Am I obligated to finish my post-doc term before starting my tenure-track job? I am currently a postdoc at a research university. I had originally intended to stay at my postdoc for two years, and this was the understanding I had with my postdoc adviser.

However, I decided to speculatively apply for a few faculty jobs, intending to defer any offers for a year. I have now obtained a faculty job offer to start at the end of one year of my postdoc.

After much reflection, I believe that it is better for my family life not to ask for a deferral and to start the faculty job earlier.
In the event that I take the faculty job offer,
I still intend to continue and finish the research work
which I have started with my postdoc adviser,
so I will not be hurting my adviser by starting but not finishing my postdoc work.

Question: Do I have an ethical obligation to stay at my postdoc for two years, because that was my initial plan and was what I told my postdoc adviser?
Should I expect him to be understanding when I choose what is best for my family? 

The following timeline may explain the sequence of events more clearly:


March 2014 I was applying for postdocs, and I told my potential postdoc advisor that I was intending to stay for two years at the university if he were to give me an offer.
April 2014 I was hired with a one-year contract starting August 2014. My adviser explains that there is a good chance my contract will be renewed at the end of the year.
Sept 2014 I decide to apply opportunistically for a few faculty positions just in case I can get a good offer, and intend to ask for a deferral, so that I would only start my faculty job in August 2016.
I told my postdoc adviser that I was applying opportunistically only for a few jobs,
and that if I got a job offer, I would ask for a deferral.
Jan 2015 I get a faculty job offer to start in August 2015.
By this time I changed my mind and feel that starting in 2015 is better for me and my family.


Additional relevant details:


My research area is applied mathematics,
so the only requirements for my research are
a decent computer and an Internet connection.
I can collaborate productively with my postdoc adviser effectively
even if I am not physically in the same location as he is.
I did tell my postdoc adviser that I received a job offer.
However, my postdoc contract has not yet been renewed for the second year,
and we have not yet had a conversation about when to renew the contract.
Congrats on getting a tenure track position! 

You are not the first person to whom this has happened.  Of course, there may be specific circumstances that make the early departure more difficult - say, if you are the only person who can operate a certain instrument in a lab.  But people do leave post-docs early, from time to time - it's not at all unheard of.  And, if you applied opportunistically, you probably have a job that you are happy to take. 

My advice is to break the news to your advisor as soon as possible - once you have made the decision to accept the offer. Your current institution might still be able to find a replacement for next year, which would be good for them. If you reflect on it, it's hard to see real advantages of telling your advisor later.  And there is a good chance they will be happy for you that you got a tenure track position. 

Remember that you will still be in touch your advisor professionally, so you want to try to keep the relationship on the best terms that you can.