The Importance Of Greenhouse Gasses...

We observe that the Earth's surface is on average around +15 degrees C. However, without the greenhouse gasses that we find here, the surface would be -18 degrees C as explained by the amount of energy that hits the surface and the amount that is reflected back to space. That is to say, that greenhouse gasses account and hold in enough energy for an increase in 32 degrees C resulting in a surface so that it can sustain a multitude of life as compared to one that would be devoid of most life that know today.

In a very rough approximation the following trace gases contribute to the greenhouse effect:

60% water vapor
20% carbon dioxide (CO2)
The rest (~20%) is caused by ozone (O3), nitrous oxide (N2O), methane (CH4), and several other species.

So, as the temperature of the surface of the earth rises, water vapor in the air increases resulting in more greenhouse effect in a potentially never ending cycle. However, increases in water vapor also potentially increase the occurrence of clouds that both absorb energy and reflect it back to space reducing the impact of the Sun's heating effect. Additionally, in order to form water droplets, dust must be present since each water droplet can only form around a piece of dust. The amount of dust in the Earth's atmosphere is also variable dependent upon many factors.

As a result of this interaction and interplay between all of the greenhouse gasses and formation of clouds, the statement applies....

One of the major problems in climate research at present is the fact that we still cannot realistically reproduce the formation of clouds in currently available climate models. Therefore an exact prediction of the influence exerted by water vapor and a prediction of the warming on the whole, remains very doubtful. This area of research calls for an enormous amount of scientific work.

We conclude:
Due to the so called "greenhouse effect" - caused by atmospheric trace gases such as carbon dioxide (CO2), ozone (O3), and water vapor (H2O) - infrared radiation from the earth is stored temporarily in the atmosphere. Of all these trace gases, water vapor represents the most important constituent. It contributes to the natural greenhouse warming process by approximately 60%. Water vapor amplifies the anthropogenic contribution to greenhouse warming through a positive feedback. This amplification is counteracted by the increased reflection off clouds. How these two factors combine in the real atmosphere still remains an open question.

So, with the incomplete science in all the interactions between factors inputted in climate models, how do we know that increases in CO2 will definitely lead to catastrophic changes in climate?

you can read more about water vapor and its impact on climate, here.