Andrew Revkin at the New York Times has written an accessible (well, as accessible as it gets) article on why we live in a bathtub.
We're running water (carbon dioxide and other greenhouse gases) into the bathtub, and, currently, we have a number of "sinks" that can absorb carbon, the drain from the bathtub. We're adding more carbon than is being removed, so the "water in the bathtub" is rising. As well, the "sinks" are starting to reach their limit in absorbing carbon (the drain is getting plugged).
If we get to a situation where global water in = global water out, we'll still have a bathtub with a lot of water in it. As Christian at Greenpeace puts it: "Turning the taps off doesn't solve the entire problem - because it's actually the amount of water in the bath which warms the planet, not how much the taps are on."
We have to stop running too much water into the bathtub.