Water vapour, although it is not commonly associated with climate change, is a greenhouse gas that contributes to global warming. Water vapour is not bad for the environment when it is in moderation. However, as the earth warms up due to human emissions of manmade greenhouse gases more water vapour enters the atmosphere and thus warming our planet.
As our planet warms even more water vapour enters the atmosphere and contributes to global warming. As you can see, this reaction works in a loop. According to recent studies conducted by researchers at NASA water vapour has the potential to produce double the heating effect of that of CO2.
On average, however, water vapour traps about 2 watts worth of heat per square. This means that water vapour at max could trap 1.018 billion watts worth of heat, making water vapour a small but significant section of global warming.