Melting produced at the surface of subfreezing permeable firn does not freely percolate but instead refreezes near its point of origin, raising the firn temperature and decreasing the pore space. If sufficient meltwater is introduced, the firn will warm to 0 ¿C, and subsequent water will fill the remaining pore space without freezing. Only after the residual water content of the firn is exceeded will water flow through the firm and possibly escape as runoff. This process prevents summer melt on high Arctic glaciers and ice caps from escaping in its entirety, and it must be accounted for in modeling glacier runoff on the basis of surface energy balance. A model is presented here which describes in a simple way the transient process of infiltration, refreezing, and runoff in a future warming climate. The model is applied to Greenland, for which predictions of runoff-induced sea level rise that do not consider the refreezing process could be as much as 5.0 cm too high over 150 years. ¿ American Geophysical Union 1991