How strongly have human-made aerosols cooled the climate?
In plain words
Pollution particles reflect sunlight and make clouds brighter, offsetting part of the warming from greenhouse gases. The size of this offset is poorly known, which blurs what past warming says about future warming.
Precise statement
The industrial-era total aerosol effective radiative forcing has a 68 percent range of about $-1.6e3 \text{ to } -0.65e3\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}^2)$ (Bellouin and coauthors, Reviews of Geophysics 2020, https://doi.org/10.1029/2019RG000660), dominated by aerosol-cloud interactions (droplet number, liquid-water-path and cloud-fraction adjustments). Determine this forcing with a 68 percent range narrower than about $4e2\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}^2)$. An answer is a value supported by process-level observations of cloud adjustments and consistent with the historical and interhemispheric temperature record.
What would settle it
Satellite and field measurements of cloud liquid-water and cloud-fraction responses to known aerosol perturbations (ship tracks, volcanic eruptions, emission cuts) used to constrain models.
Status in the literature
Unverified note
As of 2024 to 2025 the 2020 cut in shipping-fuel sulfur is being analyzed as a natural experiment for aerosol-cloud forcing.