Uncertainty in aerosol-cloud radiative forcing is driven by clean conditions

Abstract

Abstract. Atmospheric aerosols and their impact on cloud properties remain the largest uncertainty in the human forcing of the climate system. By increasing the concentration of cloud droplets (Nd), aerosols reduce droplet size and increase the reflectivity of clouds (a negative radiative forcing). Central to this climate impact is the susceptibility of cloud droplet number to aerosol (β), the diversity of which explains much of the variation in radiative forcing in global climate models. This has made measuring β a key target for developing observational constraints of the aerosol forcing. While the aerosol burden of the clean, pre-industrial atmosphere has been demonstrated as a key uncertainty for the aerosol forcing, here we show that the behaviour of clouds under these clean conditions is of equal importance for understanding the spread in radiative forcing estimates between models and observations. This means that the uncertainty in the aerosol impact on clouds is, counterintuitively, driven by situations with little aerosol. Discarding clean conditions produces a close agreement between different model and observational estimates of the cloud response to aerosol, but does not provide a strong constraint on the radiative forcing from aerosol-cloud interactions. This makes constraining aerosol behaviour in clean conditions a key goal for future observational studies.

Aerosol Effects on Water Cloud Properties in Different Atmospheric Regimes

“…Under the context of poorly characterized ACI process in global climate models (GCM), our findings provide useful information to improve ACI process in GCM. As revealed from the literature review, Twomey effect is considered while parameterizing ACI process in GCM (e.g., Gryspeerdt et al., 2023; Zelinka et al., 2014). However, our study underscores the necessity of considering atmospheric regime dependent ACI process (Twomey effect and Anti‐Twomey effect) in GCM to reduce uncertainties related to aerosol led impacts on present climate system, hydrological cycle, and future climate projection.…”

Frontiers in Satellite‐Based Estimates of Cloud‐Mediated Aerosol Forcing

“…Furthermore, since aerosol effects on clouds are relative to the fractional change in aerosols, small non‐discernible changes in absolute aerosol amounts in very clean situations can be large fractional changes that can lead to correspondingly large differences in cloud properties. This uncertainty in the aerosol signal and hence CCN in low aerosol (clean) conditions drives the diversity of observational estimates of the N d susceptibility to aerosol, as well as providing a significant source of variation in GCM estimates of the ERF aci (Gryspeerdt et al., 2023).…”

A New Method for Size‐Resolved Aerosol CCN Activity Measurement at Low Supersaturation in Pristine Atmosphere

“…The atmospheric radiative budget is influenced by aerosol particles more easily in pristine atmosphere than in polluted regions, and the indirect aerosol effect is particular sensitive to the CCN abundance (Carslaw et al., 2013, 2017; Fan et al., 2018; Garrett & Zhao, 2006; Sherwood, 2002; Zaveri et al., 2022; C. Zhao et al., 2020). Therefore, understanding the impact of the chemical‐physical properties of aerosols on the radiation transfer and cloud microphysics in pristine atmosphere is essential for accurately assessing the aerosol climate forcing and improving future climate predictions (Andreae et al., 2018; Carslaw et al., 2013; Gryspeerdt et al., 2023; Gulev et al, 2021). Moreover, a pristine atmosphere may experience accelerated changes (2–3 times faster) under global warming (Andreae et al., 2018; Blunden & Arndt, 2019; Chapman & Walsh, 1993; Serreze & Barry, 2011; Zábori et al., 2015), and aerosol‐cloud interactions likely play a significant role (Burkart et al., 2017; Chang et al., 2022; Garrett & Zhao, 2006; Lubin & Vogelmann, 2006; Schmale et al., 2021).…”