Arctic Sea Ice Volume in June - 1980-2012
The loss of the Arctic ice throughout the summer would have a warming effect roughly equivalent to all human activity to date. That is to say, with the ice gone in summer, the planet would have an additional heating effect just as large as the heating effect of all human CO2 and other greenhouse gasses to date.

Figure 8 - Arctic sea ice volume in June tells a much worse story than coverage alone. June ice volume in 2012 was less than half of the ice volume seen in 1980. June ice volume loss appears to be accelerating.
Accelerated Warming The second thing to fear about loss of Arctic sea ice is the potential to accelerate climate change on a global basis. A black object gets hotter in the sun than a white object. That much is common sense. Earlier, in describing how melting ice accelerated the melt of more ice, I talked about the fact that dark sea waters absorb up to 90% of the sun’s energy that strikes them, while snow-covered ice absorbs only 10 to 20% of that same energy. The exposure of darker waters speeds up heating of the Arctic, and thus the loss of more ice. But the impact is larger than that. And indeed, it’s large enough to make a difference on a global scale. In June, the Arctic ice cap covers around 2% of the Earth’s surface – around 11 million square kilometers of Arctic ice cap out of a total of 510 million square kilometers of Earth’s land and oceans. And that 2% of the Earth’s surface, for a period of roughly two months, receives more solar energy per day than even the sunniest areas on the equator. Analyzing this, Peter Wadhams of the Global Oceans Physics Program at Cambridge calculates that the loss of the Arctic ice throughout the summer would have a warming effect roughly equivalent to all human activity to date. That is to say, with the ice gone in summer, the planet would have an additional heating effect just as large as the heating effect of all human CO2 and other greenhouse gasses to date. In other words, the complete meltdown of the Arctic could roughly double the rate of warming of the planet as a whole.
URL:     http://blogs.scientificamerican.com/guest-blog/2012/09/21/arctic-sea-ice-what-why-and-what-next/
(The text for the image(s) on this Web page was taken from the above source.)