>>890148The fundamental physics were first established in the 19th century by Joseph Fourier, with refinements and early experimentation by Joseph Tyndall and Svante Arrhenius. They established that the Earth requires greenhouse gases for the surface to remain at the temperature that it is. Otherwise, any solar radiation striking the Earth would just escape into space, the our planet's surface would be frozen even in the equator.
Why is CO2 especially important? It has to do with the residency time in the atmosphere, water vapour is dependent on the temperature and not the other way around. It can be a positive or negative feedback in the climate system, but it can't affect it You can't pump water vapour into the atmosphere endlessly to increase the water vapour concentration, because at some point it turns into clouds and rain. CO2 will eventually saturate too, but that's what Venus looks like (96% concentration versus 400 ppm on Earth).
A modern paper modelling a world without any CO2 finds that a CO2-less world will be too cold for life, but increasing the H2O concentration just results in the climate returning to an equilibrium after it rains out:
http://science.sciencemag.org/content/330/6002/356http://www.tellusb.net/index.php/tellusb/article/view/19734 