Tenured professor wanting to go back to school for a PhD in a different field
Tenure means you are set for life. If you have this kind of thoughts, you may not like the way you are set. A tenured professor has a lot of autonomy (especially in US). They have a lot of freedom in choosing their research. Among the things you can do as a tenured professor is switching fields. I'd advise trying to do that rather than get a PhD. 

As a professor, people already know you are an experienced researcher and/or teacher, so it would be more likely for them to want to start collaborating with you. Plus, you have access to resources unavailable to graduate students. 

What I would do:


Use my contacts (or go to conferences, write emails, whatever) to get in touch with an environmental psychologist. 
Visit your new contact and sketch a work plan. Then see how you can find funding for what you plan to do together. You could also be a visiting professor for a while there and start working on a project you find interesting.
Apply for a grant with your friend and if you get it
Hire a postdoc. You can learn a lot by working with the postdoc.


I would add 5. live happily ever after as an environmental psychologist.

I would not do a PhD again if I were you, especially if it meant quitting my tenured job. Your family might not take it very well if you suddenly decide to forfeit your status as a tenured faculty. I think this will complicate things for you so much that it will offset all the advantages of learning the subject in graduate school.