Ships spray fine sea-salt aerosol into the marine boundary layer. Those particles act as extra cloud condensation nuclei (CCN): the same liquid water in the stratocumulus deck now gets divided among more, smaller droplets, which is the Twomey effect. More, smaller droplets scatter sunlight more efficiently, so cloud albedo rises without adding any water to the cloud.
Nd = Nd0 + Nmax·(rate·coverage)/(K + rate·coverage)
∂A/∂ln(Nd) = ⅓·A·(1−A) (Twomey susceptibility)
ΔF = −(A − A0)·S0 (regional shortwave forcing)
dT/dt = (T0 + λ·ΔF − T)/τ (mixed-layer relaxation)
- Fleet spray rate — how much sea-salt aerosol mass the ships inject per unit time; more spray means more CCN, but with diminishing returns once droplets are already numerous (saturating Michaelis–Menten term).
- Seeded area coverage — fraction of the target ocean patch actually reached by the spray plumes; a small, concentrated fleet cannot brighten a whole basin.
- Twomey effect — because albedo depends logarithmically on droplet number, the first doses of aerosol brighten the cloud far more than later ones — the curve visibly flattens as Nd climbs.
- Overseeding demo — cranks spray rate to an extreme to show the flat top of the logistic albedo curve: past a point, more aerosol buys almost no extra cooling while still costing fuel and risking suppressed rainfall / drizzle under the seeded patch.
Real-world relevance: MCB is the leading "fast, reversible, regional" solar radiation management proposal — unlike stratospheric aerosol injection it targets low marine clouds directly and its effect switches off within days of stopping, but its efficacy is capped by exactly this saturation and by how much ocean area a realistic fleet can actually cover.