Well, depending on where you are you can also vote for the local green party. If you're in a proportional election system then that the bigger the green voting block is the better. If you're in a country with a first-past-the-post electoral system like the US, UK, or Australia then things are much tougher and you should also be campaigning for electoral reform to make your country more democratic.

Assuming your local green party representative doesn't think that shutting down a safe nuclear power plant has higher priority than CO2/Methane...

The UK had a referendum on introducing a different voting system and there was no change as the general view seemed to be that "first past the post" gave strong governments.

Yeah, I wouldn't call the current British government strong but I would call the past six NZ governments strong. Each of those governments have been made up of at least three parties working together.

The Australian electoral system is first past the post too and is similarly as chaotic as the UK.

Note Australia is not first past the post. It is preferential.

In practice this probably means you feel more like your vote matters, but eventually goes to one of the major parties.