The suns brightness has absolutely nothing to do with temperature. I know that the suns output fluctuates every few thousand years, but that's only about, half a degree. Your talking about a small change, but even then, it go's down. There are alot of other reasons why the earth could burn. Like plate tectonics and volcanic activity or even the earths tilt. I agree with the fact that burning fossil fuels wold absolutely rape earth, but if ten times brighter, it would make almost no difference. For the sun to make an impact on us, we would need to move nearly 0.7 AU towards the sun.