WATER VAPOR

March 21 12013: The importance of aircraft emission in climate change. While air travel today accounts for just 3% of worldwide greenhouse gas emissions, the CO2 and other pollutants that come out of jet exhaust contribute disproportionately to increasing surface temperatures below because the warming effect is amplified in the upper atmosphere. The Intergovernmental Panel on Climate Change (IPCC) reports that CO2 emitted by jets can survive in the atmosphere for upwards of 100 years, and that its combination with other gas and particulate emissions could have double or four times the warming effect as CO2 emissions alone. Modern jet engines are not that different from automobile engines; both involve internal combustion and burn fossil fuels. But instead of gasoline or diesel, jet fuel is primarily kerosene. Just like car engines, jets emit CO2, nitrogen oxides, sulfur oxides and soot. Beyond their contributions to global warming, airplane emissions can also lead to the formation of acid rain and smog, as well as visibility impairment and crop damage down on the ground. link

According to NOAA water vapor is the most abundant greenhouse gas in the atmosphere. However, changes in its concentration are 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. link 

Currently general agreement is that 95% of GHG (Greenhouse Gases) are caused by water vapor, and 99.999% of that is of natural origin. We can do little about it.             

This point is so crucial to the debate over global warming that whether water vapor is or isn't factored into an analysis of Earth's greenhouse gases makes the difference between describing a significant human contribution to the greenhouse effect, or a negligible one. Charts on its inclusion by percentage impact can be reviewed here: but many people arguing against Al Gore's theories contest that he hasn't taken water vapor into account, and he can be more readily dismissed.
But this is an area clearly not understood, and not withstanding that water vapor could be the most serious GHG, it isn't predominantly caused by human activity. It could be argued that it is more of a "constant" factor, and increasing CO2, methane etc. are the only causes we can, and must, reduce.

October 2010: Water vapor and clouds are the major contributors to Earth's greenhouse effect, but a new atmosphere-ocean climate modeling study shows that the planet's temperature ultimately depends on the atmospheric level of carbon dioxide. The NASA study identified non-condensing greenhouse gases - such as CO2, methane, nitrous oxide, ozone and fluorocarbons - as providing the core support for the terrestial greenhouse effect. link 

February 2010: Water vapor slows global warming during last decade. Over the last 10 years, global temperatures rose about 25% more slowly than scientists had previously predicted. A study published in the journal Science reports that that's because the climate models factored in increases in CO2 and other greenhouse gases but didn’t count on a decrease in water vapor - the most abundant greenhouse gas - in the middle part of the atmosphere. link

January 2010: Study reveals water vapour caused one-third of global warming in 1990s. The research, led by one of the world's top climate scientists, suggests that almost one-third of the global warming recorded during the 1990s was due to an increase in water vapour in the high atmosphere, not human emissions of greenhouse gases. A subsequent decline in water vapour after 2000 could explain a recent slowdown in global temperature rise, the scientists add. The experts say their research does not undermine the scientific consensus that emissions of greenhouse gases from human activity drive global warming, but they call for "closer examination" of the way climate computer models consider water vapour. It was not clear why the water vapour levels had swung up and down, but suggested it could be down to changes in sea surface temperature, which drives convection currents and can move air around in the high atmosphere. link

December 2009: Aircraft vapor trails responsible for 15-20% of Arctic warming. The first analysis of emissions from commercial airline flights shows that they are responsible for 4–8% of surface global warming since surface air temperature records began in 1850. This study is yet more strong evidence that we need a high priority global strategy to sharply reduce black carbon.  link 

March 2011: A new study on contrails finds that all those aircraft condensation trails you see across the sky may, on any given day, be warming the planet more than all the CO2 emitted by all the planes since the Wright Brothers’ first flew over a century ago. The question arises as to whether changing the flight pattern of aircraft or perhaps their engine technology could ameliorate this problem. link  

Contrails are clouds formed when water vapor condenses and freezes around small particles (aerosols) that exist in aircraft exhaust. Some of that water vapor comes from the air around the plane; and, some is added by the exhaust of the aircraft. Contrails, especially persistent contrails, represent a human-caused increase in high thin clouds in the Earth's atmosphere, and are likely to be affecting climate and ultimately our natural resources. link