Even though our star seems to be in its comfortable middle age it is, in fact, slowly heating up. Every ton of Hydrogen gas that gets converted into Helium forces the Sun to contract just a little bit and that raises its temperature just a bit. When scientists examine computer models for a future climate under the revved up Sun they see a Greenhouse effect gone wild. The polar ice caps will be history, and much of the fertile land will be flooded. As the Sun's output continues to increase so much water is evaporated into the atmosphere that even the stratosphere gets wet.
Sunlight can then break apart the water molecules allowing the hydrogen atoms to escape into space.
No more water, no more life. The world as we know it will have ended. One of the greatest revolutions in our understanding of the last thirty years is that biospheres are delicate things. While we don't have to start packing yet it is clear that the Earth does not have a comfortable five billion year tenure as life bearing planet. The McGraw-Hill Companies.