A recent paper published in the Proceedings of the National Academy of Sciences of the United States of America addressed this question. The paper, Upper-tropospheric moistening in response to anthropogenic warming, by Chung et al. (doi:10.1073/pnas.1409659111) examined this question to determine the source of the rising water vapor. What they did was to take the observed changes in water vapor and compared that record to projected changes due to various causes. In their conclusion, they state,
Note that the observational estimate for the period 2000 – 2010 lies within the distribution of model simulations only when anthropogenic forcing is included, further indicating that the observed changes in upper-tropospheric water vapor are a direct result of anthropogenic warming.
One of the coauthors, Brian Soden, a professor at the University of Miami, was quoted by Eos as saying,
"If we consider natural variations plus CO2 plus fossil fuels, the only tenable explanation we can come up with is that the increase in water vapor is due to manmade burning of fossil fuels."
The amount of atmospheric water vapor is critically important. As noted above, it is a potent greenhouse gas and is much more efficient at trapping IR radiation than CO2. The level of atmospheric water vapor has been increasing because CO2 is trapping IR radiation and warming the planet and a warmer atmosphere will hold more water vapor. But, then that increased water vapor will lead to an increase in the global temperature and will result in even more atmospheric water vapor, and so on.
Ultimately, this all goes back to burning fossil fuels and is one more piece of evidence that we need to change our way of doing business.