Surface Reflectivity

Sea ice covered with snow reflects about 85-90% of sunlight, while ocean water reflects just 10%. Thus, as sea ice melts, revealing more and more of the ocean beneath, the increasing absorption of solar radiation adds to global warming, which causes more melting, which in turn causes more warming, and so on…

Source: ACIA Impacts of a Warming Arctic: Arctic Climate Impact Assessment  (2004),
 Key Finding #2, p.34