| Marine low clouds reflect solar radiation and cool the Earth
as a result (Hartmann and Short, 1980; Ramanathan et al.,
1989). The solar radiation reflected by marine low clouds
(albedo) increases with the amount of liquid water they contain and as the size of cloud droplets decreases (Stephens,
1978). Twomey (1974, 1977) showed that, for a fixed liquid
water path (LWP), cloud albedo increases with the concentration of cloud droplets (Nd). Thus anthropogenic aerosol pollution increases cloud albedo and cools climate. A total of 4
decades of subsequent research has established the “Twomey
effect” as being the largest contributor to the overall cooling
impact of aerosols on climate (Zelinka et al., 2014; Bellouin
et al., 2020).
In recent decades, evidence showing cloud macrophysical adjustments to aerosol increases has mounted. Albrecht (1989) suggested that reduced droplet sizes would
lead to suppressed collision–coalescence, greater retention of
water, and an augmentation of the Twomey effect. Modeling and observations both show precipitation suppression by
aerosol in warm clouds (Ackerman et al., 2004; Sorooshian
et al., 2010; Terai et al., 2015), and yet observations of ship
tracks (Coakley and Walsh, 2002; Toll et al., 2019), pollution
plumes (Toll et al., 2019; Trofimov et al., 2020), and largescale shipping lanes (Diamond et al., 2020) reveal LWP reductions in the mean. Modeling has shown that aerosols can
cause both positive and negative LWP adjustments (Ackerman et al., 2004; Wood, 2007), with the sign of the change
dependent on meteorological and aerosol conditions. Reduced LWP stems from increased cloud-top entrainment of
dry free-tropospheric air due to smaller cloud droplets and/or
turbulent invigoration of the boundary layer caused by suppressed precipitation (Wang et al., 2003; Ackerman et al.,
2004; Bretherton et al., 2007; Wood, 2007). A recent paper
by Glassmeier et al. (2021) illustrates that the sign of LWP
adjustments depends not only on the meteorological conditions but also on the number of aerosol particles, which cause
positive adjustments when the aerosol number is small, and
precipitation suppression increases the condensate retention.
Negative adjustments are found when the aerosol number is
large, due to the aforementioned entrainment drying. Studies using shipping and land-based pollution sources suggest
that mean LWP decreases may offset the Twomey response
to a degree that ranges from 3 % (Trofimov et al., 2020) to
perhaps 20 % (Toll et al., 2019; Diamond et al., 2020). LWP
adjustments in low clouds are poorly handled in large-scale
models (Malavelle et al., 2017), which almost universally
show LWP increases in simulations of anthropogenic aerosol
impacts (Lohmann and Feichter, 2005; Isaksen et al., 2009;
Bellouin et al., 2020). Global models also tend to show cloud
cover increases in response to aerosol, but these appear to be
small compared with the Twomey responses and LWP adjustments (Zelinka et al., 2014). Cloud cover adjustments are
difficult to constrain using observations (e.g., Gryspeerdt et
al., 2016; Possner et al., 2018) |