Which statement best defines radiative forcing?

• A. Radiative forcing is the emission rate of solar energy.
• B. Radiative forcing is the cooling effect of aerosols.
• C. Radiative forcing is the change in net energy balance at the top of the atmosphere due to a forcing agent, measured in W/m^2; e.g., increased CO2 raises positive forcing and warms the climate.
• D. Radiative forcing is a measure of heat absorbed by groundwater.

Answer: (C)

Radiative forcing measures how a perturbation changes the Earth’s energy balance at the top of the atmosphere, expressed as the net change in energy flux in watts per square meter. It captures the difference between the energy coming in and going out after a forcing agent acts, so a positive value means more energy is retained and the climate tends to warm, while a negative value means cooling. For example, increasing CO2 raises positive forcing, because the extra greenhouse gas traps more longwave radiation, leading to a warmer climate. The other descriptions don’t capture this net, TOA-centered balance: one option describes only the rate of solar energy arriving, not the resulting net balance; another focuses only on a cooling effect of aerosols, whereas radiative forcing includes both warming and cooling effects depending on the agent; and the last option refers to groundwater heat, which is unrelated to this atmospheric energy balance.