An Atmospheric General Circulation Model (AGCM) is coupled to three land surface schemes (LSSs) individually and in combination (i.e., LSSs receive the same AGCM forcing and the average surface fluxes are passed back to the AGCM) to study the uncertainties of simulated climatology and variability caused by different LSSs. The three LSSs produce significantly different surface fluxes over most of the land, no matter whether they are coupled individually (different forcing to land) or in combination (same forcing to land).