Let's talk Sci-Fi here - lets just say to have our cake and eat it too, we....

[Taverner]

....build a large metal shell around Earth. All lighting will be directed downward, with the lights switching off at night.

The climate is controlled via machine, and the carbon produced goes out into space, but far away from our filtered world.

In addition this constant filtering of our air removes all bacteria (nanotechnology) so if an epidemic even starts, nanobots are sent via the 'Earth Climate System' cleans it out...

And then this wonderful system breaks....

[kentauros]

1. You can only have your cake and eat it too

if you start out with Fairy Cake and then create an Infinite Improbability drive out of thin air, using the future existence of that drive to help create itself.

[Taverner]

2. Well, yeah. That's the point of fiction, right?

[kentauros]

3. We'd have to ask Douglass Adams about that point...

but he's dead ;)

Why not just go ahead and create a true Dyson Sphere, instead of one on only a planetary scale? There's plenty to go wrong with just a Ringworld, too

[baldguy]

4. You don't want to build a shell around the earth.

You want to build it around the Sun.

http://en.wikipedia.org/wiki/Dyson_sphere

[targetpractice]

5. It will become a terrarium...

...that ultimately only supports lifeforms that can exploit geo-thermal energy... We humans will exist for a while, but eventually it will become a stinky goopy mess of bizarre beings (like you would find at the bottom of the ocean near a volcanic vent)... Stinky, goopy, and boring... just like a terrarium.