Ultimately I'm not sure it matters if we detect a supernova that will kill us within a year. That's not enough time to do anything about it, but enough time for worldwide panic and massive suffering before we all die anyway.
We'd really need to know at least 10-20 years out. Enough time to rally the world to find a solution but not enough time that it ceases to be a concern (like thousands of years).
That's an interesting thought. Severe consequences of climate change can be expected within decades, but apparently that is still to far off and hard to imagine for many people, and hard to sell for politicians. This leaves me wondering if people would accept decades of worsening quality of life (moving into caves etc.) even if the alternative was certain death by gamma rays.
I think the climate change thing has little to do with how far off it is. It's just that the people who stand to benefit from not mitigating climate change are both powerful politically and wealthy enough that the effects, even if they happened much sooner than predicted, wouldn't likely effect them. Sure, they may lose one of their homes on the coastline. The mass migration away from coastlines will be devastating to humanity, but not to those people or their children. Heck, even worst case scenarios like massive social unrest or resource wars aren't likely to effect these people. Their kids certainly won't be going off to war!
In other words, it's not a problem of being able to imagine the effects. I suspect the powerful people resisting climate change mitigation can imagine the effects, they just know those effects won't touch them.
We'd really need to know at least 10-20 years out. Enough time to rally the world to find a solution but not enough time that it ceases to be a concern (like thousands of years).