Although my response to the flossing story is (confession time) more “see I told you” and less “eww, gross…” I can’t agree that it is comparable to climate change. The meta-study found 12 studies totalling 582 people for up to three months. Those studies were inconsistent between finding positive and no effect. Unless your critieria for flossing being worth the oppurtunity cost is “it needs be able to create clear consistent results with in three months with sample size of under 600 people”, this is far from being a finding of no effect. Its a finding that more research needs to be done to find out if it has an effect.
In contrast, climate change has upwards of 3,000 papers published a year, with >90% coming to the same conclusion.