What is Earth’s energy imbalance; climate feedback parameter?

Earth’s energy imbalance $N$ is the net flux of energy into the climate system at the top of the atmosphere, averaged over the globe and over a year or more, in $\mathrm{W}\,\mathrm{m}^{-2}$: absorbed sunlight minus outgoing infrared. It is the rate at which the ocean, the land, the ice and the air gain heat, per square metre of the Earth’s surface.