It's a reduced relative clause in which "open" is used as an adjective. Here's the full thing: "We should build an environmental governance system *that is* open to all sectors of society,". As is often the case, I prefer the reduced version, as succinctness is a virtue.

hey Shana. Here is an example: Please fill in the blank ” The University is open ….. overseas students, primarily from American universities, who may enroll in study abroad programs during the summer months. “ Here we can use both for/to (but for is better in this particular case) University is open for/to overseas students. But the meanings will be slightly different: for: we are accepting overseas students right now, we are ready and we are waiting to: in general – our school accepts student from overseas (that means not only students from one particular country, but also from different countries) I hope it helps :)

For my part, “open” is an adjective used to qualify the system.

The meaning of open in this context is accessible so its describing the system.