Sea ice melts during the summer when solar radiation heats the ice surface. The amount of solar radiation absorbed by the ice depends on surface albedo, a characteristic described in the previous section (see Albedo).
Sea ice reflects about half of the incoming solar radiation. This reflection prevents the ice from warming up as quickly as open ocean would, but radiation can nonetheless heat the ice enough to initiate melting. Thick sea ice covered with snow reflects even more radiation, approximately 90%. As a result, sea ice with snow takes even longer to melt. After the snow starts to melt, melt ponds form, and because water has a lower albedo than snow, the surface albedo of sea ice with snow and melt ponds drops to about 0.75 (75% solar radiation reflected). As the melt ponds grow and deepen, the albedo continues to decrease, leading to higher absorption of solar radiation and an increased rate of melting.
Energy to melt ice can come from sources besides direct solar energy. Water that is under the ice and that has a temperature above the freezing point causes the bottom surface of the ice to melt. Warm surface waters cause the edges of the ice to melt, particularly in leads and polynyas. For more on these formations, see Features in the Characteristics section.
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