What is an indirect effect of aerosols on climate?

• A. Aerosols act as cloud condensation nuclei, altering cloud reflectivity, lifetime, and precipitation
• B. They directly heat the surface
• C. They reduce ozone
• D. They permanently remove moisture from air

Answer: (A)

The indirect effect being tested is that aerosols change climate by altering clouds. When aerosols act as cloud condensation nuclei, they change cloud microphysics: there are more tiny cloud droplets, which makes clouds brighter (more reflective) and can also affect how long clouds last and how rain forms. This cloud-mediated change in the planet’s energy balance is how aerosols indirectly influence climate.

So, aerosols acting as cloud condensation nuclei, and thereby changing cloud reflectivity, lifetime, and precipitation, best captures this indirect pathway. The other options describe direct heating, chemical changes to ozone, or moisture removal—mechanisms that aren’t the indirect cloud-mediated effect in question.