finding 18.3 : key-message-18-3

The Northeast’s urban centers and their interconnections are regional and national hubs for cultural and economic activity. Major negative impacts on critical infrastructure, urban economies, and nationally significant historic sites are already occurring and will become more common with a changing climate. (High Confidence)

This finding is from chapter 18 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

It is understood that authors for a regional assessment must have scientific and regional credibility in the topical areas. Each author must also be willing and interested in serving in this capacity. Author selection for the Northeast chapter proceeded as follows:

First, the U.S. Global Change Research Program (USGCRP) released a Call for Public Nominations. Interested scientists were either nominated or self-nominated and their names placed into a database. The concurrent USGCRP Call for Public Nominations also solicited scientists to serve as chapter leads. Both lists were reviewed by the USGCRP with input from the coordinating lead author (CLA) and from the National Climate Assessment (NCA) Steering Committee. All regional chapter lead (CL) authors were selected by the USGCRP at the same time. The CLA and CL then convened to review the author nominations list as a “first cut” in identifying potential chapter authors for this chapter. Using their knowledge of the Northeast’s landscape and challenges, the CLA and CL used the list of national chapter topics that would be most relevant for the region. That topical list was associated with scientific expertise and a subset of the author list.

In the second phase, the CLA and CL used both the list of nominees as well as other scientists from around the region to build an author team that was representative of the Northeast’s geography, institutional affiliation (federal agencies and academic and research institutions), depth of subject matter expertise, and knowledge of selected regional topics. Eleven authors were thus identified by December 2016, and the twelfth author was invited in April 2017 to better represent tribal knowledge in the chapter.

Lastly, the authors were contacted by the CL to determine their level of interest and willingness to serve as experts on the region's topics of water resources, agriculture and natural resources, oceans and marine ecosystems, coastal issues, health, and the built environment and urban issues.

On the due diligence of determining the region’s topical areas of focus

The first two drafts of the Northeast chapter were structured around the themes of water resources, agriculture and natural resources, oceans and marine ecosystems, coastal issues, health, and the built environment and urban issues. During the USGCRP-sponsored Regional Engagement Workshop held in Boston on February 10, 2017, feedback was solicited from approximately 150 online participants (comprising transportation officials, coastal managers, urban planners, city managers, fisheries managers, forest managers, state officials, and others) around the Northeast and other parts of the United States, on both the content of these topical areas and important focal areas for the region. Additional inputs were solicited from other in-person meetings such as the ICNet workshop and American Association of Geographers meetings, both held in April 2017. All feedback was then compiled with the lessons learned from the USGCRP CLA-CL meeting in Washington, DC, also held in April 2017. On April 28, 2017, the author team met in Burlington, Vermont, and reworked the chapter’s structure around the risk-based framing of interest to 1) changing seasonality, 2) coastal/ocean resources, 3) rural communities and livelihoods, 4) urban interconnectedness, and 5) adaptation.

Description of evidence base:

The urban built environment and related supply and management systems are at increased risk of disruption from a variety of increasing climate risks. These risks emerge from accelerated sea level rise as well as increased frequency of coastal and estuarine flooding, intense precipitation events, urban heating and heat waves, and drought.

Coastal flooding can lead to adverse health consequences, loss of life, and damaged property and infrastructure.641ac0a3-aad2-4422-a632-f07117fe694a Much of the region’s major industries and cities are located along the coast, with 88% of the region’s population and 68% of the regional gross domestic product.9f559c9b-c78e-4593-bcbe-f07661d29e16 High tide flooding is also increasingly problematic and costly.c66bf5a9-a6d7-4043-ad99-db0ae6ae562c Rising sea level and amplified storm events can increase the magnitude and geographic size of a coastal flood event. The frequency of dangerous coastal flooding in the Northeast would more than triple with 2 feet of sea level rise.dd5b893d-4462-4bb3-9205-67b532919566 In Boston, the areal extent of a 1% (1 in 100 chance of occurring in any given year) flood is expected to increase multifold in many coastal neighborhoods.4d61fbc8-2282-49e8-bb8c-e7d87075f424 However, there will likely be notable variability across coastal locations. Using the 2014 U.S. National Climate Assessment’s Intermediate-High scenario for sea level rise (a global rise of 1.2 meters by 2100), the median number of flood events per year for the Northeast is projected to increase from 1 event per year experienced today to 5 events by 2030 and 25 events by 2045, with significant variation within the region.5f4de85b-be39-4ffd-ac94-1950932c0140

Intense precipitation events can lead to riverine and street-level flooding affecting urban environments. Over recent decades, the Northeast has experienced an increase of intense precipitation events, particularly in the spring and fall.9131626f-95e5-4b4c-8a4e-08183ff2fe12 From 1958 to 2016, the number of heaviest 1% precipitation events (that is, an event that has a 1% chance of occurring in any given year) in the Northeast has increased by 55%.e8089a19-413e-4bc5-8c4a-7610399e268c A recent study suggests that this trend began rather abruptly after 1996, though uniformly across the region.9131626f-95e5-4b4c-8a4e-08183ff2fe12

Urban heating and heat waves threaten the health of the urban population and the integrity of the urban landscape. Due to the urban heat island effect, summer surface temperatures across Northeast cities were an average of 13°F to 16°F (7°C to 9°C) warmer than surrounding rural areas over a three-year period, 2003 to 2005.6b78125d-611b-402d-ab56-c409b15d52aa This is of concern, as rising temperatures increase heat- and pollution-related mortality while also stressing energy demands across the urban environment.5d45fe96-3a2f-4f4c-b989-406af17fc9af However, the degree of urban heat island intensity varies across cities depending on local factors such as whether the city is coastal or inland.8d5cd278-b5eb-4a23-aba3-79fc4b0f5544 Recent analysis of mortality in major cities of the Northeast suggests that the region could experience an additional 2,300 deaths per year by 2090 from extreme heat under RCP8.5 (compared to an estimated 970 deaths per year under the lower scenario, RCP4.5) compared to 1989–2000.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94 Another study that considered 1,692 cities around the world suggested that without mitigation, total economic costs associated with climate change could be 2.6 times higher due to the warmer temperatures in urban versus extra-urban environments.e904b5f2-2c5e-4e55-8365-2ba748291939

Changes in temperature and precipitation can have dramatic impacts on urban water supply available for municipal and industrial uses. Under a higher scenario (RCP8.5), the Northeast is projected to experience cumulative losses of $730 million (discounted at 3% in 2015 dollars) due to water supply shortfalls for the period 2015 to 2099.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94 Under a lower scenario (RCP4.5), the Northeast is projected to sustain losses of $510 million (discounted at 3% in 2015 dollars).0b30f1ab-e4c4-4837-aa8b-0e19faccdb94 The losses are largely projected for the more southern and coastal areas in the region.

New information and remaining uncertainties:

Projecting changes in urban pollution and air quality under a changing climate is challenging given the associated complex chemistry and underlying factors that influence it. For example, fine particulates (PM2.5; that is, particles with a diameter of or less than 2.5 micrometers) are affected by cloud processes and precipitation, amongst other meteorological processes, leading to considerable uncertainty in the geographic distribution and overall trend in both modeling analysis and the literature.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94 Land use can also play an unexpected role, such as planting trees as a mitigation option that may lead to increases in volatile organic compounds (VOCs), which, in a VOC-limited environment that can exist in some urban areas such as New York City, may increase ozone concentrations (however, it is noted that most of the Northeast region is limited by the availability of nitrogen oxides).5b52af56-61c6-4663-9d7d-302e8570800f

Interdependencies among infrastructure sectors can lead to unexpected and amplified consequences in response to extreme weather events. However, it is unclear how society may choose to invest in the built environment, possibly strengthening urban infrastructure to plausible future conditions.

Assessment of confidence based on evidence:

There is high confidence that weather-related impacts on urban centers already experienced today will become more common under a changing climate. For the Northeast, sea level rise is projected to occur at a faster rate than the global average, potentially increasing the impact of moderate and severe coastal flooding.c66bf5a9-a6d7-4043-ad99-db0ae6ae562c

By the end of the century and under a higher scenario (RCP8.5), Coupled Model Intercomparison Project Phase 5 (CMIP5) models suggest that annual average temperatures will increase by more than 9°F (16°C) for much of the region (2071–2100 compared to 1976–2005), while precipitation is projected to increase, particularly during winter and spring.75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1

Extreme events that impact urban environments have been observed to increase over much of the United States and are projected to continue to intensify. There is high confidence that heavy precipitation events have increased in intensity and frequency since 1901, with the largest increase in the Northeast, a trend projected to continue.75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 There is very high confidence that extreme heat events are increasing across most regions worldwide, a trend very likely to continue.75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 Extreme precipitation from tropical cyclones has not demonstrated a clear observed trend but is expected to increase in the future.75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1,4f1e7aa1-0c36-4220-ac77-7d55bcb33061 Research has suggested that the number of tropical cyclones will overall increase with future warming.4230c77b-1f8c-4452-aa52-5690aac2a72e However, this finding is contradicted by results using a high-resolution dynamical downscaling study under a lower scenario (RCP4.5), which suggests overall reduction in frequency of tropical cyclones but an increase in the occurrence of storms of Saffir–Simpson categories 4 and 5.75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1

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