Ice sheets and glaciers have been observed to respond rapidly to climate warming. Cryohydrologic warming, a new mechanism proposed by Phillips et al., could explain why. Meltwater can flow through networks of crevasses and fractures that develop or expand within glaciers as the climate warms. Some liquid water can remain in a glacier without refreezing after the summer melt season ends. Heat exchange between this englacial water and the ice can accelerate warming of ice sheets, the authors explain.