Melting of white Arctic ice, currently at its lowest level in recent history, is causing more absorption. Thirty years ago there was typically about eight million square kilometres of ice left in the Arctic in the summer, and by 2007 that had halved, it had gone down to about four million, and this year it has gone down below that. The volume of ice in the summer is only a quarter of what it was 30 years ago and that's really the prelude to this final collapse.
The polar ice cap acts as a giant parasol, reflecting sunlight back into the atmosphere in what is known as the albedo effect. But white ice and snow reflect far more of the sun's energy than the open water that is replacing it as the ice melts. Instead of being reflected away from the Earth, this energy is absorbed, and contributes to warming.