How much faster has Earth warmed since the Arctic Ocean had less and less ice?

Albedo is the fraction of solar energy (shortwave radiation) reflected from the Earth back into space. It is a measure of the reflectivity of the earth's surface. Ice, especially with snow on top of it, has a high albedo: most sunlight hitting the surface bounces back towards space.

How much faster will Earth heat when the Arctic Ocean turns black with oil?

Dutch Royal Shell is not ready for what is coming and neither is the rest of humanity. Paper towels doesn't work on ice. There is no enforcement of 'clean up standards' that will matter. 

There is no cleaning up the Arctic Ocean. It is not the Gulf of Mexico. Dutch Royal Shell probably doesn't even realize it's operations in the Gulf can't be repeated in the Arctic.

The spills will occur during the summer months and then it will be abandoned to cold temperatures. The life within the Arctic Ocean doesn't go on vacation over the winter.

The oil spilled in the Arctic Ocean will ultimately make it's way to the North Atlantic between Greenland and Canada. The fisheries won't exist because of genetic deterioration when exposed to oil.