Premelting increases the rate of regelation by an order of magnitude

Glacier sliding over small obstacles relies on melting on their upstream sides and refreezing downstream. Previous treatments have appealed to 'pressure melting' as the cause of the spatial variations in melting temperature that drive this regelation process. However, we show that typical liquid pressure variations across small obstacles are negligible and therefore variations in ice pressure closely approximate variations in effective stress. For a given change in effective stress, the equilibrium melting temperature changes by an order of magnitude more than when the pressure of ice and liquid both change by an equal amount. In consequence, the temperature gradients that drive heat flow across small obstacles are larger than previously recognized and the rate of regelation is faster. Under typical conditions, the transition wavelength at which ice deformation and regelation contribute equally is of m-scale, several times longer than previous predictions, which have been reported to underestimate field inferences.