18 Water Vapor
SkepticalScience
The amount of water vapor in the atmosphere exists in direct relation to the temperature. If you increase the temperature, more water evaporates and becomes vapor, and vice versa. So when something else causes a temperature increase (such as extra CO2 from fossil fuels), more water evaporates. Then, since water vapor is a greenhouse gas, this additional water vapor causes the temperature to go up even further—a positive feedback.
How much does water vapor amplify CO2 warming? Studies show that water vapor feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C change caused by CO2, the water vapor will cause the temperature to go up another 1°C. When other feedback loops are included, the total warming from a potential 1°C change caused by CO2 is, in reality, as much as 3°C.
The other factor to consider is that water is evaporated from the land and sea and falls as rain or snow all the time. Thus the amount held in the atmosphere as water vapour varies greatly in just hours and days as result of the prevailing weather in any location. So even though water vapour is the greatest greenhouse gas, it is relatively short-lived. On the other hand, CO2 is removed from the air by natural geological-scale processes and these take a long time to work. Consequently CO2 stays in our atmosphere for years and even centuries. A small additional amount has a much more long-term effect.
Generally it is held that the WV condenses out within a period of 14days.
As atmospheric CO2 levels rise relentlessly week on week, month on month, year on year, atmospheric water vapour reaches a rapid equilibration according to its vapour pressure in relation to the atmospheric presure and temperature.
Water vapour isn’t 10x more effective a GC than CO2. Despite the fact that the water vapour concentration of the atmosphere is 5 times that of CO2 (around 0.3% by mass for water vapour cf around 0.06% by mass for CO2), the contribution of CO2 to the greenhouse effect is at least 10% (and more like 25-30% with the water vapour feedback).
Atmospheric CO2 doesn’t fall out of the atmosphere. As we pump CO2 into the atmosphere it accumulates day by day, month by month, year by year. That can’t happen with water vapor.
Atmospheric CO2 concentrations rise cumulatively (and very very quickly now). The water vapour that we pump into the atmosphere is a tiny supplement to the natural evaporative/precipitation cycle, and since this comes straight out of the lower atmosphere within a week or two it can (a) have only a very small effect and (b) caanot be cumulative.
Man can’t “add” water vapour to the atmosphere. The atmospheric water vapour levels are essentially “defined” by the atmospheric temperature and pressure. What happens to all of that water (e.g the vast amount from natural evaporation)?. It all comes straight out as precipitation. What stays in the atmosphere is what the atmosphere can support in relation to the atmospheric temperature and pressure. In fact the research indicates that the atmosphere tends to maintain a relatively constant relative humidity.
ACS
It’s true that water vapor is the largest contributor to the Earth’s greenhouse effect. On average, it probably accounts for about 60% of the warming effect. However, water vapor does not control the Earth’s temperature, but is instead controlled by the temperature. This is because the temperature of the surrounding atmosphere limits the maximum amount of water vapor the atmosphere can contain. If a volume of air contains its maximum amount of water vapor and the temperature is decreased, some of the water vapor will condense to form liquid water. This is why clouds form as warm air containing water vapor rises and cools at higher altitudes where the water condenses to the tiny droplets that make up clouds.
The greenhouse effect that has maintained the Earth’s temperature at a level warm enough for human civilization to develop over the past several millennia is controlled by non-condensable gases, mainly carbon dioxide, CO2, with smaller contributions from methane, CH4, nitrous oxide, N2O, and ozone, O3. Since the middle of the 20th century, small amounts of man-made gases, mostly chlorine- and fluorine-containing solvents and refrigerants, have been added to the mix. Because these gases are not condensable at atmospheric temperatures and pressures, the atmosphere can pack in much more of these gases . Thus, CO2 (as well as CH4, N2O, and O3) has been building up in the atmosphere since the Industrial Revolution when we began burning large amounts of fossil fuel.
If there had been no increase in the amounts of non-condensable greenhouse gases, the amount of water vapor in the atmosphere would not have changed with all other variables remaining the same. The addition of the non-condensable gases 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. The warming due to increasing non-condensable gases causes more water vapor to enter the atmosphere, which adds to the effect of the non-condensables.
There is also a possibility that adding more water vapor to the atmosphere could produce a negative feedback effect. This could happen if more water vapor leads to more cloud formation. Clouds reflect sunlight and reduce the amount of energy that reaches the Earth’s surface to warm it. If the amount of solar warming decreases, then the temperature of the Earth would decrease. In that case, the effect of adding more water vapor would be cooling rather than warming. But cloud cover does mean more condensed water in the atmosphere, making for a stronger greenhouse effect than non-condensed water vapor alone – it is warmer on a cloudy winter day than on a clear one. Thus the possible positive and negative feedbacks associated with increased water vapor and cloud formation can cancel one another out and complicate matters. The actual balance between them is an active area of climate science research.
Trenberth
It is the well-known positive feedbacks within the climate system that play off the changes otherwise forced on the climate system and amplify the changes. They depend on the temperature changes and heating already going on. Most notable is water vapour feedback.
As the Earth and its oceans warm up, the water-holding capacity of the atmosphere increases at a rate of about 7% per degree Celsius (or about 4% per degree Fahrenheit). And the record high sea temperatures ensure that there is more moisture in the form of water vapour in the atmosphere as a result. Estimates are 5 to 15% relative to prior to the 1970s, when global temperature increases began in earnest.
But water vapour is a powerful greenhouse gas. The increases have likely increased global heating by an amount comparable to that from increases in carbon dioxide. And we are seeing the consequences!
In the current climate, for average all-sky conditions, water vapour is estimated to account for 50% of the greenhouse effect, carbon dioxide 19%, ozone 4%, and other gases 3%; while clouds make up a quarter of the greenhouse effect.
The main greenhouse gases of carbon dioxide, ozone, methane, and nitrous oxide do not condense and precipitate as water vapour does. The result is orders of magnitude differences in the lifetime of these gases (decades to centuries) compared with about nine days for water vapour. It is because of this vigorous hydrological cycle the average atmospheric life of a water vapour molecule is just 9 days.
Although the main water vapour increases may be near the surface, upper tropospheric water vapour is more critically important for the net greenhouse effect.