Currently the human race has only one planet. Earth. It's biosphere must sustain 100% of human life; long term survival outside of the biosphere is not possible. If that biosphere undergoes a shift that does not favor human life than the volume of human life that can be supported will be reduced. Perhaps by as much as 100%.

The cause of shifts in the type of life favored by the biosphere are largely academic. What matters is that our considerable scientific and industrial might be mustered and directed toward conserving a biosphere that is favorable to our species. 

I do not use the word conserve by accident. I am a conservative woman and I look around at culture, at society, at economic and fiscal systems and I see many things worth conserving. But I also look at the planet and I see something that must be conserved; oceans must be conserved, forests must be conserved, the ozone layer must be conserved and a certain temperature range within which humans can exist must also be conserved. 

Perhaps there will come a day when our survival as a species is no longer dependent on one planets biosphere. But we have not yet arrived at that time. For now control over the environment must be seized and it must be locked into place and held there until the human race has advanced beyond it's dependency on a single biosphere.