Global Dimming & Brightening: Aerosol Masking of Greenhouse Warming

Year

Year 1980

1950aerosol optical depth2020

Aerosol

Type

Override with manual optical depth

Optical depth (manual) 0.085

Readouts

Surface solar radiation— W/m²

Net radiative forcing— W/m²

Implied ΔT vs 1750— °C

How it works

Two competing effects drive global-mean temperature. Greenhouse gases add a logarithmic forcing as CO₂ rises above its 1750 baseline of 280 ppm:

ΔF_GHG = 5.35 · ln(CO₂ / 280)   [W/m²]

Aerosols do the opposite: sunlight passing through a haze layer of optical depth τ is attenuated by Beer–Lambert extinction, and the scattered/absorbed fraction depends on the single-scattering albedo ω₀ (near 1 = pure scattering, e.g. sulfate; well below 1 = absorbing, e.g. soot):

Transmission  T = e^(−τ)
Surface solar  = (S₀/4) · T,   S₀ = 1361 W/m²
ΔF_aerosol ≈ −F₀ · τ · (2ω₀ − 1)

When ω₀ is close to 1 the aerosol layer mostly reflects sunlight back to space and cools the surface — this is the historical "global dimming" measured by sunlight sensors from the 1950s onward, at its strongest in the 1970s–80s as sulfate pollution from coal and oil peaked. Clean-air laws (US 1970, EU/China later) cut SO₂ emissions and optical depth fell, so more sunlight reached the ground again — "global brightening" — which unmasked greenhouse warming that had been partly hidden underneath it the whole time. When ω₀ drops below about 0.5 (soot-heavy smoke, which absorbs strongly), the sign flips and the aerosol layer adds warming instead of cooling.

Year slider / Play — steps through a schematic 1950–2020 history of atmospheric CO₂ and aerosol optical depth built from published dimming/brightening and ice-core/Mauna Loa records.
Aerosol type — sets ω₀: sulfate ≈0.98 (near-pure scattering), soot ≈0.55 (strongly absorbing), mixed ≈0.85.
Manual optical depth — overrides the year-driven history so you can test any haze thickness directly.
Falling particles are photons: yellow ones punch straight through to the ground, pale ones scatter back out of the haze layer, and dark red ones are absorbed inside it and fade — the ratio between the three tracks T and ω₀ live.

Real-world relevance: the IPCC's best estimate of present-day aerosol effective radiative forcing is roughly −1.1 W/m² (net cooling), meaning aerosols have offset a meaningful share of the ~2.9 W/m² added by greenhouse gases since 1750 — one reason warming has not yet caught up with the full committed forcing.

1980

Transmitted photon Scattered (reflected) Absorbed (heat)

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