The scientific evidence for human-caused global climate change has become quite compelling in recent years. Global average temperatures have been rising, and human activities have changed the composition of the atmosphere significantly enough that we can now confidently say that the climate will continue to change. Along with the projected future warming, there will be changes in atmospheric and oceanic circulation, and in the hydrologic cycle, leading to altered patterns of precipitation and runoff. There also will likely be impacts on other physical and biological systems. For example, a warmer climate will make sea level rise inevitable. There also will be new stresses on ecological systems, including forests and riparian zones as well as coastal and freshwater aquatic systems. Such stresses may affect the regulatory environment for water utilities. Water provision for environmental needs is an important policy consideration in many locations, and climate change may make it more difficult to achieve a satisfactory balance between human water uses and environmental stewardship.
Scientists agree on some of the important broad-scale features of the expected hydrologic changes, the most likely of which will be an increase in global average precipitation and evaporation as a direct consequence of warmer temperatures. That, however, does not mean that there will be more precipitation everywhere or that runoff and recharge would increase in proportion to precipitation.
At the regional scale, precipitation predictions are less certain. Changes in circulation patterns will be critically important in determining future changes in precipitation and water availability, and climate models can provide only a crude picture of how those patterns may change. The currently available evidence suggests that arctic and equatorial regions may have a tendency to become wetter, and that subtropical regions may experience drying. Projections of precipitation changes for temperate regions are less consistent.
The water supply for any utility will depend on the quantity and timing of local and regional precipitation, both of which may change with global climate change. While it is impossible to make reliable predictions of changes in the overall quantity of precipitation for a particular region, scientific theory suggests an intensification of the global hydrological cycle, leading to more intense but possibly less frequent periods of precipitation.
In other words, we may see longer periods of drought alternating with spells of heavy rainfall and runoff. Such changes could create a number of difficulties for water utility planning and operations. For example, greater runoff variability could make it more difficult to maintain optimal reservoir levels, which could reduce the reliability of water storage. In addition, increased reliance on groundwater resources during extended dry spells could reduce aquifer levels and discharge to surface water bodies, which could cause unintended damage to freshwater ecosystems.
The direct effects of temperature changes on water supplies are also significant, particularly for the timing of runoff. For example, in mountainous regions, there will likely be shorter snow accumulation periods – especially in lower elevation areas, possibly leading to reduced annual snowpacks, earlier spring melting and reduced late summer flows. Warmer temperatures during the winter will affect the form of precipitation, with a larger fraction of total precipitation coming as rain rather than snow. However, when it does snow, warmer temperatures and increased moisture availability may result in heavier snowfalls. A temperature change of only a few degrees during the melting season would have a substantial effect on the timing of spring runoff. Less snowpack in the late spring means that there will be a smaller supply in late summer, when water is scarcest and demand is high.
Unfortunately, at the regional and local scales that are relevant for water utilities, current scientific understanding does not yet allow confident projections of the magnitude or precise nature of climatic changes. Therefore, important uncertainties remain regarding how regional and local climates, hydrology and ecosystems will change in the coming century. Because regional and local variables are what matter for municipal water management, the prospect of climate change has imposed a new level of uncertainty on water managers. This suggests that they will need to examine and adapt their methods of planning to account for the fact that past hydrological patterns may become an increasingly unreliable guide to the future.
Why is climate change of particular interest to water resource planners? First, given the nature of the industry, decisions made in the near term will affect system reliability well into the future. For instance, utilities build costly water infrastructure with the expectation that investments will meet future requirements for decades to come. Likewise, policies designed to improve the efficiency of water use take time to implement, and it takes a long time to achieve their full benefits. Second, long-term planning in the context of uncertainty is already standard practice in this industry. Water utilities must account for many future uncertainties when formulating long-term plans, such as potential changes in water consumption patterns due to demographic and socio-economic changes. Climate change is an additional source of uncertainty that will become increasingly relevant to water resource managers in the 21st century. Just as with any other source of uncertainty, best practice requires understanding as much as possible about the changes that can occur and their implications for operation and management of the utility.