Without greenhouse effect, the average temperature of earth surface would be

The correct answer is B) -18°C. This is the standard figure in NCERT-level and NASA-style explanations. It comes from a simple energy balance. Sunlight reaching Earth (solar constant about 1361 W/m², albedo about 0.3) is balanced by infrared radiation leaving Earth. That balance gives an effective radiating temperature of about 255 K, which is roughly -18°C.

The greenhouse effect is what lifts the actual average surface temperature to about 15°C, a rise of roughly 33°C. Water vapour, CO2, methane and other gases absorb outgoing infrared and re-radiate part of it back to the surface.

• A) 0°C is the freezing point of water. It is a tempting but unrelated reference point.
• C) S°C is not a usable physical value. It looks like a misprint in the paper, and no standard estimate matches it.
• D) -20°C is close but not the accepted value. Stick to -18°C.

Core concept: radiative equilibrium and the greenhouse effect. For Mains (GS III, environment), this is the base for explaining why greenhouse gases are necessary for life, and why enhanced greenhouse gas concentrations cause warming.

Trap: students who remember that the greenhouse effect is 'bad' sometimes pick a number that is too low or too high. Remember that the natural greenhouse effect is what keeps Earth about 33°C warmer than it would otherwise be.