By Anna Mische John
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The widespread global presence of mountainous areas invites comparative study of the effects of climate change on a global scale. Several studies suggest that high elevation environments are among those most sensitive to such climate change on a global scale, due to the unique interactions of air and water at the increased air pressure of high elevations (Diaz et al, 2003). This sensitivity provides a sort of "fast forward" view of climatological effects, because environmental changes on mountainous ecosystems are incrementally amplified in comparison of those found at lower levels. For hydrological study, mountains are of particular interest: besides representing approximately 25% of the earth's land area, mountains are the source of freshwater for millions. The Rocky Mountains of the United States can be a site of globally applicable field research because they typify the topography and hydrology of high elevation environments.
A mountain range's complex topography and rapid change in elevation over short distances creates characteristic temperature patterns and hydrology. The resulting topographic lifting is know as the orographic effect. As air parcels gain elevation while passing over mountains, they become increasingly saturated because of the decrease in temperature and increasing air pressure. Air parcels become saturated when they reach the dew point-a variable of temperature and air pressure-and precipitation is then released. The area over which where precipitation occurs is dependant on the present temperature conditions. The orographic cooling of air causes a "rain shadow" that results in windward slopes receiving more moisture than leeward slopes. With temperature increases and all other variables constant, the air parcels will have to travel higher up the mountain to release their moisture in the solid form. This has been observed and is generalized by the formula: for every temperature increase of 1° Celsius snowline retreats 150 meters uphill. A change in the "beginning" of the mountain's hydrologic cycle has effects throughout the ecosystem.
The temperature threshold between the phase changes of water plays a particularly important role in the hydrology of mountainous areas. Under current climatic conditions, mountains in temperate regions tend to have snow pack near the melting point (Beniston, 2003), so even a minor increase in ambient temperature would produce dramatic results. Typically in the Rocky Mountains, most precipitation falls in the solid form and remains in reservoirs of snow pack or glaciers for a period of time until the ambient temperature warms the frozen parcel of water throughout to the melting point. The warming period predictably occurs in the spring, and the reservoir of water is released by a phase change into liquid. The runoff is channeled into streams which increase in volume down the mountain as the products of other frozen water reservoirs converge. Most of the streams in the Rockies reach their greatest volumes in the spring months due to snow runoff, and then drastically retreat once the frozen reservoirs have melted. Demand for water in the areas supplied by Rocky Mountain runoff is at its peak when precipitation is at its lowest during the summer months of July and August, at the same time the mountain runoff is decreasing. Urgency for this study arises out of the scarcity of water in the arid West of the United States: mountains provide between 90% and 100% of the freshwater used for drinking, irrigation, and industrial uses to the surrounding areas.
The Rocky Mountain area has been developed despite the lag time between precipitation and demand for water. The human systems developed to adjust for demand with peak spring stream flow are based on relatively recent historical trends of climate patterns. A small increase in mean annual temperature of even 1 degree would cause an earlier snow melt in the mountains when the demand for the water by the lowland populations isn't high, further throwing off the lag time of supply and demand and contribute to seasonal flooding and water shortages.
Such an increase in mean annual temperatures would result in less solid precipitation to begin with; rather than being stored as snow until the warmer seasons, soil absorption and runoff would occur as the precipitation fell, not gradually as in the current pattern. If global temperatures increase, mountain regions will receive an increased proportion of precipitation as liquid rather than solid snow. The consequences of this change in the hydrologic cycle would affect the mountain watersheds and low-lying areas by causing new patterns of erosion and flooding.
An increase in temperature is also likely to cause increased melting of small mountain glaciers and create additional runoff which would add to stream volume and groundwater levels. Glaciers in the Rockies are relatively small, and the smaller the glacier the more it will be affected by an increase in annual temperature (Beniston, 2003).
Mountain studies, including those dealing with hydrology, are currently included in an initiative of the United Nations dealing with the concern of global climate change. The results from mountain research on climate change are expected to help predict hydrologic changes for the remaining 75% of the earth's land mass because they are so sensitive to environmental changes and are affected so dramatically. The change in water resource distribution - both chronologically and geographically - is something that climate change will influence across land masses, and observation in the Rockies can provide a good predictive model for results elsewhere. Besides mountainous communities and those directly serviced by mountain runoff, the information gathered by studying climate change on high elevation ecosystems is relevant globally by applying the principles learned by studying accelerated environmental change.
References Cited --
Henry F. Diaz, Martin Grosjean and Lisa Graumlich, 2003. Climate Variability and Changes in High Elevation Regions:Past, Present and Future. Climatic Change, Volume 59, Issue 1-2, Pages 1-4
M. Beniston, 2003. Climatic change in mountain regions: A review of possible impacts. Climatic Change, Volume 59, Issue 1-2, Pages 5-31
*Image taken from www-class.unl.edu/ geol101i/15a_climate.htm