Precipitation Driver Response Model Intercomparison Project (PDRMIP) to address how climate drivers impact California wintertime precipitation. Consistent with recent work, GHGs including carbon dioxide and methane, as well as solar forcing, yield a robust increase in California wintertime precipitation. We also find robust California precipitation responses to aerosols but with opposite responses for sulfate versus black carbon aerosol. Sulfate aerosol increases California wintertime precipitation, whereas black carbon reduces it. Moreover, California precipitation is more sensitive to aerosols, particularly regional emissions from Europe and Asia, than to GHGs. These precipitation responses are consistent with shifts in the jet stream and altered moisture fluxes. Although the idealized nature of PDRMIP simulations precludes a formal attribution, our results suggest that aerosols can perturb precipitation and fresh water resources along the West Coast of the United States.

## Reducing the aerosol forcing uncertainty using observational constraints on warm rain processes

J Mülmenstädt, C Nam, M Salzmann, J Kretzschmar, TS L’Ecuyer, U Lohmann, P-L Ma, G Myhre, D Neubauer, P STIER, K Suzuki, M Wang, J Quaas

## Aerosols Enhance Cloud Lifetime and Brightness along the Stratus-to-Cumulus Transition

Proceedings of the National Academy of Sciences of USA National Academy of Sciences (2020)

M CHRISTENSEN, W Jones, P STIER

## Diurnal cycle of the semi-direct effect from a persistent absorbing aerosol layer over marine stratocumulus in large-eddy simulations

Atmospheric Chemistry and Physics European Geosciences Union 20 (2020) 1317–1340-

R Herbert, N Bellouin, E Highwood, A Hill

<p>The rapid adjustment, or semi-direct effect, of marine stratocumulus clouds to elevated layers of absorbing aerosols may enhance or dampen the radiative effect of aerosol&ndash;radiation interactions. Here we use large-eddy simulations to investigate the sensitivity of stratocumulus clouds to the properties of an absorbing aerosol layer located above the inversion layer, with a focus on the location, timing, and strength of the radiative heat perturbation. The sign of the daily mean semi-direct effect depends on the properties and duration of the aerosol layer, the properties of the boundary layer, and the model setup. Our results suggest that the daily mean semi-direct effect is more elusive than previously assessed. We find that the daily mean semi-direct effect is dominated by the distance between the cloud and absorbing aerosol layer. Within the first 24&thinsp;h the semi-direct effect is positive but remains under 2&thinsp;W&thinsp;m<sup>&minus;2</sup>&nbsp;unless the aerosol layer is directly above the cloud. For longer durations, the daily mean semi-direct effect is consistently negative but weakens by 30&thinsp;%, 60&thinsp;%, and 95&thinsp;% when the distance between the cloud and aerosol layer is 100, 250, and 500&thinsp;m, respectively. Both the cloud response and semi-direct effect increase for thinner and denser layers of absorbing aerosol. Considerable diurnal variations in the cloud response mean that an instantaneous semi-direct effect is unrepresentative of the daily mean and that observational studies may underestimate or overestimate semi-direct effects depending on the observed time of day. The cloud response is particularly sensitive to the mixing state of the boundary layer: well-mixed boundary layers generally result in a negative daily mean semi-direct effect, and poorly mixed boundary layers result in a positive daily mean semi-direct effect. The properties of the boundary layer and model setup, particularly the sea surface temperature, precipitation, and properties of the air entrained from the free troposphere, also impact the magnitude of the semi-direct effect and the timescale of adjustment. These results suggest that the semi-direct effect simulated by coarse-resolution models may be erroneous because the cloud response is sensitive to small-scale processes, especially the sources and sinks of buoyancy.</p>

## Constraining uncertainty in aerosol direct forcing

Geophysical Research Letters American Geophysical Union 47 (2020) e2020GL087141

D Watson-Parris, N Bellouin, L Deaconu, N Schutgens, M Yoshioka, L Regayre, K Pringle, J Johnson, C Smith, K Carslaw, P Stier

The uncertainty in present-day anthropogenic forcing is dominated by uncertainty in the strength of the contribution from aerosol. Much of the uncertainty in the direct aerosol forcing can be attributed to uncertainty in the anthropogenic fraction of aerosol in the present-day atmosphere, due to a lack of historical observations. Here we present a robust relationship between total present-day aerosol optical depth and the anthropogenic contribution across three multi-model ensembles and a large single-model perturbed parameter ensemble. Using observations of aerosol optical depth, we determine a reduced likely range of the anthropogenic component and hence a reduced uncertainty in the direct forcing of aerosol.

## Planet Hunters TESS I: TOI 813, a subgiant hosting a transiting Saturn-sized planet on an 84-day orbit

Monthly Notices of the Royal Astronomical Society Oxford University Press 494 (2020) 750-763

N Eisner, O Barragan Villanueva, S Aigrain, C Lintott, G Miller, N Zicher, TS Boyajian, C Briceño, EM Bryant, JL Christiansen, AD Feinstein, LM Flor-Torres, M Fridlund, D Gandolfi, J Gilbert, N Guerrero, JM Jenkins, K Jones, M Christensen, A Vanderburg, AR López-Sánchez, C Ziegler, DM Bundy, LD Melanson, I Terentev

We report on the discovery and validation of TOI 813b (TIC 55525572 b), a transiting exoplanet identified by citizen scientists in data from NASA's Transiting Exoplanet Survey Satellite (TESS) and the first planet discovered by the Planet Hunters TESS project. The host star is a bright (V = 10.3 mag) subgiant (USDR_\star=1.94\,R_\odotUSD, USDM_\star=1.32\,M_\odotUSD). It was observed almost continuously by TESS during its first year of operations, during which time four individual transit events were detected. The candidate passed all the standard light curve-based vetting checks, and ground-based follow-up spectroscopy and speckle imaging enabled us to place an upper limit of USD2 M_{Jup}USD (99 % confidence) on the mass of the companion, and to statistically validate its planetary nature. Detailed modelling of the transits yields a period of USD83.8911_{ - 0.0031 } ^ { + 0.0027 }USD days, a planet radius of USD6.71 \pm 0.38R_{\oplus}$, and a semi major axis of$0.423_{ - 0.037 } ^ { + 0.031 }USD AU. The planet's orbital period combined with the evolved nature of the host star places this object in a relatively under-explored region of parameter space. We estimate that TOI-813b induces a reflex motion in its host star with a semi-amplitude of USD\sim6USD msUSD^{-1}USD, making this system a promising target to measure the mass of a relatively long-period transiting planet.

## Cloudy-sky contributions to the direct aerosol effect

Atmospheric Chemistry and Physics Copernicus GmbH 20 (2020) 8855-8865

G Myhre, BH Samset, CW Mohr, K Alterskjær, Y Balkanski, N Bellouin, M Chin, J Haywood, Ø Hodnebrog, S Kinne, G Lin, MT Lund, JE Penner, M Schulz, N Schutgens, RB Skeie, P Stier, T Takemura, K Zhang

&lt;jats:p&gt;Abstract. The radiative forcing of the aerosol&#x2013;radiation interaction can be decomposed into clear-sky and cloudy-sky portions. Two sets of multi-model simulations within Aerosol Comparisons between Observations and Models (AeroCom), combined with observational methods, and the time evolution of aerosol emissions over the industrial era show that the contribution from cloudy-sky regions is likely weak. A mean of the simulations considered is 0.01&#xB1;0.1&#x2009;W&#x2009;m&#x2212;2. Multivariate data analysis of results from AeroCom Phase&#xA0;II shows that many factors influence the strength of the cloudy-sky contribution to the forcing of the aerosol&#x2013;radiation interaction. Overall, single-scattering albedo of anthropogenic aerosols and the interaction of aerosols with the short-wave cloud radiative effects are found to be important factors. A more dedicated focus on the contribution from the cloud-free and cloud-covered sky fraction, respectively, to the aerosol&#x2013;radiation interaction will benefit the quantification of the radiative forcing and its uncertainty range. &lt;/jats:p&gt;

## Author Correction: Weak average liquid-cloud-water response to anthropogenic aerosols.

Nature 577 (2020) E3-E5

V Toll, M Christensen, J Quaas, N Bellouin

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

## Bounding global aerosol radiative forcing of climate change

Reviews of Geophysics American Geophysical Union 58 (2019) e2019RG000660

N Bellouin, J Quaas, S Kinne, P Stier, D Watson-Parris, O Boucher, KS Carslaw, M Christensen, A-L Daniau, JL Dufresne, G Feingold, S Fiedler, P Forster, A Gettelman, JM Haywood, U Lohmann, F Malavelle, T Mauritsen, DT McCoy, G Myhre, J Muelmenstaedt, A Possner, M Rugenstein, O Sourdeval, V Toll

Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth's radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable, and arguable lines of evidence, including modeling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol‐radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol‐driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed‐phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of ‐1.6 to ‐0.6 W m−2, or ‐2.0 to ‐0.4 W m−2 with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted toward more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial‐era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds.

## Constraining the Twomey effect from satellite observations: issues and perspectives

Atmospheric Chemistry and Physics Discussions European Geosciences Union (2020)

J Quaas, A Antti, B Cairns, M Christensen, H Deneke, AML Ekman, G Feingold, A Fridlind, E Gryspeerdt, O Hasekamp, Z Li, A Lipponen, P-L Ma, J Muelmenstaedt, A Nenes, J Penner, D Rosenfeld, R Schroedner, K Sinclair, O Sourdeval, P Stier, M Tesche, B van Dieedenhoven, M Wendisch

The Twomey effect describes the radiative forcing associated with a change in cloud albedo due to an increase in anthropogenic aerosol emissions. It is driven by the perturbation in cloud droplet number concentration (ΔNd,ant) in liquid-water clouds and is currently understood to exert a cooling effect on climate. The Twomey effect is the key driver in the effective radiative forcing due to aerosol–cloud interactions which also comprises rapid adjustments. These adjustments are essentially the responses of cloud fraction and liquid water path to ΔNd,ant and thus scale approximately with it. While the fundamental physics of the influence of added aerosol particles on the droplet concentration (Nd) is well described by established theory at the particle scale (micrometres), how this relationship is expressed at the large scale (hundreds of kilometres) ΔNd,ant remains uncertain. The discrepancy between process understanding at particle scale and insufficient quantification at the climate-relevant large scale is caused by co-variability of aerosol particles and vertical wind and by droplet sink processes. These operate at scales on the order of 10s of metres at which only localized observations are available and at which no approach exists yet to quantify the anthropogenic perturbation. Different atmospheric models suggest diverse magnitudes of the Twomey effect even when applying the same anthropogenic aerosol emission perturbation. Thus, observational data are needed to quantify and constrain the Twomey effect. At the global scale, this means satellite data. There are three key uncertainties in determining ΔNd,ant, namely the quantification (i) of the cloud-active aerosol – the cloud condensation nuclei concentrations (CCN) at or above cloud base –, (ii) of Nd, as well as (iii) the statistical approach for inferring the sensitivity of Nd to aerosol particles from the satellite data. A fourth uncertainty, the anthropogenic perturbation to CCN concentrations, is also not easily accessible from observational data. This review discusses deficiencies of current approaches for the different aspects of the problem and proposes several ways forward: In terms of CCN, retrievals of optical quantities such as aerosol optical depth suffer from a lack of vertical resolution, size and hygroscopicity information, the non-direct relation to the concentration of aerosols, the impossibility to quantify it within or below clouds, and the problem of insufficient sensitivity at low concentrations, in addition to retrieval errors. A future path forward can include utilizing colocated polarimeter and lidar instruments, ideally including high spectral resolution lidar capability at two wavelengths to maximize vertically resolved size distribution information content. In terms of Nd, a key problem is the lack of operational retrievals of this quantity, and the inaccuracy of the retrieval especially in broken-cloud regimes. As for the Nd – to – CCN sensitivity, key issues are the updraught distributions and the role of Nd sink processes, for which empirical assessments for specific cloud regimes are currently the best solutions. These considerations point to the conclusion that past studies using existing approaches have likely underestimated the true sensitivity and, thus, the radiative forcing due to the Twomey effect.

## Open cells exhibit weaker entrainment of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer

Atmospheric Chemistry and Physics Copernicus Publications 20 (2020) 4059-4084

SJ Abel, PA Barrett, P Zuidema, J Zhang, M Christensen, F Peers, JW Taylor, I Crawford, KN Bower, M Flynn

<br>This work presents synergistic satellite, airborne and surface-based observations of a pocket of open cells (POC) in the remote south-east Atlantic. The observations were obtained over and upwind of Ascension Island during the CLouds and Aerosol Radiative Impacts and Forcing (CLARIFY) and the Layered Smoke Interacting with Clouds (LASIC) field experiments. A novel aspect of this case study is that an extensive free-tropospheric biomass burning aerosol plume that had been transported from the African continent was observed to be in contact with the boundary layer inversion over the POC and the surrounding closed cellular cloud regime. The in situ measurements show marked contrasts in the boundary layer thermodynamic structure, cloud properties, precipitation and aerosol conditions between the open cells and surrounding overcast cloud field.</br> <br>The data demonstrate that the overlying biomass burning aerosol was mixing down into the boundary layer in the stratocumulus cloud downwind of the POC, with elevated carbon monoxide, black carbon mass loadings and accumulation-mode aerosol concentrations measured beneath the trade-wind inversion. The stratocumulus cloud in this region was moderately polluted and exhibited very little precipitation falling below cloud base. A rapid transition to actively precipitating cumulus clouds and detrained stratiform remnants in the form of thin quiescent veil clouds was observed across the boundary into and deep within the POC. The subcloud layer in the POC was much cleaner than that in the stratocumulus region. The clouds in the POC formed within an ultra-clean layer (accumulation-mode aerosol concentrations of approximately a few cm−3) in the upper region of the boundary layer, which was likely to have been formed via efficient collision–coalescence and sedimentation processes. Enhanced Aitken-mode aerosol concentrations were also observed intermittently in this ultra-clean layer, suggesting that new particle formation was taking place. Across the boundary layer inversion and immediately above the ultra-clean layer, accumulation-mode aerosol concentrations were ∼ 1000 cm−3. Importantly, the air mass in the POC showed no evidence of elevated carbon monoxide over and above typical background conditions at this location and time of year. As carbon monoxide is a good tracer for biomass burning aerosol that is not readily removed by cloud processing and precipitation, it demonstrates that the open cellular convection in the POC is not able to entrain large quantities of the free-tropospheric aerosol that was sitting directly on top of the boundary layer inversion. This suggests that the structure of the mesoscale cellular convection may play an important role in regulating the transport of aerosol from the free troposphere down into the marine boundary layer.</br> <br>We then develop a climatology of open cellular cloud conditions in the south-east Atlantic from 19 years of September Moderate Resolution Imaging Spectroradiometer (MODIS) Terra imagery. This shows that the maxima in open cell frequency (> 0.25) occurs far offshore and in a region where subsiding biomass burning aerosol plumes may often come into contact with the underlying boundary layer cloud. If the results from the observational case study applied more broadly, then the apparent low susceptibility of open cells to free-tropospheric intrusions of additional cloud condensation nuclei could have some important consequences for aerosol–cloud interactions in the region.</br>

## Up to two billion times acceleration of scientific simulations with deep neural architecture search

CoRR abs/2001.08055 (2020)

MF Kasim, D Watson-Parris, L Deaconu, S Oliver, P Hatfield, DH Froula, G Gregori, M Jarvis, S Khatiwala, J Korenaga, J Topp-Mugglestone, E Viezzer, SM Vinko

Computer simulations are invaluable tools for scientific discovery. However, accurate simulations are often slow to execute, which limits their applicability to extensive parameter exploration, large-scale data analysis, and uncertainty quantification. A promising route to accelerate simulations by building fast emulators with machine learning requires large training datasets, which can be prohibitively expensive to obtain with slow simulations. Here we present a method based on neural architecture search to build accurate emulators even with a limited number of training data. The method successfully accelerates simulations by up to 2 billion times in 10 scientific cases including astrophysics, climate science, biogeochemistry, high energy density physics, fusion energy, and seismology, using the same super-architecture, algorithm, and hyperparameters. Our approach also inherently provides emulator uncertainty estimation, adding further confidence in their use. We anticipate this work will accelerate research involving expensive simulations, allow more extensive parameters exploration, and enable new, previously unfeasible computational discovery.

## Atmospheric energy budget response to idealized aerosol perturbation in tropical cloud systems

Atmospheric Chemistry and Physics Copernicus GmbH 20 (2020) 4523-4544

G Dagan, P Stier, M Christensen, G Cioni, D Klocke, A Seifert

<jats:p>Abstract. The atmospheric energy budget is analysed in numerical simulations of tropical cloud systems to better understand the physical processes behind aerosol effects on the atmospheric energy budget. The simulations include both shallow convective clouds and deep convective tropical clouds over the Atlantic Ocean. Two different sets of simulations, at different dates (10–12 and 16–18 August 2016), are simulated with different dominant cloud modes (shallow or deep). For each case, the cloud droplet number concentration (CDNC) is varied as a proxy for changes in aerosol concentrations without considering the temporal evolution of the aerosol concentration (for example due to wet scavenging, which may be more important under deep convective conditions). It is shown that the total column atmospheric radiative cooling is substantially reduced with CDNC in the deep-cloud-dominated case (by ∼10.0 W m−2), while a much smaller reduction (∼1.6 W m−2) is shown in the shallow-cloud-dominated case. This trend is caused by an increase in the ice and water vapour content at the upper troposphere that leads to a reduced outgoing longwave radiation, an effect which is stronger under deep-cloud-dominated conditions. A decrease in sensible heat flux (driven by an increase in the near-surface air temperature) reduces the warming by ∼1.4 W m−2 in both cases. It is also shown that the cloud fraction response behaves in opposite ways to an increase in CDNC, showing an increase in the deep-cloud-dominated case and a decrease in the shallow-cloud-dominated case. This demonstrates that under different environmental conditions the response to aerosol perturbation could be different. </jats:p>

## Effects of aerosol in simulations of realistic shallow cumulus cloud fields in a large domain

ATMOSPHERIC CHEMISTRY AND PHYSICS 19 (2019) 13507-13517

G Spill, P Stier, PR Field, G Dagan

## tobac 1.2: towards a flexible framework for tracking and analysis of clouds in diverse datasets

GEOSCIENTIFIC MODEL DEVELOPMENT 12 (2019) 4551-4570

M Heikenfeld, PJ Marinescu, M Christensen, D Watson-Parris, F Senf, SC van den Heever, P Stier

## Efficacy of climate forcings in PDRMIP models

Journal of Geophysical Research: Atmospheres American Geophysical Union 124 (2019) 12824-12844

TB Richardson, PM Forster, CJ Smith, AC Maycock, T Wood, T Andrews, O Boucher, G Faluvegi, D Flaeschner, O Hodnebrog, M Kasoar, A Kirkevåg, J-F Lamarque, J Mülmenstädt, G Myhre, D Olivié, RW Portmann, BH Samset, D Shawki, D Shindell, P Stier, T Takemura, A Voulgarakis, D Watson-Parris

Quantifying the efficacy of different climate forcings is important for understanding the real‐world climate sensitivity. This study presents a systematic multi‐model analysis of different climate driver efficacies using simulations from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP). Efficacies calculated from instantaneous radiative forcing deviate considerably from unity across forcing agents and models. Effective radiative forcing (ERF) is a better predictor of global mean near‐surface air temperature (GSAT) change. Efficacies are closest to one when ERF is computed using fixed sea surface temperature experiments and adjusted for land surface temperature changes using radiative kernels. Multi‐model mean efficacies based on ERF are close to one for global perturbations of methane, sulphate, black carbon and insolation, but there is notable inter‐model spread. We do not find robust evidence that the geographic location of sulphate aerosol affects its efficacy. GSAT is found to respond more slowly to aerosol forcing than CO2 in the early stages of simulations. Despite these differences, we find that there is no evidence for an efficacy effect on historical GSAT trend estimates based on simulations with an impulse response model, nor on the resulting estimates of climate sensitivity derived from the historical period. However, the considerable intermodel spread in the computed efficacies means that we cannot rule out an efficacy‐induced bias of ±0.4 K in equilibrium climate sensitivity to CO2 doubling (ECS) when estimated using the historical GSAT trend.

## The impact of ship emission controls recorded by Cloud Properties

Geophysical Research Letters American Geophysical Union 46 (2019) 12547-12555

E Gryspeerdt, T Smith, E O'Keeffe, M Christensen, F Goldsworth

The impact of aerosols on cloud properties is one of the leading uncertainties in the human forcing of the climate. Ships are large, isolated sources of aerosol creating linear cloud formations known as shiptracks. These are an ideal opportunity to identify and measure aerosol-cloud interactions. This work uses over 17,000 shiptracks during the implementation of fuel sulfur content regulations to demonstrate the central role of sulfate aerosol in ship exhaust for modifying clouds. By connecting individual shiptracks to transponder data, it is shown that almost half of shiptracks are likely undetected, masking a significant contribution to the climate impact of shipping. A pathway to retrieving ship sulfate emissions is demonstrated, showing how cloud observations could be used to monitor air pollution.