Which of the following is an indirect effect of aerosols on climate?

• A. Reflecting sunlight to cool surfaces (sulfate).
• B. Absorbing sunlight to warm surfaces (black carbon).
• C. Altering cloud properties and lifetimes (indirect effect).
• D. They have no effect on climate.

Answer: (C)

Aerosols influence climate not only by directly interacting with sunlight but also by changing clouds. The indirect effect happens when aerosols act as cloud condensation nuclei, producing clouds with more numerous, smaller droplets. This makes clouds brighter (higher albedo) and can extend their lifetimes, influencing how much solar energy is reflected back to space and how long rain might be suppressed or altered. In short, aerosols change cloud properties and how long clouds persist, which then feeds back into the climate system. That’s why altering cloud properties and lifetimes is the indirect effect. The other ideas describe direct interactions with radiation, or claim no effect, which doesn’t capture the indirect pathway.