I think that's a tough question. Here's how I justify it to myself, and I think most people share these values. You can tell they share them because they don't

I think that's a tough question. Here's how I justify it to myself, and I think most people share these values. You can tell they share them because they don't want to die or suffer. If you grant that conscious existence has value, then the continued existence of the human race is worthwhile. If you grant that life has value, then preventing the destruction of eath's environment is worthwhile. If you don't value those things, it's difficult to make a compelling argument. However, I would argue that consciousness matters because it's the only thing that can create "meaning" to existence. And life matters because it's the only thing (we know of) that produces consciousness.