finding 12.1 : key-message-12-1

A reliable, safe, and efficient U.S. transportation system is at risk from increases in heavy precipitation, coastal flooding, heat, wildfires, and other extreme events, as well as changes to average temperature (high confidence). Throughout this century, climate change will continue to pose a risk to U.S. transportation infrastructure, with regional differences (high confidence).

This finding is from chapter 12 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

We sought an author team that could bring diverse experiences and perspectives to the chapter, including some who have participated in prior national-level assessments within the sector. All are experts in the field of climate adaptation and transportation infrastructure. The team represents geographic expertise in the Northeast, Mid-Atlantic, South, Central, and Western regions, including urban and rural as well as coastal and inland perspectives. Team members come from the public (federal and city government and academia) and private sectors (consulting and engineering), with practitioner and research backgrounds.

The chapter was developed through technical discussions of relevant evidence and expert deliberation by the report authors at several workshops and teleconferences and via email exchanges. The authors considered inputs and comments submitted by the public, the National Academies of Sciences, Engineering, and Medicine, and federal agencies. For additional information on the overall report process, see Appendix 1: Process. The author team also engaged in targeted consultations with transportation experts during multiple listening sessions.

Because the impacts of climate change on transportation assets for the United States and globally have been widely examined elsewhere, including in the Third National Climate Assessment (NCA3),dd5b893d-4462-4bb3-9205-67b532919566 this chapter addresses previously identified climate change impacts on transportation assets that persist nationally, with a focus on recent literature that describes newly identified impacts and advances in understanding. Asset vulnerability and impacts are of national importance because there are societal and economic consequences that transcend regional or subregional boundaries when a transportation network fails to perform as designed; a chapter focus is the emerging understanding of those impacts. Further, place-based, societally relevant understanding of transportation system resilience has been strongly informed by numerous recent local and state assessments that capture regionally relevant climate impacts on transportation and collectively inform national level risks and resilience. The chapter synthesizes the transportation communities’ national awareness of and readiness for climate threats that are most relevant in the United States.

Description of evidence base:

Global mean sea level has risen since 1900 and is expected to continue to rise.c66bf5a9-a6d7-4043-ad99-db0ae6ae562c High tide flooding is increasing91aeffdb-e82f-4645-abe9-f6ea6909e979 and is projected to continue increasing.91aeffdb-e82f-4645-abe9-f6ea6909e979 The peak storm surge levels are expected to rise more than the rise in sea level; models show that if the depth of storm flooding today is A and the rise in sea level between now and a future occurrence of an identical storm is B, then the resulting future storm surge depths can be greater than A + B.b19545a1-2e63-458c-8497-32a6d023aa89 The U.S. roads and bridges in the coastal floodplainaae26529-edab-4278-8fe1-5763251ddb97 are vulnerable today, as storms are repeatedly causing damage.c4151050-1289-41b6-a2ac-b760afe3c98b,cc670795-0251-48b0-8bf3-2f87fc67f8fd,e3ecbd2a-576b-4626-a05d-1cc3f1183446,3f0c9740-aae2-4250-8f2a-edef68695594 Sea level rise is also projected to impact ports,34d996c7-e76f-455d-b975-1dc8693fff76 airports,e192e196-23b1-417f-b4a3-ce2a8ef52268 and roads.d339d85e-f249-4ab4-acbb-eb605b777dd9,823d4e36-015c-4288-92e9-833a8c9d6ce0,d09c22ad-256c-4fc1-998b-cf888a93fa58 High tide flooding currently makes some roads impassable due to floodingec58e058-9bec-479d-83b8-679f27aa4361,b4808700-a94a-44da-b2bb-d360a83146f1 and is very likely to increase transportation disruptions in the future.b4808700-a94a-44da-b2bb-d360a83146f1

In most parts of the United States, heavy precipitation is increasing in frequency and intensity, and more severe precipitation events are anticipated in the future.e8089a19-413e-4bc5-8c4a-7610399e268c Inland transportation infrastructure is highly vulnerable to intense rainfall and flooding.bde3292e-b7bb-4a48-b2ea-40a594f37eb5,e8089a19-413e-4bc5-8c4a-7610399e268c,ccb1b544-9a86-4b57-a3d7-9499227d67c7,3fed1df6-1ec6-4f8b-a3b7-6bb715cee3ee,0d131f9a-8a41-4607-808d-1514525c668e,b86b6e3d-2579-4e53-a2ca-a257d04c8df9 In the western United States, large wildfires have increased and are likely to increase in the future,a29b612b-8c28-4c93-9c18-19314babce89 escalating the vulnerability of transportation infrastructure to severe precipitation events.b65e9759-8397-48fc-bb41-fca6d6036994,7ada6479-bc35-4d04-8916-638e6a6ffbbe

The frequency of summer heat waves has increased since the 1960s, and average annual temperatures have increased over the past three decades; these temperature changes are projected to continue to increase in the future.29960c69-6168-4fb0-9af0-d50bdd91acd3 Warming temperatures have increased costscd7183d0-7e06-4d08-bba2-3765b2eba3fe and reduced the performance of roads,7088ab60-2e3b-484f-811e-3590848901e6 bridges,50d04578-d18d-4401-9b14-87b507319741,b05cd14d-f90c-42ba-92d7-ab8235603a3c railways,50d04578-d18d-4401-9b14-87b507319741,b05cd14d-f90c-42ba-92d7-ab8235603a3c,88a0560f-7655-43aa-8f5b-e6fd9a4a0818 and air transport.bde3292e-b7bb-4a48-b2ea-40a594f37eb5,8acf6392-b55f-4fbe-a660-1777e06b6607,8dbd70c4-cd8c-4dce-b27c-bfb412901e58 Future temperature increases are projected to reduce infrastructure lifetimea37337e2-9a0a-439d-bfc5-fcb396ac056b,bab690fd-db14-4554-96c3-1d8a92b23a48,d67e92b2-0e74-45a2-8e13-5cb22ea12623 and increase road costs.b0fc2727-11d7-4627-84ac-33c201875b58 Milder winters will likely lengthen the shipping season in northern inland ports,488a66dd-d4b8-4675-b48a-e7f4b9a60833,295090ab-21ab-488e-b320-314adb122c2c benefit transportation safety,b7b33c40-58c1-4a5d-a6fa-f850a96d0981,351fbf4f-480e-450e-9cc3-9cecd429f564,e1b34455-2f79-4bb8-8983-521b2e1c3f82,ccb1b544-9a86-4b57-a3d7-9499227d67c7,14204bc4-38ef-4f94-8bc3-5e3cb561a676 and reduce winter maintenance.50d04578-d18d-4401-9b14-87b507319741,b0fc2727-11d7-4627-84ac-33c201875b58,0b40389d-5528-4924-983d-d803f66b26d8 In Alaska, however, permafrost thawing will damage roadsdf6fcad4-f0ea-4c60-97e1-ae2a40455f51 and increase the cost of roads (Ch. 26: Alaska).

New information and remaining uncertainties:

Peer-reviewed literature on climate impacts to some assets is limited. Most literature addresses local- or regional-scale issues. Uncertainty in the ranges of climate change projection leads to challenges to quantifying impacts on transportation assets, which have long lifetimes.

Impacts to transportation infrastructure from climate change will depend on many factors, including population growth, economic demands, policy decisions, and technological changes. How these factors, with their potential compounding effects, as well as the impacts of disruptive or transformative technologies (such as automated vehicles or autonomous aerial vehicles), will contribute to transportation performance in the future is poorly understood.

The relationship among increases in large precipitation events and flood-induced infrastructure damage is uncertain because multiple factors (including land use, topography, and even flood control) impact flooding.a36df8f5-949c-412c-8371-e5a5b139c757,bc4f3fef-d1f5-465a-9376-6aa2aaa731a1,84167440-9325-4f42-8bc1-e4261323c584,68583faf-24fb-47e8-b1b5-d066cee5d485 Hirsch and Ryberg (2012)a7f8dbf5-3ec8-4ee1-8740-014006b72bfd found limited evidence of increasing global mean carbon dioxide concentrations resulting in increasing flooding in any region of the United States. Archfield et al. (2016)22b8177d-1789-4177-90d5-2f425b344b46 found that flood changes to date are fragmented and that a climate change signal on flood changes was not yet clear.

Assessment of confidence based on evidence:

There is very high confidence that sea level rise and increases in flooding during coastal storms and astronomical high tides will lead to damage and service reductions with coastal bridges, roads, rails, and ports.

There is high confidence that heavy precipitation events have increased in intensity and frequency since 1901 (with the largest increase seen in the Northeast); this trend is projected to continue.e8089a19-413e-4bc5-8c4a-7610399e268c There is medium confidence that precipitation increases will lead to surface and rail transit delays in urban areas. There is medium confidence that flood-induced damages to roads and bridges will increase.

Rising temperatures and extreme heat (high confidence) will damage pavement and increase railway and air transit delays. However, the actual magnitude of those impacts will depend on technological advancements and policy decisions about design and operations.

References :

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