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. Aerosol–cloud interactions represent the leading uncertainty in our ability to infer climate sensitivity from the observationalrecord. The forcing from changes in cloud albedo driven by increases in cloud droplet number (Nd) (the firstindirect effect) is confidently negative and has narrowed its probable range in the last decade, but the sign and strength offorcing associated with changes in cloud macrophysics in response to aerosol (aerosol–cloud adjustments) remain uncertain. Thisuncertainty reflects our inability to accurately quantify variability not associated with a causal link flowing from the cloudmicrophysical state to the cloud macrophysical state. Once variability associated with meteorology has been removed, covariancebetween the liquid water path (LWP) averaged across cloudy and clear regions (here characterizing the macrophysical state) andNd (characterizing the microphysical) is the sum of two causal pathways linking Nd to LWP:Nd altering LWP (adjustments) and precipitation scavenging aerosol and thus depleting Nd. Only theformer term is relevant to constraining adjustments, but disentangling these terms in observations is challenging. Wehypothesize that the diversity of constraints on aerosol–cloud adjustments in the literature may be partly due to notexplicitly characterizing covariance flowing from cloud to aerosol and aerosol to cloud. Here, we restrict our analysis to theregime of extratropical clouds outside of low-pressure centers associated with cyclonic activity. Observations from MAC-LWP (Multisensor Advanced Climatology of Liquid Water Path)and MODIS are compared to simulations in the Met Office Unified Model (UM) GA7.1 (the atmosphere model of HadGEM3-GC3.1 andUKESM1). The meteorological predictors of LWP are found to be similar between the model and observations. There is alsoagreement with previous literature on cloud-controlling factors finding that increasing stability, moisture, and sensible heatflux enhance LWP, while increasing subsidence and sea surface temperature decrease it. A simulation where cloud microphysicsare insensitive to changes in Nd is used to characterize covariance between Nd and LWP that isinduced by factors other than aerosol–cloud adjustments. By removing variability associated with meteorology and scavenging, weinfer the sensitivity of LWP to changes in Nd. Application of this technique to UM GA7.1 simulations reproducesthe true model adjustment strength. Observational constraints developed using simulated covariability not induced by adjustmentsand observed covariability between Nd and LWP predict a 25 %–30 % overestimate by the UM GA7.1 in LWP changeand a 30 %–35 % overestimate in associated radiative forcing.

Untangling causality in midlatitude aerosol–cloud adjustments