Water Vapor

Water vapor is a misunderstood greenhouse gas. In some ways more significant even than carbon dioxide to global warming, but perhaps over-rated by those who consider climate change to be a hoax. Hopefully this page will answer some of the mysteries while much scientific study remains.
Water vapor is a very effective absorber of heat energy in the air, but it does not accumulate in the atmosphere in the same way as other greenhouse gases. Water vapor is the largest contributor to Earth’s greenhouse effect. On average it probably accounts for 60% of the warming effect; however it does not control the Earth’s temperature, but is controlled by the Earth’s temperature. The addition of the non-condensable gases (CO2, methane, nitrous oxide and ozone) causes the temperature to increase and this leads to an increase in water vapor that further increases the temperature. This is an example of a positive feedback effect. link


  • General information
  • Contrails
 General information

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 terrestrial greenhouse effect. link

January 2010: Is water vapor in the stratosphere slowing global warming? Stratospheric water vapor declined by 10% since 2000, based on satellite and balloon measurements, yet that was enough to appreciably affect temperatures at ground level according to climate models “Reduce the water vapor and you have less long-wave radiation coming back down to warm the troposphere,” said Karen Rosenlof of the National Oceanic and Atmospheric Administration’s. Conversely, an apparent increase in water vapor in this region in the 1980s and 1990s exacerbated global warming. The amount of water vapor in the atmosphere is also affected by another potent greenhouse gas – methane – which has unexpectedly failed to increase in recent years. link

January 2010: Study reveals water vapour caused one-third of global warming in1990s. 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

Human activities contribute slightly to water vapor concentrations through farming, manufacturing, power generation, and transportation. However, these emissions are so dwarfed in comparison to emissions from natural sources we can do nothing about, that even the most costly efforts to limit human emissions would have a very small, perhaps undetectable, effect on global climate.

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.


Contrails:  Jet engines spew out very hot air. And, because water vapor is one of the byproducts of the exhaust, the air is also very humid. However, high in the atmosphere where these jets fly, the air is typically very cold, often lower than -40 degrees Fahrenheit. Additionally, the atmosphere up there is often of low vapor pressure, or the force exerted by a gas on the surrounding environment. When a jet engine is spewing out hot, humid air into an atmosphere that is cold and has low vapor pressure, the result is condensation. The water vapor coming out of the engine quickly condenses into water droplets and then crystallizes into ice. The ice crystals are the clouds that form behind the engine. This is why the streaks are called contrails, short for “condensation trails.”  However contrails consist of not just ice crystals and water vapor but also other byproducts of engine exhaust. These include carbon dioxide, nitrogen oxides, sulfate particles and soot. Some point out that these, in addition to the extra cloud cover, can have negative environmental effects. 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

March 2013: 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

            For information on chemtrails, see Geoengineering page