Water vapor feedback

In Eq. (LL), A// is the heat of vaporization of water and R is the gas constant. Thus the vapor pressure of water has an exponential dependence on temperature. This suggests that there may be a water vapor feedback associated with global climate change. If the atmosphere warms, for example due to increased greenhouse gases such as C02, increased concentrations of gaseous water are expected in accordance with Eq. (LL). The increased water vapor traps more thermal infrared radiation, warming the atmosphere further (e.g., Raval and Ramanathan, 1989 Stenchikov and Robock, 1995). 

Hall A., Manabe S. (1999) The role of water vapor feedback in unperturbed climate variability and global warmingj. Clim. 12, 2327-46. 

H20 Greenhouse Feedback. As the lower atmosphere (the troposphere) warms, it can hold more water vapor. The enhanced water vapor traps more IR radiation and amplifies the greenhouse effect. Ramanathan [36] indicates that, based on studies with one-dimensional climate models, this feedback amplifies the air temperature by a factor of about 1.5 and the surface warming by a factor of about 3. The IPCC [23] determined a surface temperature amplification factor of 1.6 for water vapor feedback. 

Inamdar and Ramanathan (1998) have shown that there are considerable geographical variations in water vapor feedback, with the dominating effect in the ec]uatorial ocean region. In this area the greenhouse feedback exceeds the blackbody emission, reproducing the so-called super-greenhouse effect (Ramanathan and Collins, 1991). The overall results demonstrate the importance of realistically reproducing the three-dimensional atmospheric circulation and the associated water distribution for a credible water vapor feedback. 

Water present in the atmosphere in gaseous form the source of all forms of condensation and precipitation. Water vapor, clouds, and carbon dioxide are the main atmospheric components in the exchange of terrestrial radiation in the troposphere, serving as a regulator of planetary temperatures via the greenhouse effect. Approximately 50 percent of the atmosphere s moisture lies within about 1.84 km of the earth s surface, and only a minute fraction of the total occurs above the tropopause. water vapor feedback... 

Wordsworth et al 2010 [367] made a three dimensional global circulation model (GCM) of the early martian climate. In their model CO2 condensation, cloud formation and a water cycle was included. Local water vapor feedbacks compensate reduced CO2 warming effects. In general CO2 clouds lead to a substantial warming. 

One such feedback is the influence of clouds and water vapor. As the climate warms, more water vapor enters the atmosphere. But how much And which parts of the atmosphere, high or low And how does the increased humidity affect cloud formation While the relationships among clouds, water vapor, and global climate are complicated in and of themselves, the situation is further complicated by the fact that aerosols exert a poorly understood influence on clouds. 

The nature of such processes can be depicted as a feedback loop, as shown in Fig. 17-4. Using the nomenclature in this figure and continuing with enhanced evaporation of water vapor as our physical example of a feedback that is completely internal to the climate system, we... 

CFC-12. These manmade chemicals absorb infrared radiation in a part of the spectrum where water vapor and CO2 do not already have strong bands. On the other hand, the manmade increase of CO2 is so large (currently ca. 25% since the mid-1800s - see Chapter 11) that it is the largest anthropogenic input to the greenhouse effect (not counting feedbacks). 

Although thermodynamically it is relatively simple to determine the amount of water vapor that enters the atmosphere using the Clausius-Clapeyron equation (see, e.g.. Chapter 6, Equation (1)), its resultant atmospheric residence time and effect on clouds are both highly uncertain. Therefore this seemingly easily describable feedback is very difficult to quantify. 

Another family of feedbacks involving biota arise via the process of evapotranspiration in which the rate of water vapor is transferred from the land surface to the atmosphere is mediated by plants. Several consequences have been proposed that include influences of biota on the greenhouse effect of water vapor as well as relative humidity and clouds. Lovelock (1988) suggested that tropical forests might be kept cool by increasing cloud cover in response to higher relative humidity released through enhanced evapotranspiration (via the clouds influences on albedo). Yet another connection arises because tree-covered land has different turbulence properties above it than bare soil, which also influences the cloud cover above. 

Since feedbacks may have a large potential for control of albedo and therefore temperature, it seems necessary to highlight them as targets for study and research. Besides the simple example above of cloud area or cloud extent, there are others that can be identified. High-altitude ice clouds, for example, (cirrus) have both an albedo effect and a greenhouse effect. Their occurrence is very sensitive to the amount of water vapor in the upper troposphere and to the thermal structure of the atmosphere. There may also be missing feedbacks. 

High atmospheric COj leads to fester evapotranspiration rates, providing another mechanism by which water vapor levels increase. This is singularly important as water vapor changes are now recognized by the IPCC as the largest feedback affecting climate sensitivity. 

Feedbacks Water Vapor, Clouds, and the Supergreenhouse Effect ... 

Hashimoto G. L. and Abe Y. (2001) Predictions of a simple cloud model for water vapor cloud albedo feedback on Venus. J. Geophys. Res. 106, 14675—14690. 

Soden B. J., Wetherald R. T., Stenchikov G. L., and Robock A. (2002) Global cooling following the eruption of Mt. Pinatubo a test of chmate feedback by water vapor. Science 296, 727-730. 

However, this cannot happen in the present atmosphere so the direct warming effect at the surface, assuming no feedback, would amount to about 1.3 K (Ramanathan, 1981). Now it appears that the atmosphere is close to conserving relative humidity, so a warming would increase the water vapor in the atmosphere and hence further increase the warming, thus creating a positive feedback effect. It is interesting to note that even Arrhenius (1896) included the feedback from water vapor. 

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