In addition, water vapor is the most abundant of the greenhouse gases in the atmosphere and the most important in establishing the Earth's climate. Greenhouse gases allow much of the Sun's shortwave radiation to pass through them but absorb or trap the longwave, infrared radiation emitted by the Earth's surface. Without water vapor and other greenhouse gases in the air, surface air temperatures would be well below freezing.
The concentration of water vapor in the atmosphere reflects the number of molecules of water compared with the total number of air molecules (mainly nitrogen and oxygen). There are many ways to define the atmospheric water vapor concentration. In this text we will use mainly mixing ratio and relative humidity. Mixing ratio is the measure of the mass of water vapor in a kilogram of air. Relative humidity reflects the ratio of the actual pressure of water vapor in a sample of air to the pressure necessary to saturate that air at a given temperature.
Warm air can sustain a higher concentration of water vapor than cooler air without becoming saturated. Consequently, as air warms, for whatever reason, more evaporation may take place and the concentration of water vapor may increase. An increase in water vapor enhances the greenhouse effect and gives rise to further warming. This positive feedback, warming from increased greenhouse gases leading to an increase of water vapor and therefore even more warming, is a feature of climate models used for estimating the effect of increased greenhouse gases. According to the 1990 Intergovernmental Panel on Climate Change (IPCC) report, Climate Change: The IPCC Scientific Assessment [Houghton et al., 1990], this feedback could amplify the temperature change due to a doubling of carbon dioxide by some 60%. The IPCC update, scheduled for release in 1996, does not change this conclusion. Thus an understanding of the mechanisms distributing water vapor through the atmosphere and of water vapor's effects on atmospheric radiation and circulation is vital to estimating long-term changes in climate.
Recent theoretical and observational advances have put a new focus on water vapor in the climate system but also have raised new questions.
The weight of the atmosphere's water vapor contributes only about one quarter of one percent of the total sea level pressure of all the gases. If all the water vapor in the air at a particular time were to condense and fall as rain, it would amount to a depth of only about 2.5 cm. This is called precipitable water. Because water vapor is not evenly distributed globally, there would be about 5 cm near the equator and less than one tenth as much at the poles. The average precipitation over the globe is about 1 m annually, so there must be a rapid turnover of water in the air; the average water molecule spends about 9 days in the air before precipitating back to the surface.
This rapid turnover, combined with the variation of temperature with height and geography, causes water vapor to be distributed unevenly in the atmosphere, not only horizontally but vertically as well. Figure 1 shows the mean vertical distribution of temperature and the mixing ratio of water vapor in the atmosphere. The lower scale shows that water vapor decreases rapidly with height as the atmosphere gets colder. Nearly half the total water in the air is between sea level and about 1.5 km above sea level. Less than 5-6% of the water is above 5 km, and less than 1% is in the stratosphere, nominally above 12 km. Relative humidity (not shown) also tends to decrease with height, from an average value of about 60-80% at the surface to 20-40% at 300 mbar (9 km). Despite the small amount of water vapor in the upper troposphere (above about 5 km) and stratosphere, recent research has shown that upper tropospheric water vapor is very important to the climate.
The effect of clouds on the climate system is complicated. Clouds reflect sunlight, which reduces solar radiation input to the Earth-atmosphere system. However, clouds also trap longwave radiation emitted by the Earth, as does water vapor. Clouds are highly interactive with the Earth's surface. They regulate the amount of sunlight received by the surface and so influence evaporation from the surface, which in turn influences cloud formation. Precipitation from clouds, in turn, influences soil moisture and evaporation rates. Soil moisture content and sunshine regulate the type of vegetation that covers the surface, which also influences evaporation rates. As the Earth's climate changes, we cannot predict whether the net effect of these interactive changes in cloudiness and other elements of the climate system will tend to amplify or reduce the change in climate.
The mechanics by which convective cloud systems transport water vapor vertically in the atmosphere are poorly understood. Cloud updrafts have long been thought to carry moisture to higher elevations, but evidence also suggests that cloud microphysical processes and cloud dynamics may dry the upper troposphere.