[Mars]

What could slowly render Earth uninhabitable (over to course of a couple hundred years, not millennia) to humans who have the technology to build a self sustaining colony on Mars? Anything? Or would such technology cover just about everything slow?

Man made, natural catastrophe suggestions are all good.

[Jeff Vader]

I'd count the modern man as such a catastrophe.

[Bob-san]

Dyson sphere or similar. Take it apart piece by piece.

[Dark Hunter]

Hmmm... what about a supervirus? The descolada virus from the Ender series might fit this (it was less a virus and more a genetic supersplicer that rewrote entire global ecosystems).

[Mobius]

Total extinction of all kinds of bees, as Einstein said. It's far worse than you could ever imagine.

(please note that I'm serious)

[BloodEagle]

Hmmm... what about a supervirus?

That sounds like something that she would come up with. 

[Flipside]

What's with today's obsession with planetary disasters?

[Scuddie]

They're more fantastical than local disasters, perhaps.

[tinfoil]

Because creating one seems to be in the human DNA somewhere. Plus it is interesting to think that we could be wiped out one day with nothing left to cause any more stupid problems. In fact the "Ultimate Solution" would probably be to simply wipe ourselves off the face of the earth.

[Flipside]

The downside being, of course, that there wouldn't be anyone left to say 'I told you so.'

[tinfoil]

you mean only Nuke

[watsisname]

What's with today's obsession with planetary disasters?

Just wait, I bet we start discussing universal disasters before this is done.  Lol, Big Crunch.

[FUBAR-BDHR]

Some sort of mutation occurring on the Martian colony or in space that transfers back to the Earth.  You have your super virus, new bacteria, all the way up to a Human mutation resulting in sterility after a few generations.  A lot can happen with all that cosmic radiation.   

[tinfoil]

Just wait, I bet we start discussing universal disasters before this is done.  Lol, Big Crunch.

Or maybe a Gnab Gib

[Bobboau]

if earth's magnetic field collapses, the atmosphere would get striped away over the desired time frame.

only problem would be it would be a lot easier to setup livable shelters on earth than mars, simply because we would have all the equipment and resources here already, and we'd have plenty of time to take advantage of the atmosphere before it got eroded.

I supose a better scenario would be if the output of the sun rose for some reason at a steady rate for a few hundre years, this would have the effect of making earth too hot, and mars less cold.

[Mars]

For those wondering I'm contemplating a story. I'd need a convinient disaster to be slowly sapping the life out of Earth, but not threatening Mars.

My problem, as Bobboau stated, is that if you can build biospheres etc on Mars, than Earth should be easy.

[BloodEagle]

The Earth's core could start rapidly (still relatively slow) cooling, for whatever reason.

[Dark RevenantX]

Turn on the LHC.

If that doesn't work, then just pile up all of the nukes of the world in one place and set them off at the same time.

[Mars]

Slow is kind of a big thing here

EDIT

Although, a nuclear winter could work

[Kosh]

What's with today's obsession with planetary disasters?

There seems to be an increasing trend as of late to believe in "the end is near" BS.

Turn on the LHC

I seriously hope you're joking, the LHC could never destroy the Earth.