Actions that work in the possible world in which it collapses soon are actively harmful in possible worlds in which it doesn’t. Acting as if a threat will happen only makes sense if the action isn’t significantly harmful in cases where it doesn’t, where significantly is based on the harm of not being prepared and the chance of it happening.
If the Gulf Stream will collapse by 2025, the response isn’t to be more eco-friendly. In fact, it’s the opposite. Everyone in the north should prepare to burn a lot more fuel, and concern for global warming would definitely be reduced. Global warming is something you can only afford to give a shit about when temperatures haven’t just dropped by 3.5C and you haven’t just lost 78% of your arable land (UK figures, because that’s where I live).