how might adding carbon dioxide to the atmosphere couse flooding in some areas?

Answer 1

Carbon dioxide is a greenhouse gas. It takes incoming sunlight and traps the infrared (heat) in the atmosphere. This can have effects on the oceans and cause sea levels to rise:

1) By warming the temperature of the oceans, causing water to expand.

2) By melting the polar ice caps, releasing the water into the oceans.

Either way, sea levels would rise by a few feet.

Answer 2

I would like to add to the otherwise excellent first answer that, of course, the Arctic ice already floating, won't increase the sea level by melting. But the great amount of ice on Greenland, will.
But the picture is more complex than that. The air, at the standard atmosphere of 15°C can sustain about 14 grams of water per cubic meter. With a global increase of the temperature, there will be more water in the atmosphere. That will, in turn, reduce the heating of the earth since clouds will reflect the sun radiations. The temperature is observed to increase more at the poles than the equator. It means that the difference, which is the cause of the violence of the storms, is decreasing. This is the reason the temperate zones are more storm-full in the winter, when the temperature difference between the pole and equator is greater, than in the summer, when it is less.
So it may not be a doomsday prophecy as some want us to believe. But it will still be a big change with uncertain results and that is enough for us to be environmental conscious and reduce as much as possible the release of carbon dioxide and other polluting agents.