The Fate of the Universe

Some say the world will end in fire,
Some say in ice.
From what I've tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice. -Robert Frost
Sure, if all we're talking about is the Earth, that one's a no-brainer. It's fire.

Our Sun, very slowly, is burning up its Hydrogen fuel into Helium, through the process of nuclear fusion. But over the course of billions of years, it starts to burn its remaining fuel at an ever increasing rate. Over the next billion or two years, the Sun's output will slowly increase, forcing the Earth's temperature to creep upwards, ever so slowly.

(Image credit: S. Alvarez and National Geographic.)
The temperature will rise until it's hot enough that -- believe it or not -- the oceans boil. Not from lava vents, as shown above, but from the Sun heating the oceans up past their boiling point!
And if that weren't dastardly enough, the Sun won't be done with us yet.

Although it will continue to burn hotter and hotter for billions of years ...