Changing fields after PhD, master or post-doc?
I don't expect this to be the best answer (I am not from these fields), however what I'd do in your place would be researching websites of environmental science departments and research groups in order to find some topics/projects for which your qualification up to now makes some sense or adds some relevant competence. There even may be postdoc positions, however maybe you can ask for some project or internship work. Maybe also there are some biologically oriented temporary teaching jobs in such departments, which could enable you to get into the community.
In the meantime try to read and learn, and try to get into contact with people. I can't guarantee it of course but sometimes opportunities open up when looking for them in a determined way. And surely try to make use of your already achieved qualifications, try to connect things somehow.