*** ANECDOTE WARNING ***
In a completely non-scientific study, I have just been for a walk on my lunch break. In the middle of December, in the south of the UK in a short sleeved shirt. This has led me to believe that it is considerably warmer at this time of year than it used to be, as I remember fondly the winters of my youth being warm jackets and skidding around on patches of ice.
This "winter" I have worn my warm jacket once, we had a cold snap, it lasted a few days it is now back to being about the same weather that it was in early October.
No science here, only opinion.
I don't -know- what is 'causing' it, it might be natural, I feel with the way the human race treats things (other humans, other animals, the planet) I would not be at all surprised, if somehow, it is our fault.