The short version is that plate tectonics combines with weather to keep temperatures between boiling and freezing points of water over the very long term. In Earth's early history, when the sun was much more feeble, there was much more CO2 in the atmosphere.
According to the article I read, or heard, the sun was already hot enough that critical amounts of CO2 will have to be removed from the atmosphere just to keep temps from soaring and oceans from boiling away. Soon, the Earth will remove so much CO2 that plant life will be reduced. Long before the sun goes to Red Giant stage, advandced life on Earth will be blotted out. The article said this was already beginning to happen, and that we don't have 4.5 billion years, but much closer to 4.5 THOUSAND years, before this becomes a noticiable and increasingly serious problem.
What, no more salad bars? What will the veg heads do then?