Effective radiative forcing (ERF) is evaluated in a General Circulation Model (GCM) with fixed land and sea-surface-temperatures as well as sea-ice. The 4xCO2 ERF is 8.0 Wm-2. In contrast, a typical ERF experiment with only fixed sea-surface-temperatures (SST) and sea-ice gives rise to an ERF of only 7.0 Wm-2. This difference arises due to the influence of land warming in the commonly used fixed-SST ERF experimental design, which results in: (i) increased emission of longwave radiation to space from the land surface (-0.45 Wm-2) and tropospheric temperatures (-0.90 Wm-2), (ii) reduced land snow-cover and albedo (+0.17 Wm-2), (iii) increased water-vapour (+0.49 Wm-2), and (iv) reduced stability and cloudiness over land (increased ERF), and increased lower tropospheric stability and marine cloudiness over oceans (reduced ERF) (global-mean cloud adjustment -0.26 Wm-2). The sum of these radiative adjustments to land warming is to reduce the 4xCO2;ERF in fixed-SST experiments by ~1.0 Wm-2. CO2 stomatal effects are quantified and found to contribute just over half of the land warming effect and adjustments in the fixed-SST ERF experimental design in this model. The basic physical mechanisms in response to land warming are confirmed in a solar ERF experiment. We test various methods that have been proposed to account for land warming in fixed-SST ERFs against our GCM results and discuss their strengths and weaknesses.