Water Vapor is the most abundant greenhouse gas in the atmosphere, which is why it is addressed here first. However, changes in its concentration is also considered to be a result of climate feedbacks related to the warming of the atmosphere rather than a direct result of industrialization. The feedback loop in which water is involved is critically important to projecting future climate change, but as yet is still fairly poorly measured and understood.
As the temperature of the atmosphere rises, more water is evaporated from ground storage (rivers, oceans, reservoirs, soil). Because the air is warmer, the relative humidity can be higher (in essence, the air is able to 'hold' more water when its warmer), leading to more water vapor in the atmosphere. As a greenhouse gas, the higher concentration of water vapor is then able to absorb more thermal IR energy radiated from the Earth, thus further warming the atmosphere. The warmer atmosphere can then hold more water vapor and so on and so on. This is referred to as a 'positive feedback loop'. However, huge scientific uncertainty exists in defining the extent and importance of this feedback loop. As water vapor increases in the atmosphere, more of it will eventually also condense into clouds, which are more able to reflect incoming solar radiation (thus allowing less energy to reach the Earth's surface and heat it up). The future monitoring of atmospheric processes involving water vapor will be critical to fully understand the feedbacks in the climate system leading to global climate change. As yet, though the basics of the hydrological cycle are fairly well understood, we have very little comprehension of the complexity of the feedback loops. Also, while we have good atmospheric measurements of other key greenhouse gases such as carbon dioxide and methane, we have poor measurements of global water vapor, so it is not certain by how much atmospheric concentrations have risen in recent decades or centuries, though satellite measurements, combined with balloon data and some in-situ ground measurements indicate generally positive trends in global water vapor.
With the dominance of water in retaining reflected heat, it might stand to reason that increased atmospheric moisture from man-made sources could dominate any warming from carbon dioxide. With this in mind I asked myself "what man-made means is there to increase water vapor?"
How about the paving of lands that formerly absorbed water and the storing of that water in retention ponds? I postulate that pond water is more likely to evaporate than water that soaks into the ground. Could this be an alternative means towards man-made climate change – blame it on concrete and asphalt, any good environmentalist can get excited about that.
On the other hand, applying the Law of Unintended Consequences:
What we don’t know is what the affect is of using retention ponds instead of letting the water run away to rivers, lakes, oceans, etc. The irony here is that the environmental mandate for storage of runoff may be a major contributor to global warming - the worst environmental disaster of all time.