TL;DR the Sun's slowly getting brighter, which will eventually (and LONG before the much-talked-about-as-the-end-of-the-world "Sun expands and eats the Earth" thing) break the carbonate-silicate cycle. This will affect availability of carbon to the biosphere. There are a few pathways for photosynthesis, and these will, basically one at a time, stop being viable over a period of one or two hundred million years, with the biosphere coming under more and more stress (=lots of stuff dying, biomass reducing). At some point, diversity among and quantity of large life forms will shrink tremendously, and finally there will be no (or damn near no—perhaps some lobster-type things will survive near undersea vents) more complex life on Earth, due to lack of available energy.