Having read various pieces of documentation, it's still not completely clear to me what dictates the difficulty rise, and how Ethereum difficulty levels differ to Bitcoin.

In the past week difficulty has jumped from around 11 to over 17, whilst GH/s has fluctuated between 800.00 - 1400.00 but with no clear correlation between the two.

Has the "difficulty bomb" now been introduced? And if yes, is there any way for approximating or calculating how much difficulty will likely rise by in the next 6-12 months?

Related: If Serenity / PoS is unlikely to be introduced until early next year, has there been any suggestion from the dev team that the effects of this "difficulty bomb" will be reduced in either the homestead or metropolis releases?