our greenhouse gases are the problem. i read that only 4% of land animals were left in one of the extinctions because there was a bacteria that produced greenhouse gases,at the rate that we are. it was during the span of a like 2 or so thousand years that this certain type of bacteria reproduced, giving off greenhouse gases, rain became acidic, the air un-breathable, and this domino affect took place where almost all land animals were wiped out along with many sea creatures as well.
I believe the earth is safe for another billion years, give or take a few days...