finding 3.2 : key-message-3-2

Deteriorating water infrastructure compounds the climate risk faced by society (high confidence). Extreme precipitation events are projected to increase in a warming climate (high confidence) and may lead to more severe floods and greater risk of infrastructure failure in some regions (medium confidence). Infrastructure design, operation, financing principles, and regulatory standards typically do not account for a changing climate (high confidence). Current risk management does not typically consider the impact of compound extremes (co-occurrence of multiple events) and the risk of cascading infrastructure failure (high confidence).

This finding is from chapter 3 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

Chapter authors were selected based on criteria, agreed on by the chapter lead and coordinating lead authors, that included a primary expertise in water sciences and management, knowledge of climate science and assessment of climate change impacts on water resources, and knowledge of climate change adaptation theory and practice in the water sector.

The chapter was developed through technical discussions and expert deliberation among chapter authors, federal coordinating lead authors, and staff from the U.S. Global Change Research Program (USGCRP). Future climate change impacts on hydrology, floods, and drought for the United States have been discussed in the Third National Climate Assessment3ff0e30a-c5ee-4ed9-8034-288be428125b and in the USGCRP’s Climate Science Special Report.e8089a19-413e-4bc5-8c4a-7610399e268c,a29b612b-8c28-4c93-9c18-19314babce89 Accordingly, emphasis here is on vulnerability and the risk to water infrastructure and management presented by climate variability and change, including interactions with existing patterns of water use and development and other factors affecting climate risk. The scope of the chapter is limited to inland freshwater systems; ocean and coastal systems are discussed in their respective chapters in this report.

Description of evidence base:

Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since about 1900 and are projected to continue to increase over this century, with important regional differences (Ch. 2: Climate).e8089a19-413e-4bc5-8c4a-7610399e268c,b37557ac-ee97-4c28-98ca-4f1f1afe163b Detectable changes in some classes of flood frequency have occurred in parts of the United States and are a mix of increases and decreases (Ch. 2: Climate).3ff0e30a-c5ee-4ed9-8034-288be428125b,e15600d0-290f-44e2-9b58-9ffd295ee6d2 However, formal attribution approaches have not established a significant connection of increased riverine flooding to human-induced climate change, and the timing of any emergence of a future detectable anthropogenic change in flooding is unclear (Ch. 2: Climate). There is considerable variation in the nature and direction of projected streamflow changes in U.S. rivers (Ch. 2: Climate).3ff0e30a-c5ee-4ed9-8034-288be428125b,64014404-d26e-45c7-9b33-8e2253a9ca04

Infrastructure systems are typically sized to cope with extreme events expected to occur on average within a certain period of time in the future (for example, 25, 50, or 100 years), based on historical observations.112eb5c3-41b2-4a81-a07f-d467711c41eb There is substantial concern about the impacts of future changes in extremes on the existing infrastructure. However, the existing operational design and risk assessment frameworks (for example, rainfall intensity–duration–frequency, or IDF, curves and flood frequency curves) are based on the notion of time invariance (stationarity) in extremes.882957f7-f48f-4cd0-a294-acd47609938d,05bd57a9-fd03-40d1-b2e1-d2261b9266c2

Variability in sea surface temperatures influences atmospheric circulation and subsequently affects the occurrence of regional wet and dry periods in the United States.78211ce6-158f-4caa-b7f8-0f80d32c895c,e6cb8869-cd7b-4635-9c84-a009468a4962,87c37b27-962f-4c4f-959f-70ae474d526f,51d2b0cb-d63e-424d-a23e-812e4d69bc81,e7f5961d-d245-4c99-9dc6-b87ea8237ad1 Reconstructed streamflow data capture the extreme dry/wet periods beyond the instrumental record, but a limited literature has considered their application for water management.18dc1019-1d08-4de4-beec-2b32149d8c50,7e8aa607-09d4-4313-8cb3-dca9bda256a9

A number of models have been developed to incorporate the observed and/or projected changes in extremes in frequency analysis and risk assessment.8ed13492-803d-432e-a09f-1ec2e3b5c035,fecebf8c-4efd-438f-b5d2-6ca8970817b1,ec212544-37bc-4752-8885-9dc143bb051e,40238086-eb6b-43ec-9203-6e0e815638e6,b3d303f6-9153-48ab-9211-b2eaace11db7,e0ef030a-67cc-4e80-bc36-0c5b38565145,2d7eac05-0cb2-4046-8ffb-59d369bde630,f12a311b-5481-4992-99aa-91feb2f9ff6e The appropriateness of a fixed return period for IDF curves or for flood/drought frequency analysis is also questioned in the literature.a5e05170-d9cb-45fe-8c73-7aeaa4e9a5bc,4e34060a-8368-4e07-80ca-05fbd4e89c26,31bf15ab-c374-4466-8b4c-894a527813cb,b5ce2baf-ce26-4597-8d78-25c52f6d170c This chapter has not evaluated the existing methods in the literature that account for temporal changes in extremes, and the issue warrants more investigation in the future.

Previous studies show that compound extreme events can have a multiplier effect on the risks to society, the environment, and built infrastructure.2ec30e37-5594-44e2-acd4-a7a8b3964027,faea1d4f-493d-4545-bea1-1703ad92ac95 Current design frameworks ignore this issue and mainly rely on one variable at a time.48840ae4-7ca3-4663-b406-49f2dc42ee95,faea1d4f-493d-4545-bea1-1703ad92ac95,e8a46b45-5e9f-46cc-aed3-7855cd849140 For example, coastal flood risk assessment is primarily based on univariate methods that consider changes in terrestrial flooding and ocean flooding separately.f5f8a8bf-0d94-4699-9b48-36bbe84ce0f7,882957f7-f48f-4cd0-a294-acd47609938d,ebf174d9-9c08-4902-8a26-4fea81be11a9 Few studies have offered frameworks for considering multiple hazards for the design and risk assessment of infrastructure.2ec30e37-5594-44e2-acd4-a7a8b3964027,faea1d4f-493d-4545-bea1-1703ad92ac95 Expected changes in the frequency of extreme events and their compounding effects can have significant consequences for existing infrastructure systems.

New information and remaining uncertainties:

There are high uncertainties in future floods because of uncertainties in future long-term regional/local precipitation and uncertain changes in land use/land cover, water management, and other non-climatic factors that will interact with climate change to affect floods. There also are high uncertainties in future water supply estimates because of uncertainties in future precipitation. Drought increase due to combined precipitation and temperature change has a moderate uncertainty.

Assessment of confidence based on evidence:

There is high confidence in the presence of a strong relationship between precipitation and temperature, indicating that changes in one will likely alter the statistics of the other and hence the likelihood of occurrence of extremes. The aging nature of the Nation’s water infrastructure is well documented. Not all aging infrastructure is deteriorating, however, and many aging projects are operating robustly under changing conditions. Unfortunately, no national assessment of deteriorating infrastructure or the fragility of infrastructure relative to aging exists. For example, the U.S. Army Corps of Engineers (USACE) has assessed how climate change projections with bias correction compare with the nominal design levels of USACE dams; however, this represents only a fraction of the Nation’s 88,000 dams. While age may be an imperfect proxy for deterioration, it is used here to call attention to the general concern that many elements of the Nation’s water infrastructure are likely not optimized to address changing climate conditions. There is high confidence that deteriorating water infrastructure (dams, levees, aqueducts, sewers, and water and wastewater treatment and distribution systems) compounds the climate risk faced by society.

Studies show that compound extreme events will likely have a multiplier effect on the risk to society, the environment, and built infrastructure. Sea level rise is expected to increase in a warming climate. Sea level rise adds to the height of future storm tides, reduces pressure gradients that are important for transporting fluvial water to the ocean, and enables greater upstream tide/wave propagation and coastal flooding.

There is high confidence in the existence of the interannual and decadal cycles but medium confidence in the ability to accurately simulate the joint effects of these cycles and anthropogenic climate change for water impacts.

Currently, coastal flood risk assessment is primarily based on univariate methods that consider changes in terrestrial flooding and ocean flooding separately, which may not reliably estimate the probability of interrelated compound extreme events. The expected changes in the frequency of extreme events and their compounding effects will likely have significant consequences for existing infrastructure systems. Because of the uncertainties in future precipitation and how extreme events compound each other, there is medium confidence in the effects of compound extremes (multiple extreme events) on infrastructure failure.

References :

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