Climate Change Impacts

Water resources are at risk from climate change. These risks can occur individually or cumulatively at specific locations and pose challenges to federal, state, tribal, and local government water managers. According to the National Action Plan, which heavily relies on reports by the U.S. Global Change Research Program, they include:

Air and Water Temperature Increases

Warmer air and water temperatures will have significant impacts on water resources and aquatic habitats, including:

• shifts in aquatic species distribution and population
• increased concentrations of some pollutants
• increased production of excess nutrients that stimulate excessive plant growth (eutrophication)

Water temperatures have been rising and increases have been observed in both saltwater and freshwater.

Changes in Amounts and Distribution of Rainfall and Snowfall

As the climate warms, some areas will receive more precipitation while others will receive less, particularly in the western United States. Warmer temperatures will shift the form of precipitation from snow to rain and also result in earlier melting of snowpacks. These changes are expected to lead to decreases in the size of snowpacks and bring about earlier runoff in areas where seasonal cycles of runoff have historically been dominated by snowmelt. Such loss of snowpack storage is expected to result in a decrease in the amount of reliable water supply in areas where snow has been a major component o f the hydrologic system.

Over the past 50 years, precipitation has increased an average of about 5 percent across the nation, with widespread temperature related reductions in snowpack in the West and a transition to more rain and less snow in some areas.

Increased Intensity of Rainfall and Storms

As the climate warms, the hydrologic system becomes more dynamic, the intensity of storms increases and rainfall occurs more often as a downpour. The amount of rain falling in the heaviest downpours has increased approximately 20 percent on average, in the past century, and this trend is very likely to continue. Climate models also project continued increases in the frequency and intensity of the heaviest downpours during this century. Heavy downpours that are now 1-in-20-year occurrences are projected to occur about every 4 to 15 years by the end of this century, depending on location, and the intensity of heavy downpours is also expected to increase.

Sea Level Rise

Rising sea levels resulting from a warming climate are likely to cause a complex set of interrelated impacts on coastal freshwater aquifers and the threat of inundation to water-related infrastructure (e.g., sewage treatment plants and drinking-water treatment plants).

After at least 2,000 years of little change, sea level rose by roughly 8 inches over the past century. Satellite data available over the past 15 years show sea level rising at a rate roughly double the rate observed over the past century. Recent observations confirm that trend; a 2011 study shows that the melting of the Antarctic and Greenland ice sheets is accelerating. If they continue at the current accelerating rates, global sea level could rise as much as 1 foot within the next 40 years.

Indirect Impacts

It is likely that climate change mitigation actions will lead to adoption of low carbon fuel and energy technologies. Some of these technologies may have indirect impacts on water resources. For example, production of biofuels can result in increased competition for water supplies.