Most of the greenhouse effect takes place in the upper atmosphere where water vapor levels are relatively much lower. (realclimate.org)
Therefore, at current levels, CO2 could be said to be somewhat of a stabilizer of the greenhouse effect, taking up part of the slack when water vapor levels drop too low. (realclimate.org)
However, to support the assertion that global warming is responsible for a great deal of damage from such events, it is sufficient to show that such events have the «signature» of global warming — for example, that specific global warming - related factors such as abnormally high sea surface temperatures, elevated water vapor levels, and altered jet stream patterns contributed to making Hurricane Sandy what it was — even if those factors can not be precisely quantified. (realclimate.org)