= Solution
For a liquid <cloud> at fixed <water> content and geometrical thickness, more <cloud condensation nuclei> can produce more, smaller droplets. The increased total extinction area raises <optical depth> and reflected sunlight: the <Twomey effect> gives \b[negative shortwave <radiative forcing>]. It is an indirect <aerosol> effect mediated by <cloud> microphysics, rather than direct <aerosol> scattering.<The Influence of Pollution on the Shortwave Albedo of Clouds by S. Twomey>[Journal of the Atmospheric Sciences 34 (1977), pp. 1149–1152, fixed-water-content droplet-number and reflectance effect]{ref}
Smaller droplets can also delay <collision-coalescence> and drizzle, potentially increasing liquid-water retention, <cloud> lifetime or coverage; that adjustment would usually strengthen cooling for low reflective <clouds>. It is not universal: evaporation and mixing with dry air can reduce <water> content or <cloud> cover, and dynamical changes can oppose the simple lifetime argument. Absorbing particles can heat an atmospheric layer, sometimes evaporating <cloud> and producing a positive semi-direct forcing, but their vertical position can instead stabilize a <cloud> layer and reverse the response. <Aerosols> can additionally alter ice nucleation and mixed-phase precipitation, with no single forcing sign independent of <cloud> type. The net response combines shortwave reflection and longwave thermal effects, and depends on <cloud> height, <optical depth> and adjustments. Thus \b[the fixed-water Twomey sign is robust within its assumptions; adjustment and ice-cloud signs require context].<The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity by Piers Forster and others>[IPCC AR6 Working Group I, Chapter 7, <aerosol>–<cloud> forcing and cloud-adjustment assessment]{ref}
Back to article page
