Impacts of climate change on water supply
The impacts of climate change on the freshwater surface and groundwater systems that are the principal sources of urban water supply are primarily due to expected increases in temperature, increases in worldwide mean precipitation and its variability, and higher sea levels.
The most recent Intergovernmental Panel on Climate Change (IPCC) report, in 2007, indicates the following expected changes in climate variables for the period 2090-2099 relative to the period 1980-1999:
- Global surface temperatures are expected to increase by as much as 3.2 - 7.0°F (1.8 - 4.0°C).
- Global sea level is expected to rise by 7 - 23 inches (0.18 - 0.59 meters), a range that may be conservative, because it excludes possible future rapid changes in ice flow.
- An increase in mean global precipitation is expected with more floods and droughts, more intense precipitation events and possibly more intense tropical storms.
These forecasts suggest substantial pressures on the operation, management, and capacity of urban water systems world-wide. These pressures will be added to the additional demands on urban water systems posed by increasing population and incomes. Cities now account for one half of the world’s 6.6 billion population; by 2030, it is expected that almost 5 billion people will live in cities. These changes will be seen particularly in developing areas, many of which have highly inadequate urban water supply and wastewater treatment systems now. United Nations (UN) population projections suggest that by 2030 each of the major developing regions of the world will have more urban than rural dwellers, and by 2050 it is expected that two thirds of the inhabitants of the developing world will live in cities. Thus climate impacts on cities, and in particular on urban water supply and treatment, will be one of the principal impacts of climate change world-wide. Moreover, the effects will be distributed unevenly because of expected differences in regional temperature and precipitation changes and in coping capacities.
Factors affecting water supply
Higher temperatures will impact urban water supply systems in a variety of ways. Higher temperatures result in greater evapotranspiration, a factor in the expected increase in drought events in many areas. Higher temperatures also result in earlier snowmelt, which impacts urban water systems that depend on water supplies from snowpack storage in mountainous areas. There is evidence that this effect is already occurring in the Western and Northeastern United States. Moreover, higher water temperatures have generally negative effects on water quality, potentially increasing pollutants such as sediments, nutrients, dissolved organic carbon, pathogens, pesticides, salt, and thermal pollution. Estuary and coastal waters may also be degraded by more combined sewer overflows from more frequent and intense flooding.
Mean and variability of precipitation
The distribution of precipitation changes is expected to increase in tropical areas of high rainfall (such as monsoon areas), to decrease generally in the subtropics, and to increase in higher latitudes. Many arid and semi-arid areas are expected to experience decreases in precipitation together with increased evapotranspiration, putting intense pressures on urban water supply. In some cases urban areas will benefit from more available water, although the increased intensity of rainfall that is expected means that urban systems will be subject to increased flooding, and may experience more difficulty in capturing precipitation in storage. The increased variability of precipitation means that, paradoxically, both more droughts and more floods can be expected to impact urban systems, as more intense rainfall will be accompanied by longer periods between precipitation events.
Sea level rise
Urban water systems in coastal areas will also be impacted by rising sea levels in several ways. These challenges are already underway, as global sea level rose by about 7 inches in the 20th century. Sea level rise impacts urban water systems through saltwater intrusion into aquifers, river withdrawals for water use through the upstream encroachment of the salt front, and the discharge of treated effluents from wastewater treatment systems located at sea level. The latter will experience more frequent flood outages because storm surges will be superimposed on a higher sea level. The consequences of a rising sea level will be felt most acutely in low-lying neighborhoods of the world’s major coastal cities with millions of inhabitants, like New York City, Los Angeles, London, Tokyo, Shanghai, Hong Kong, Mumbai (Bombay), Bangkok, and Rio de Janeiro, to name just a few.
These effects, taken together, pose very substantial challenges to the management of urban water and wastewater treatment systems world-wide, which are often stressed already. On the whole, it is expected that the impacts of climate change on urban water supply will be negative, and that even in areas with increased precipitation, this benefit may be wholly or largely offset by negatives such as increased precipitation variability and flood risks.
In planning for adaptation to climate change, water system managers depend on scenarios of climate change in their regions. The IPCC results are generated from Global Climate Models (GCMs) that do not generally produce results that are sufficiently focused for urban planning. GCMs model physical atmospheric and ocean processes within grid boxes on short time scales. These are typically from 100 km (62.5 miles) to 400 km (250 miles) on a side at the equator, with width narrowing as the grid box system moves poleward. There is thus a need to downscale IPCC GCM results to smaller geographical areas. This can be done by approximation methods that typically examine grid boxes over and adjacent to the area of interest, weighting temperature and precipitation results in a way that gives greater importance to the nearer gridboxes. It should be recognized that this is an approximation device, as the GCMs do not model the smaller areas explicitly. Other models, called Regional Climate Models (RCMs), model smaller areas and are driven by GCM conditions at their boundaries. These are more resource-intensive than downscaling processes.
Impacts and adaptation
The challenge of dealing with climate impacts on urban water systems has begun to be confronted in some countries of the developed world including Canada, USA, UK, Germany, Australia, and the Netherlands, although there are few examples to date of implemented adaptations. In the United States, states, counties, and cities have taken the lead in the absence of a strong Federal policy on adaptation. Leading examples at each level of government are California, Washington State, King County, and New York City. As a general statement the field of adaptation of urban water supply to climate change is in its early stages, but there is a reasonable expectation that many suitable adaptations can be made in time to deal with climate change stresses. The situation is different in many urban areas in developing countries, particularly in coastal regions, which are already highly stressed and lack sufficient resources for the ordinary challenges of maintaining a clean water supply. Urban water supply systems in developing areas will be a concern for many decades to come.
The range of potential adaptations available where resources are sufficient includes policy, management, and infrastructure investment and rehabilitation. Policy changes include major decisions such as the willingness to collaborate and share water with neighboring systems through interconnections and joint operations. Management and operations changes will utilize modeling and revised operating rules to reflect changing conditions and, in general, a shift from decisions based on historical hydrologic data to using future scenarios as the basis for planning future operations. Changes may include reallocation of storage between flood control and water supply, adaptive management of changing water quality, and increased water banking in groundwater reservoirs. Infrastructure investment and rehabilitation options are wide-ranging. Flood walls may be needed for coastal wastewater treatment plants; additional upstream water storage may be required; upstream provision for inland flood protection may be needed; and drainage systems may be expanded and redesigned to adapt to more intense rainfall events. Many adaptations, such as groundwater banking, that have been developed to deal with water stresses other than climate, are suitable also for use in adapting to climate change. As a result, some adaptations for climate change will have co-benefits in providing resiliency toward other sources of variability.
Adaptation of urban water systems to climate change has progressed to the extent that some useful guidelines for incorporating climate change in infrastructure design and management can be listed. These include the following steps:
- Conduct audit of existing infrastructure, lifetimes, rehabilitation cycles
- Compare existing systems with climate change forecasts for the region/locality
- Design for thresholds and ranges of forecast temperature, sea level, hydrology
- Evaluate potential adaptations (cost/benefit, environmental impacts)
- Schedule adaptations (over decades)
- Update and revise adaptation plans periodically based on the latest climate science assessments
- Choose adaptation options that are capable of flexibility and adjustment to changing conditions
- Review climate science and parameters (every 3-5 years)
Where adequate resources are available, following these guidelines provides some assurance that urban water systems can be successfully adapted to climate change. However, planning for changes in system design and operation is a lengthy process, and prudence requires an early start.
- Greater London Authority, Adapting to climate change: Lessons for London. London, UK: Greater London Authority, 2006
- Intergovernmental Panel on Climate Change, “Summary for Policymakers,” in Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report Intergovernmental Panel on Climate Change, 2007.
- Intergovernmental Panel on Climate Change, “Freshwater resources and their management,” Chapter 3 of Climate Change 2007: Impacts, Adaptation, and Vulnerability, Contribution of Working Group II to the Fourth Assessment Report Intergovernmental Panel on Climate Change, 2007.
- Cynthia Rosenzweig, David C. Major, Kate Demong, Christina Stanton, Radley Horton, and Melissa Stults, “Managing Climate Change Risks in New York City’s Water System: Assessment and Adaptation Planning,” Mitigation and Adaptation Strategies for Global Change, 2007, DOI 1007/s11027-006-9070-5.