Your car will emit almost the same amount of Carbon Dioxide no matter if the A/C is on or not. The difference, if any, would be very slight.

Very little. When you use you ac it makes the engine work slightly harder to run the compressor. I don't see it as significant.

Depends on how new it is.

New cars have been consstructed to be more fuel efficient when running the A/C as opposed to rolling the windows down. rolling down the windows open creates a drag effect in today's aerodynamic cars, thus reducing fuel efficiency.

Yes it will, as you will consume less fuel and so lower your carbon footprint.

You may want to consider a hybrid car, if you are looking to help save the planet.

Not necessarily: you will likely burn more gas, if only slightly, at highway speeds. City speeds, you may save some.

This is because most people like to breath and not roast in their vehicle while derving down the road in hot weather. This means they will roll down one or more windows to stay somewhat cool.

Your A/C draws about 6 to 8 HP on your engine. This is regardless of speed. That means at highway speeds, where you're using more horsepower to maintain higher speeds, the draw of your A/C is a proportionally smaller (i.e. less significant load) on the engine.

However, while turning your A/C off reduces the load on your engine, rolling your windows down INCREASES your drag, which is directly proportional to the square of your speed. However, FUEL consumption is proportional to the CUBE of your speed. So, at highway speeds, the extra drag on you car requires your engine to work harder to maintain its speed than it would have with the windows up and the A/C on.

It's a trade off of some sorts: In city driving, turning off your A/C increases fuel economy measurably while rolling down your windows has negligible effect. At highway speeds, the reverse happens: The savings in HP is negligible while the drag with the windows down becomes significant.

See this like at Popular Mechanics:
http://www.popularmechanics.com/.../4264939.html

USING A/C WILL INCREASE FUEL CONSUPMTION LITTLE BIT, WHICH IN TURNS WILL PRODUCE MORE EMMISION.
NOT USING A/C WILL REDUCE THE AMOUNT OF EMMISSION.