Post physics PhD, how could I go about working in ecology, environmental science or earth sciences?
How likely is it that I could make such a large career shift?


It is very common for people who studied physics to end up doing something rather different.  Studying physics is great preparation for many different endeavors.


  How do I go about finding out more about what research is out there and what I might like specifically, given the very large quantity of research that exists??


Fun ways to do this are to attend seminars and take a look at journals in the target fields.  At this point you're mainly getting your feet wet and getting a few ideas about what's out there, and what the culture of those other disciplines is like.


  Does anyone know of any examples of people who have made similar shifts? (Partly just because right now I need the inspiration and belief I am not trapped!)


My advisor, Akira Okubo, is a good example.  You might enjoy reading this tribute and overview of his life and work: http://www.yen.biology.gatech.edu/papers/Okubo%20tribute.pdf


  What sort of things might I do directly after my PhD to build experience/learn more? Is trying to get field experience a good idea? Should I start to consider Master's programs?


Perhaps the shift would be less drastic if you started out by making a slight sideways move into environmental science.  But that might not even be necessary.

I think you can go straight into a two-year postdoc after you graduate.  As long as you choose the right sort of person or group to do it with, I don't think you'll need to do another degree or more coursework first, because of your undergrad broad span of sciences and the lab biology projects.  If you find you have some gaps, you can fill them on your own or by auditing some classes while you're in the postdoc.


  Am I completely as doomed as I think I could be?


There are some interdisciplinary fields, such as mathematical ecology, that are new enough that many of the people in those fields originally trained in something else.

Final comment: the tail end of a PhD can be a desolate time, and a stressful time.  So please take this growing ennui with a grain of salt.