Proton conductance in fresh and aged Nafion ionomer membranes has been studied using current sensing atomic force microscopy which measures the proton current flow from a nanometer-sized tip in contact with the membrane surface to an electrode attached to the other side of the membrane. By scanning the tip across the membrane, the measured conductance maps out the local proton conductance variations, revealing the distributions of ionic domains on the membrane surface. For fresh Nafion membranes, the local proton conductance always displays a single-peaked distribution, while, for membranes after being aged for about three years in an ambient environment, the local proton conductance shows a distribution with two peaks, one at high current corresponding to the variation of contact and the conductive path of proton current and the other at near zero current which corresponds to the appearance of large nonconductive domains on the membranes. A detailed analysis reveals that the aging causes significant changes in the ionic domains or channels exposed on the membranes' surfaces.