Water vapor is the most abundant and probably the most important naturally occurring greenhouse gas. Human activities contribute very little to the concentration of water vapor in our atmosphere. Scientists consider water vapor to be a more critical factor in the greenhouse effect than carbon dioxide. One reason is that water vapor concentrations vary considerably within a given time frame and by location.
Image from the National Water and Climate Center
The water cycle is the endless recycling of water on Earth. To trace the movement of water through the cycle pictured above, begin at the far right side. There the Sun's energy is evaporating water from the sea and land to the atmosphere in the form of water vapor. Evaporation from soil and inland bodies of as well as transpiration from plants adds large amounts of water vapor to the atmosphere. Man, animals, and machines add insignificant amounts by means of respiration and combustion.
Air masses (top of diagram) carry the water vapor across the land, where it is warmed causing the moisture laden air to rise. This results in the water vapor condensing to form clouds and precipitation. This precipitation may fall as rain, snow, sleet, or hail. Some precipitation evaporates while falling and returns to the atmosphere. A small amount is intercepted and held by plants, paved surfaces, buildings, vehicles, and other structures until it evaporates back into the atmosphere.
Some of the precipitation soaks into the soil where it may enter into streams or it may become part of our ground water system. Some of the precipitation runs off into streams and rivers which eventually into the oceans. Misuse and poor management of the soil will decrease the amount of water that soaks into the soil and increase the amount that runs off. Runoff on exposed land leads to erosion. Grass, trees, and other plants hold the soil in place and slow the runoff, allowing more water to soak into the soil.
Plants can use water that soaks into the soil. Part of the water percolates beyond the reach of plant roots down to the water table, underground reservoirs called aquifers, and to springs and artesian wells. Runoff on its way to the sea can be intercepted and stored for industrial or household use, and it can be diverted for irrigation.
Image from the National Weather Service Satellite Tutorial
The concentration of water vapor in our atmosphere will have an affect on the heat and energy budget of the Earth (shown above). Water vapor in the atmosphere can condense, warming the atmosphere and forming clouds. These clouds can either reflect sunlight, which has a cooling effect or trap infrared radiation (heat) which has a warming effect. This cooling and/or warming can change the natural radiation budget processes within our atmosphere. After cloud formation, precipitation may occur causing a cooling effect at the surface. This overall process of evaporation/condensation/precipitation is generally referred to as the water cycle. These rainfall systems control the supply of water vapor to air that is free of clouds.
There are still some uncertainties about exactly how this cycle functions, which limits the ability of general circulation models to accurately predict how much global warming will occur. The formation of clouds in response to global warming is still a major uncertainty. Scientists know that warmer air is capable of holding more moisture, but they also need to understand the distributions of clouds and water vapor on a global scale and their effects on the natural radiation budget. The understanding of water vapor and cloud distribution is critical when trying to determine precipitation patterns especially on land as this affects vegetation, agriculture, water resources, and flooding.
Scientists have theorized two possible scenarios concerning the increase of water vapor in our atmosphere. One possible scenario shows that an increase in water vapor could enhance the greenhouse effect, which would raise the temperature and allow for more water vapor to enter into the atmosphere. This positive feedback scenario is one possible outcome of increasing the average global temperature.
The other possible scenario would be a negative feedback. In this case, the increase in water vapor in the atmosphere would lead to a heavier cloud cover which in turn would reflect more incoming radiation from the Sun. This would counteract some of the greenhouse effect. Scientists are not sure which scenario is true but both need to be considered when modeling global warming.
Image from NASA’s Destination: Earth Website.
|
|
|
