什么是 Radiative forcing？

The radiative forcing of a change in the climate system — more greenhouse gas, more aerosol particles, a brighter Sun — is the change it causes in the net flow of energy entering the climate system at the top of the atmosphere, before the surface temperature has had time to respond. It is measured in watts per square metre ($\mathrm{W}/\mathrm{m}^{2}$) of the Earth’s surface. A positive forcing makes the planet gain energy and warm; a negative forcing makes it lose energy and cool.