Make an objective scientific argument in favor of the survival of the human animal as a species.
For bonus points, make sure that you do not co-incidentally argue for the preservation of all species on the planet.
All existence is subjective. I think therefore I reach. I don't really care about objectivity, because I'm not objective.
But, if I were to try, I guess I'd base it on the existence of self awareness. Science cannot exist without the self-aware mind to posit, observe, and reflect. For there to be a science or logic within which this argument can be judged valid or invalid there must, therefore exist that mind. Given that you find value in scientific objectivism, you by extension hold value in the self aware mind.
I doubt that's acceptable, as you'll probably call it circular, but, as I said initially...I'm not objective when it comes to my own existence.