Water is the most important single determinant of the earth's climate. Water covers 70 percent of the earth's surface. The oceans store heat, and they absorb CO2 and other atmospheric chemicals. Snow fields, glaciers, ice sheets, and sea ice are collectively the greatest mass of fresh water on earth. They exercise a major influence on the planet's overall albedo (surface reflectivity). Finally, water vapor is the predominant greenhouse gas. Water vapor, water droplets, and ice crystals are crucial elements in the climatic system.
Several aspects of the hydrologic cycle are important with respect to climate change. As components of the climatic system begin to warm, other factors come into play that amplify or reduce the initial warming. Some of these are atmospheric phenomena or processes directly affecting the earth's radiative balance (e.g., water vapor feedbacks and cloud feedbacks). Some are land-based phenomena or processes with impacts on radiative balance (e.g., snow and ice feedbacks and feedbacks involving surface albedo and temperature, or snow cover and soil moisture). Least well-understood are phenomena or processes that involve the biosphere (e.g., evaporation and transpiration). This chapter describes mechanisms involving the movement of water through the hydrologic cycle. Related mechanisms whose functions rely more directly on exchange of energy with the atmosphere are described in Chapter 12.
Mechanisms Involving Land Surface Hydrology
Precipitation and soil moisture content, and the resulting runoff, are important components in the climatic system. One computer simulation, which examined a shift in ground cover in the Amazon basin from forest and
savannah to pasture, showed regional climate change with a weakened hydrologic cycle exhibiting reductions in both precipitation and evaporation (Lean and Warrilow, 1989). A realistic simulation of precipitation is an important characteristic for studies of climate change. All general circulation models (GCMs) simulate broad features of the observed precipitation pattern, but they also contain significant errors. These include inadequate characterization of the Southeast Asian summer monsoon rainfall and the summer rains in the southern Zaire basin. Recent models also show large differences in estimates of the intensity of tropical ocean rainbelts (Intergovernmental Panel on Climate Change, 1990).
Soil moisture is the "control valve" of the land surface hydrology. Soil moisture is the source of water for evaporation and thus controls heat transfer from the land surface. It also is the principal absorber of heat in the surface. Precipitation and soil moisture, and the associated runoff, are directly interconnected. Soil moisture is an important factor for vegetation, including agricultural crops, and through them affects evapotranspiration, surface reflectivity, and other aspects of the climatic system.
General circulation models appear to be quite sensitive to the proper formulation of the hydrologic budgets of the land surfaces of the earth. For example, numerical experiments reviewed by Mintz (1982) have shown that large-scale changes of land surface evaporation in GCMs produce significant changes in the predicted circulation and precipitation. Smaller and more realistic soil moisture anomalies may not produce such drastic changes, but it appears (e.g., Rowntree and Bolton, 1983) that they can have considerable impact on the climate of the region surrounding the anomally. This and other evidence (e.g., Rind, 1982; Shukla and Mintz, 1982; Sud and Fennessy, 1984; Yeh et al., 1984) indicate that there is a critical need for sound parametric expressions for evaporation and related land surface processes over areas with typical length scales of hundreds of kilometers. While the details of most processes at local scales are well known and understood, as of now there is no agreement on how these hydrologic processes should best be parameterized at the scales appropriate for GCMs. General circulation models are complex in structure, and they involve intense and sophisticated computational schemes. Yet, in most instances their representation of the hydrology of the earth's land surfaces is crude and not well tested.
Climate affects ecosystems in a variety of ways. It is an important influence on processes that determine the carbon and nutrient cycles of
ecosystems. It also affects the community structure of ecosystems. Predicting how ecosystems will respond to climate change is difficult for at least three reasons. First, ecosystems contain a complex web of interactions among biological processes. Direct effects on one process may indirectly influence other processes in ways we do not yet understand. Second, the response of ecosystems to specific climatic changes depends in part on what other environmental factors are changing. A clear example is the interaction between changes in precipitation and CO2, which functions as a growth stimulant to green plants. Third, current climate change predictions are not sufficiently detailed to permit conclusions about the consequences for the biosphere for either natural ecosystems or land management practices. For many biological processes, changes in temperature extremes and the subannual patterns of temperature and moisture are more important than changes in annual mean values. Thus it is difficult to assess the interactions between the biosphere and the climatic system.
Nevertheless, there are some clear interactions. The most important short-time-scale role of the land biosphere in the climatic system is its control of evapotranspiration. Any attempt to develop a realistic land surface parameterization must separate the functioning of vegetation from that of the soil in the hydrologic cycle. Vegetation intercepts water (when precipitation evaporates from leaves before it reaches the soil), extracts water from the soil through roots, slows the transfer of water from soil into the atmosphere, alters wind patterns in ways that affect soil temperature and rates of evaporation or evapotranspiration, and causes differences in surface reflectivity (albedo).
These and other aspects of vegetation and its effects need to be characterized in sufficiently simple terms to be incorporated in GCMs. Adequate data on vegetation cover and soils would also be required.
Impact of Greenhouse Warming on the Hydroligic Cycle
The implications of greenhouse warming for changes in the hydrologic cycle and concomitant water resources were studied by the Panel on Water and Climate of the National Research Council (1977). Much of the more recent research in the 1980s has been reviewed by Gleick (1989). Two major approaches can be distinguished to analyze the problem, namely, the direct and the indirect approach.
In the direct approach, hydrologic variables such as runoff and soil moisture are part of the primary output of a climate forecasting method. Such climate forecasting can be based on paleoclimatic records, more recent (historical) records, and GCM computations. Unfortunately, however, it appears that among all forecasting variables the specifically hydrologic variables
are the ones that involve the largest degree of uncertainty. For example, even when different GCM outputs are in general agreement on changes in precipitation and temperature, they show much larger discrepancies in soil moisture and runoff. Moreover, the spatial scales of the output of the direct forecasting methods are usually one or more orders of magnitude larger than those of watersheds and catchments, which are common in hydrology.
Mainly for this reason, most hydrologists concerned with the likelihood of climate change have made use of indirect approaches. In this second class of approaches the specifically climatic outputs of climate forecasting methods are used as inputs for more detailed hydrologic models, which operate at smaller scales more appropriate for river basins and hydrologic catchments. Unfortunately, the state of the art in hydrologic modeling is probably less advanced than that of the current GCM technology. Most hydrologic simulation models, relating precipitation, runoff, and the moisture state of the catchment, are heavily parameterized and thus require an extensive data record for calibration and validation. Thus the panel concludes they cannot be trusted when they are applied to changing conditions outside their range of calibration. Furthermore, because the outputs from climate forecasting methods are generally acknowledged to be unreliable, in many instances these hydrologic models have been run with hypothetical climate scenarios as input. Usually, there is no way of knowing how internally consistent or otherwise realistic they are.
In spite of these uncertainties a few plausible results emerge from all the research so far. One is that with greenhouse warming, there is a distinct possibility over the mid-latitudes of a relative decrease in winter precipitation in the form of snow. This may in turn result in decreased water storage in the form of winter snowpack. Earlier melting of this snowpack and increased evaporation during the summer may then lead to increased aridity in many areas of the world.
In view of the potentially disastrous consequences of such scenarios, and because of the need to reduce the extreme uncertainty surrounding all this work, it is clear that a vigorous research program is urgently called for. A major effort should be directed toward a better understanding of the main hydrologic transport phenomena at scales relevant for process modeling in climate dynamics and for more regional catchment modeling in hydrology. An important component of this will be the design and execution of experiments on these scales.
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National Research Council. 1977. Climate, Climatic Change, and Water Supply. Panel on Water and Climate. Washington, D.C.: National Academy Press.
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Rowntree, P. E., and J. A. Bolton. 1983. Simulation of the atmospheric response to soil moisture anomalies over Europe. Quarterly Journal of the Royal Meteorological Society 109:501–526.
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