The Fourth National Climate Assessment: What It Says About Water and Climate Change

The US Government’s Fourth National Climate Assessment, issued in the fall of 2018, devotes a long chapter to the effects of climate change on the nation’s water. Below are excerpts from the chapter which highlight its  important features.

National Climate Assessment: Highlights from the Water Chapter

by Emily McBroom

Rising air and water temperatures and changes in precipitation are intensifying droughts, increasing heavy downpours, reducing snowpack, and causing declines in surface water quality, with varying impacts across regions. Future warming will add to the stress on water supplies and adversely impact the availability of water in parts of the United States.

Changes in the relative amounts and timing of snow and rainfall are leading to mismatches between water availability and needs in some regions, posing threats to, for example, the future reliability of hydropower production in the Southwest and the Northwest. Most U.S. power plants rely on a steady supply of water for cooling, and operations are expected to be affected by changes in water availability and temperature increases.

Groundwater depletion is exacerbating drought risk in many parts of the United States, particularly in the Southwest and Southern Great Plains.

Dependable and safe water supplies for U.S. Caribbean, Hawaii, and U.S.-Affiliated Pacific Island communities are threatened by drought, flooding, and saltwater contamination due to sea level rise.

Aging and deteriorating water infrastructure, typically designed for past environmental conditions, compounds the climate risk faced by society. Water management strategies that account for changing climate conditions can help reduce present and future risks to water security, but implementation of such practices remains limited.


Changes in Water Quantity and Quality

Changes in climate and hydrology have direct and cascading effects on water quality. Anticipated effects include warming water temperatures in all U.S. regions, which affect ecosystem health, and locally variable changes in precipitation and runoff, which affect pollutant transport into and within water bodies.

These changes pose challenges related to the cost and implications of water treatment, and they present a risk to water supplies, public health, and aquatic ecosystems.

Increases in high flow events can increase the delivery of sediment, nutrients, and microbial pathogens to streams, lakes, and estuaries; decreases in low flow volume (such as in the summer) and during periods of drought can impact aquatic life through exposure to high water temperatures and reduced dissolved oxygen.

The risk of harmful algal blooms could increase due to an expanded seasonal window of warm water temperatures and the potential for episodic increases in nutrient loading.

In coastal areas, saltwater intrusion into coastal rivers and aquifers can be exacerbated by sea level rise (or relative sea level rise related to vertical land movement), storm surges, and altered freshwater runoff. Saltwater intrusion could threaten drinking water supplies, infrastructure, and coastal and estuarine ecosystems).

Indirect impacts on water quality are also possible in response to an increased frequency of forest pest/disease outbreaks, wildfire, and other terrestrial ecosystem changes; land-use changes (for example, agricultural and urban) and water management infrastructure also interact with climate change to impact water quality.


Deteriorating Water Infrastructure at Risk

Capital improvement needs for public water systems (which provide safe drinking water) have been estimated at $384 billion for projects necessary from 2011 through 2030. Similarly, capital investment needs for publicly owned wastewater conveyance and treatment facilities, combined sewer overflow correction, and storm water management to address water quality or water quality-related public health problems have been estimated at $271 billion over a 20-year period. To date, however, there is no comprehensive assessment of the climate-related vulnerability of U.S. water infrastructure, and climate risks to existing infrastructure systems remain unquantified.

Compound extremes, such as terrestrial flooding and ocean flooding occuring at the same time, can also increase the risk of cascading infrastructure failure since some infrastructure systems rely on others, and the failure of one system can lead to the failure of interconnected systems, such as water–energy infrastructure.

Water Management in a Changing Future

Paleoclimate analyses and climate projections suggest persistent droughts and wet periods over the continental United States that are longer, cover more area, and are more intense than what was experienced in the 20th century.

The challenge is both scientific, in terms of developing and evaluating these approaches, and institutional–political, in terms of updating the regulatory, legal and institutional structures that constrain innovation in water management, planning, and infrastructure design.

Source: The Fourth National Climate Assessment: Chapter Three.