I'm worried about global warming for the simple reason that this planet is all we have. I, for one, am concerned about future generations.
Your assumption that everything is meaning less is illogical, unfounded, presumptuous and incorrect. Illogical, since a belief in a deity seems to have no link to the present or future importance of things. Unfounded, since the seems no reason for the importance of anything to change. Presumptuous, since you are attributing your own opinions to what you assume we believe, and incorrect since we do not believe what you think.
We are worried about the air supply for the simple reason that it is the only air we have, and we do not wish to die (unlike rapturists who seem to look forward to death with an unhealthy anticipation). We are worried about wars for the simple reason we do not have a deity with a history of urging his followers to genocide, and we have an innate sense of morality that tells us that killing is wrong. Your assumption that could turn into any other species merely displays your complete ignorance of the basic principles of evolution.
While it is possible that another Big Bang may occur at some point in the future, that point may be hundreds of billions of years in the future, after the demise of this present Universe, and I cannot see its relevance to why I care about this planet.
Let me in turn ask you (presumably a Christian) a question: Since your deity is going to whisk you away to heaven at any moment, where you can joyfully watch large percentages of your fellow humans writhe in agony for eternity, why do YOU care about anything on this planet?
If your answer is that you do not care, what gives you the right to destroy the planet for the rest of us?
We're all going to die. Some sooner, some later, but we all die. What you believe happens to your 'soul' after death should have no bearing on your care of the planet.
If you had been given a car as a teenager, and told that this was the only car or form of transport that you would ever be able to own: wouldn't you take care of it to make it last?