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Global Change Instruction Program The Water Cycle The water cycle is a balanced system, with water stored in many places at any one time (Table 3). The cycle involves the transfer of water in its various forms of liquid, vapor, ice, and snow through the land, air, and water environments. Both matter and energy are involved in the transfer. The transfer begins when heat from the sun warms the ocean and land surfaces and causes water to evaporate. The water vapor enters the atmosphere and generally moves with the The water cycle (Figure 21) is important in the context of biogeochemical cycling in the C, N, P, S, and O system, as well as being important in its own right. Water circulating through the ecosphere is part of a continuous hydrologic cycle that makes life on earth possible. The water cycle is the driver cycle for transport of many elements at the earth’s surface. In the atmosphere, water vapor is the most important greenhouse gas, and its behavior during a global warming is of concern. Figure 21. The global biogeochemical cycle of water (the hydrologic cycle), showing the major processes of water movement. Numbers in parentheses show the water budget in thousands of cubic kilometers per year (after Maurits la Rivière, 1990.) Atmospheric water vapor transport (40) So lar rad iat Precipitation of Snow, ice, rain (111) (71) ion Cloud Evaporation r cie Su a Gl rfa ce ru Transpiration no Evaporation (425) ff Surface water Lake River Precipitation (385) Soils Return flow (40) Percolation Land Ocean Groundwater flow 41 Understanding Global Change: Earth Science and Human Impacts circulation of the air. On a global Table 3. Distribution of water in the ecosphere scale, warm air rises in the atmosphere and cooler air descends. The Percent of total Reservoir Volume (106 km3) ________________________________________________________________ water vapor rises with the warm Ocean 1370 97.25 air. The farther from the warm planCryosphere etary surface the air travels, the (ice caps and glaciers) 29 2.05 cooler it becomes. Cooling causes Groundwater 9.5 0.68 water vapor to condense on small Lakes 0.125 0.01 particles (cloud condensation Soils 0.065 0.005 nuclei) in the atmosphere and to Atmosphere 0.013 0.001 precipitate as rain, snow, or ice and Rivers 0.0017 0.0001 fall back to the earth’s surface. When the precipitation reaches the Biosphere 0.0006 0.00004 ___________________________________________________________ land surface, it is evaporated directTotal 1408.7 100 ly back into the atmosphere, runs off or is absorbed into the ground, After Berner and Berner, 1996. or is frozen in snow or ice. Also, plants require water and absorb it, retaining some of the water in their tissue. The rest is returned to the atmosphere the planet will probably lead to more water through transpiration. Precipitation on land is vapor in the atmosphere. The increased water equal to evapotranspiration plus runoff to the vapor has the potential to absorb more infrared ocean. That is, over the land, there is more preradiation reradiated from the planetary surface cipitation than evapotranspiration. and thus lead to further warming. This is another For the ocean, the situation is reversed. Much example of a positive feedback. of the water evaporated from the ocean returns Water droplets form in the troposphere by there directly; however, a small amount (about condensation of water on cloud condensation 8% of that evaporated) is carried by atmospheric nuclei. These particles may absorb or reflect enerwinds over the continents, where it precipitates. gy. The amount of water vapor in part deterOnce on the ground, the water finds its way to mines the types and distribution of clouds that streams, lakes, or rivers in runoff or by percolaform in the atmosphere. In terms of predicting tion into and through groundwater. In due time, the effects of increasing concentrations of greenthe water will return to the ocean, mainly in house gases in the atmosphere on temperature stream and river flows and less importantly in and other climatic variables, general circulation groundwater. This return flow balances the net models (GCMs) and other types of models have loss from the ocean surface by evaporation. been used. The GCMs are very complex computSnow and ice may remain on the land for a er representations of the atmosphere or atmolong time before the water in these forms of presphere-ocean system that are used in the modelcipitation evaporates to the atmosphere or ing of global climate change. In these models, the returns via rivers or as direct glacial meltwater to effects of clouds on the radiant energy budget of the oceans. The snow and ice that feed glaciers the planet are a major source of uncertainty in may remain locked up in the cryosphere for attempting to predict future climate change. thousands of years, but finally the ice will melt, Clouds regulate the radiative heating of the and the water will travel to another part of the planet. They reflect a significant part of the hydrologic cycle. incoming solar radiation. Clouds also absorb Water vapor in the atmosphere is the princilongwave, infrared radiation emitted by the earth. pal greenhouse gas. Because the amount of water At the cold tops of clouds, energy is emitted to vapor in the atmosphere is dependent on the space. In 1984 the Earth Radiation Budget temperature of the planet, any initial warming of Experiment (ERBE) was launched. This 42 Global Biogeochemical Cycles and the Physical Climate System GCMs only perform calculations at widely separated points over the globe and relatively infrequently. Cloud formation involves very dynamic processes at short time and spatial scales. Clouds may act as a positive or negative feedback in a future earth warmed by the enhanced greenhouse effect. This ambiguity accounts in part for the range of estimates of the average temperature increase predicted by the GCMs for a doubling of the atmospheric carbon dioxide concentration. A final comment on the water cycle is that it is being significantly affected by water usage and contamination of water stocks. Currently, humans use an amount of water equivalent to about 25% of total terrestrial evapotranspiration and 55%, or 6,800 cubic kilometers per year, of the runoff of water from the continents that is accessible. Only about 20% of the world’s drainage basins have pristine water quality. There is little doubt that the world’s water resources will be significantly strained in the 21st century. experiment involves a system of three satellites that provide data on incoming and outgoing radiation. One result of this experiment so far is the demonstration that clouds have a net cooling effect on the earth. On a global scale, clouds reduce the amount of radiative heating of the planet by –13 watts per square meter. This is a large number when compared to the +2.5 watts per square meter attributed to the increase in atmospheric greenhouse gases during the last century. It is also large compared to the radiative heating that could arise from a doubling of atmospheric carbon dioxide concentrations in the next century—about +4 watts per square meter. Thus, a small change in the types and distribution of clouds may have a large effect on the radiation budget compared to the effect of changing greenhouse gas composition owing to human activities. In a world already warmed by greenhouse gases released from human activities, it is difficult to predict what will happen to the types and distribution of clouds. Too little is known about the complex processes of cloud formation and, in particular, the response of clouds to a warming earth. Also, these complex processes are difficult to simulate in the general circulation models. One difficulty lies with the problem of reproducing cloud physics in the models. A second difficulty is that 43