CSIR NET Dec 2015 Q128: Meteorology

While Earth's climate has been warming over recent decades, several surface cooling components/mechanisms partially offset this warming. Cloud radiative effects can be either warming or cooling depending on cloud type, altitude, and optical thickness: optically thin, high-altitude cirrus clouds tend to have a net warming effect (they transmit most incoming shortwave solar radiation but trap outgoing longwave radiation, acting like a greenhouse blanket), whereas optically thick, low-altitude clouds such as stratocumulus have a strong net cooling effect, since they are highly reflective to incoming shortwave solar radiation (high albedo) while being warm enough (low cloud-top height) that their longwave trapping effect is comparatively weak. The presence of optically thick stratocumulus clouds is therefore a genuine surface cooling mechanism, matching option 4.

Option 1 (water vapor-greenhouse effect feedback) is a well-known positive (warming-amplifying) feedback, not a cooling mechanism, since increased water vapor from warming enhances the greenhouse effect further. Option 2 (ice-albedo feedback) is also a positive (warming-amplifying) feedback in the context of ongoing warming: as ice melts, albedo decreases, absorbing more solar radiation and further warming the surface, rather than cooling it. Option 3 (optically thin cirrus clouds) actually contributes a net warming effect (greenhouse trapping dominates over their limited reflectivity), not cooling. Option 4 is correct: optically thick, low stratocumulus clouds reflect substantial incoming solar radiation and provide a genuine net surface cooling effect, counteracting some of the overall warming trend.