Columbia subject headings:
Harvard subject headings:
Environmental policy--Europe--History--Congresses.
Environmental policy--History--Congresses.
Environmental protection--Europe--History--Congresses.
Environmental protection--History--Congresses.
Princeton subject headings:
Environmental policy--Europe--History--Congresses.
Environmental policy--History--Congresses.
Environmental protection--Europe--History--Congresses.
Environmental protection--History--Congresses.