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Biospheric water contributes little to total global water reservoirs but serves important functions of transporting nutrients in plants, keeping plants (and hence the earth’s surface) cool and green (thereby influencing reflection of solar radiation), and serving as a source of nourishment for predators. We will see in the unit on agricultural impacts that increased atmospheric CO2 increases the water use efficiency of plants, which means they transpire less to the atmosphere. Less transpiring water means less evaporation and therefore warmer vegetation surfaces. Murray (1997) calls this the second greenhouse effect (the first being the direct warming due to greenhouse gases) and he estimates it could lead to an additional 2oC to 5oC warming of vegetation surfaces. Plants increase surface evaporation and sublimation (vaporization of snow) by suspending moisture in the form of dew, rainwater, or snow above the ground, thereby exposing much larger moisture surfaces to air movement and phase change.
7. Atmospheric Water
Atmospheric water consists of water vapor and liquid and ice components of clouds. In vapor form, water contributes to the greenhouse effect. The global distribution of water vapor is shown in Figure 14 in terms of g/kg of air, and in a second graph in terms of relative humidity (Figure 15).
8. Clouds
Clouds contribute both to global warming by absorbing infrared radiation from the surface and to global cooling by reflecting solar radiation. The role of water vapor and clouds in moving latent heat already has been discussed. They also contribute to the poleward transport of sensible heat. Atmospheric water is an important mechanism for cleansing the atmosphere of natural and anthropogenic contaminants. One such anthropogenic contaminant, sulfate aerosol, interacts with clouds to cause increased reflection of solar radiation and hence lead to global cooling (Figure 16). This will be discussed further in the unit on the Global Energy Balance .
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