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AS THE WORLD WARMS
Report by the American Water Works Association, Jan. 05, 2007
Climate change is already happening: over the past century, global average surface temperature increased by approximately 0.6 degrees Celsius and warming is expected to continue. The modest warming to date has not been evenly distributed over the surface of the globe. In particular, arctic areas have warmed more rapidly than other areas. Climate model simulations also suggest that future warming will tend to be more pronounced in the higher northern latitudes.
A change that appears most likely is that global average precipitation will increase as global temperatures rise. Evaporation will increase with warming because a warmer atmosphere can hold more moisture. The net result is basically "what goes up, must come down" in the form of more precipitation. This moisture holding capacity is governed by the exponential Clausius-Claperyon equation, which states that for a one degree Celsius increase in air temperature, the water-holding capacity of the atmosphere is increased by about seven percent. On average, current climate models suggest an increase of one to two percent per degree Celsius due to warming forced by carbon dioxide. However, an increase in global average precipitation does not mean that it will get wetter everywhere and in all seasons. Climate models tend to agree in projecting precipitation increases over high-latitude land areas, much smaller and less certain increases over equatorial regions, and decreases over some subtropical areas. Climate models differ considerably whether precipitation will increase or decrease over the middle latitudes, including the continental US.
There is a high level of confidence in projections of warmer temperatures over most land surfaces. Unlike their projections of precipitation change, climate models are fairly consistent in predictions of regional surface temperature. Because temperature is central in determining the accumulation and melting of snow and ice, these scenarios are especially relevant to regions where snowpack or glacial runoff dominate the hydrology. With rising temperatures, it is very likely that a greater portion of winter precipitation will fall as rain rather than snow, especially in areas where winter temperatures are now only slightly below freezing. An increase in rain events would increase winter runoff but result in smaller snowpack accumulations. Temperature also determines the timing of melt-off, and a warmer climate will likely result in an earlier melt season. Many regions are likely to see an increase in winter or spring flows and reduced summer flows. In fact, there is evidence that this is already occurring. Studies document the fact that the peak spring runoff has been arriving earlier in the last few decades.
One of the most important impacts of global warming is a rise in sea level. Miami-Dade County in Florida has measured a 12-inch rise in sea level since 1848. Rising sea levels impact coastal water utilities in six key ways: 1) lowland inundation and wetland displacement; 2) altered tidal range in rivers and bays; 3) changes in sedimentation patterns; 4) severe storm-surge flooding; 5) saltwater intrusion into estuaries and freshwater aquifers; and 6) increased wind and rainfall damage in regions prone to tropical cyclones. Water utility infrastructure such as water intakes, are particularly vulnerable to these effects of rising sea levels.
Given the uncertain nature of climate impact analysis, it may be tempting to disregard climate change in water resource planning. Rather, the uncertainty introduced by climate change emphasizes the importance of incorporating flexibility or no-regrets options in water resource planning.
Reference: Climate Change and Water Resources: A Primer for Municipal Water Providers. American Water Works Association and Awwa Research Foundation, No. 91120. www.awwa.org
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