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New York City Panel on Climate Change 2019 Report Chapter 3: Sea Level Rise

2019; Wiley; Volume: 1439; Issue: 1 Linguagem: Inglês

10.1111/nyas.14006

1749-6632

Vivien Gornitz, Michael Oppenheimer, Robert E. Kopp, Philip Orton, Maya K. Buchanan, Ning Lin, Radley Horton, Daniel Bader,

Geophysics and Gravity Measurements

The New York City Panel on Climate Change (NPCC, 2015) sea level rise projections provide the current scientific basis for New York City scientific decision making and planning, as reflected in, for example, the City's Climate Resiliency Design Guidelines. However, since the IPCC (2013) and NPCC (2015) reports, recent observations show mounting glacier and ice sheet losses leading to rising sea levels. Furthermore, new developments in modeling interactions between oceans, atmosphere, and ice sheets suggest the possibility of a significantly higher global mean sea level rise (GMSLR) by 2100 than previously anticipated, particularly under elevated greenhouse gas emission scenarios. Because of the potentially serious adverse consequences of soaring sea levels to people and infrastructure in low-lying neighborhoods of New York City, we introduce a new high-impact sea level rise scenario, Antarctic Rapid Ice Melt (ARIM), which includes the possibility of Antarctic Ice Sheet destabilization. An earlier “Rapid Ice Melt Scenario” (NPCC, 2010) assumed a late 21st century rate of high-end sea level rise of ∼0.39–0.47 in. per decade, based on paleo-sea level data after the last Ice Age. ARIM represents a new, physically plausible upper-end, low probability (significantly less than 10% likelihood of occurring) scenario for the late 21st century, derived from improved modeling of ice sheet–ocean behavior to supplement the current (NPCC, 2015) sea level rise projections. We briefly summarize key processes that control sea level rise on global to local scales, observed trends, and risks the city faces due to current and ongoing sea level rise. We also briefly recap the NPCC (2015) sea level rise projections for comparison with ARIM. To set the stage for ARIM, we review recent trends in land ice losses (Section 3.5) that reinforce the need to consider such an upper-end scenario. A more detailed discussion of these trends and technical details of the ARIM scenario are provided in Appendix 3.A. Multiple physical processes govern sea level rise on global to local scales. These include: (1) ocean density changes (involving temperature and salinity); (2) changes in ocean currents and circulation patterns; (3) ice mass losses from glaciers, ice caps, and ice sheets; (4) redistribution of ocean water in response to changes in the Earth's gravitation, rotation, and deformation caused by current ice mass losses (collectively referred to as “fingerprints”); (5) past ice mass losses (i.e., glacial isostatic adjustments, GIA1); (6) other vertical land movements caused by ongoing tectonic activity, sediment compaction due to loading, and subsurface extraction of water, oil, gas; and (7) changes in land water storage, for example, in dams or from groundwater mining. Thermal expansion along with losses of ice from mountain glaciers and small ice caps have historically been the major contributors to observed mean global sea level rise, but in recent decades, shrinking ice sheets have played a growing role and dominate in the higher scenarios for future GMSLR (Slangen et al., 2017, 2016; Kopp et al., 2014; Church et al., 2013, IPCC AR5). These processes interact in ways that differ from place to place, such that for any given locality the sum of the components for local sea level rise may deviate significantly from the global mean. New York City lies in a region that experiences higher than average sea level rise due to enhanced thermal expansion, mounting ice losses from the Antarctic Ice Sheet, and GIA. An additional possible factor is changes in ocean circulation. A major oceanic circulation system, the Atlantic Meridional Overturning Circulation (AMOC), could slow down due to decreased North Atlantic salinity resulting from Greenland ice losses, increased precipitation and northern river freshwater inflow, and sea ice attrition. The resulting heat build-up due to a weakened North Atlantic circulation would increase thermal expansion and redistribute water mass shoreward especially in the mid-Atlantic region, including New York City (Krasting et al., 2016; Yin and Goddard, 2013; Yin et al., 2010; 2009). While a regional sea level acceleration “hotspot” has been observed in tide gauge records along the Atlantic coast from Cape Cod to Cape Hatteras (including New York City) since the early 1990s (Sallenger et al., 2012; Boon, 2012), it is more likely that this hotspot reflects high interannual to multidecadal ocean variability than a shift in ocean circulation (Kopp, 2013; Valle-Levinson et al., 2017). Attribution of the hotspot to a weaker Gulf Stream and slowdown of the AMOC (Rahmstorf et al., 2015; Yin and Goddard, 2013) has not yet been substantiated (Böning et al., 2016; Watson et al., 2016) and is thus premature. However, this process could become important in the future (e.g., Section 3.4.2). In addition––perhaps counterintuitively, given the great distance between Antarctica and New York City––ice losses from Antarctica are amplified along the mid-Atlantic coast by the gravitational responses to this change. As the mass of the ice sheet shrinks, its gravitational attraction weakens, and water congregates farther from it. This, as well as continued GIA-related land subsidence, leads to a higher than average local sea level rise. On the other hand, gravitational effects from more nearby ice losses on Greenland and northern hemisphere glaciers mean that these ice losses raise local sea levels less than the global average. The net effect of all these processes drives New York City sea level rise above the global average (e.g., Carson et al., 2016; Slangen et al., 2014; Kopp et al., 2014.; Horton et al., 2015a). Sea level rise represents one of the most momentous consequences of climate change, potentially affecting hundreds of millions of people worldwide. In recent decades, melting ice sheets and glaciers account for over half of the total observed current rise (Dieng et al., 2017; Rietbroek et al., 2016), a fraction likely to increase with continued global warming (see Sections 3.4.1, 3.5, and 3.6). This section briefly reviews current global and local/regional trends in sea level rise, to provide context for future sea level changes, discussed in later sections. Tide gauge-based reconstructions of GMSLR between 1900 and 1990 range between 0.04 and 0.08 in./year (1 and 2 mm/year) (Dangendorf et al., 2017; Jevrejeva et al., 2017; 2014; Hay et al., 2015; Church et al., 2013; Church and White, 2011). Between 1993 and 2017, satellite altimetry shows an average GMSLR of around 0.12 in./year (3 mm/year),2 after accounting for satellite instrumental drift that affected the earlier TOPEX/Poseidon mission between 1993 and 1998 (Watson et al., 2015; Dieng et al., 2017; Beckley et al., 2017). After further accounting for the effects of the 1991 Mt. Pinatubo volcanic eruption and of strong El Niño–Southern Oscillation events, these revised estimates show clear acceleration of the sea level record, attributable to accelerated ice sheet mass loss (Chen et al., 2017; Dieng et al., 2017; Nerem et al., 2018; Fig. 3.1). Glaciers, ice caps, and ice sheets combined have raised ocean levels by 0.01 in./year (0.31 mm/year) between 1992 and 1996, increasing to 0.07 in./year (1.85 mm/year) between 2012 and 2016 (Bamber et al., 2018). Furthermore, GMSLR since the late-19th century has greatly exceeded the range of variability seen over the last three millennia (Kopp et al., 2016; Gehrels and Woodworth, 2013). These results imply two stages of global mean sea level acceleration: the first between late 19th and early 20th century to around 1990, which may in part reflect natural climate cycles, and the second from the 1990s to the present. Since 1970, anthropogenic factors may account for over 70% of the rise (Slangen et al., 2016). The local or relative3 sea level rise in New York City has averaged 0.11 in./year from 1850 to 2017 as measured by The Battery tide gauge, nearly double the 1900–1990 mean global rate (Fig. 3.2; NOAA, 2017). Local GIA-related subsidence, which accounts for roughly half of the observed relative sea level rise (Engelhart et al., 2011; Engelhart and Horton, 2012), is a key reason why New York City's rate of sea level rise is so high. As elsewhere, the historic New York City trend has increased markedly relative to the previous millennium (Kemp et al., 2017). Sea level rise has been tracked over time by the NPCC using data from both tide gauges and satellite altimetry. (For more information on tide gauges, see the NOAA Tides & Currents4 website; for satellite altimetry, see NASA Jason5 and AVISO/CNEs6 websites). New observations from these sources are included in updated analyses of sea level rise trends in each NPCC report and are included in reference to any new projected values. NPCC 2019 extends the observed record for sea level rise from NPCC 2015. In addition, NPCC 2019 analyzed how the trends in recent sea level rise compare in general to the projected changes in sea level from NPCC 2015 into the 2020s timeslice, which encompasses the time period from 2020 to 2029. Figure 3.3 shows the observed trend in sea level rise at The Battery in New York City from 1900 through 2017 compared to the NPCC 2015 projections. While NPCC3 cannot yet compare analytically projected to observed values from NPCC 2015 through 2017–2018 since we have not yet entered the onset of the 2020s time slice, nevertheless, the most recent observed trends show that sea level at The Battery has continued its upward rise since the previous NPCC report. A more comprehensive, comparative analysis will be part of the next NPCC report when a greater overlap will exist between the observed trend time period and projected values from NPCC 2015. However, such comparisons should be viewed with caution because of the role of natural variability in the short term. New York City is part of a metropolitan region (population 23.7 million7) that covers three adjacent states—Connecticut, New York, and New Jersey. Long-term sea level rise, as well as episodic coastal flooding, poses a high risk to the population, housing, and many essential New York City infrastructure facilities that line the 520 miles (837 km) of the city's waterfront. These include three major international airports, shipping infrastructure, segments of commuter and intercity bus and rail transit systems, many subway, tunnel, and bridge entrances, nearly all city wastewater treatment plants (WWTPs), oil tanks and refineries, most power plants, and telecommunication networks. The combined effects of New York City sea level rise (18 in., 45.7 cm between 1856 and 2017; Fig. 3.2, NOAA, 2017) and changes in storm climate variability (Orton et al., 2016; Lin et al., 2016; Reed et al., 2015; Talke et al., 2014; and Wahl et al., 2017) have increased the impact of coastal flood hazards (see also, Chapter 5: Mapping Climate Risk). Due to historic growth patterns and high-density shoreline development, a significant population resides within areas exposed to coastal hazards. The above-average water levels during strong hurricanes or hybrid storms, such as Sandy (Oct., 2012), Donna (Sept., 1960), Irene (Aug., 2011), and the unnamed 1788 and 1821 hurricanes, as well as “nor'easters” (e.g., Dec, 1992) resulted in substantial coastal flooding (Section 3.2.1). The location of property and key infrastructure near the shore or within the FEMA 1%-annual-chance floodplain places them at increased risk to ongoing and future sea level rise, in the absence of protective structures, such as levees, or other adaptation strategies. For example, storm surges occurring on top of higher sea level can damage wastewater treatment facilities, causing combined sewer overflows and pollution of waterways (NYC Hazard Mitigation Plan 2014). Acutely aware of this hazard, especially following Hurricane Sandy, the New York City Department of Environmental Protection has taken steps to increase resiliency and minimize potential damages. Buildings damaged by severe coastal erosion, prolonged saltwater exposure, and/or tidal flooding in low-lying areas require costly retro-fitting or even eventual relocation. In addition to high coastal storm floods and heavy rain, rising sea level is currently causing sewer surcharge and flooding streets farther away from the coast. The probability of blocked outfalls caused by poor drainage and additional backflow increases with elevated coastal storm surge superimposed on rising sea levels. Historical sea level rise in New York City (Fig. 3.2) has intensified the effects of coastal storm floods (Talke et al., 2014). In the lowest lying neighborhoods, flooding now occurs at times of high astronomical tides (tidal flooding), even in the absence of active storms. The frequency of such so-called “nuisance flooding” at The Battery has more than doubled since the 1950s (Sweet and Park, 2014; Strauss et al., 2016). Sea level rise alone will increase the severity and occurrence of New York City coastal storm-driven flooding, irrespective of changes in storm characteristics (Buchanan et al., 2017; Lin et al., 2016; Orton et al., 2016; Reed et al., 2015; Talke et al., 2014; Kemp and Horton, 2013). Further discussion on historic, current, and future flood risks is given in Chapter 4: Coastal Flooding. The Mississippi Delta and Chesapeake Bay are examples of areas already experiencing permanent land inundation due to high rates of relative sea level rise from land subsidence superimposed on global sea level rise. The Chesapeake Bay area has the highest rates of relative sea level rise on the East Coast, due to GIA and groundwater withdrawal (Eggleston and Pope, 2013), which has led to the shrinkage or loss of several small islands (Gornitz, 2013). The high relative sea level rise has led to increased tidal flooding in places such as Norfolk, Virginia (see also discussion of tidal flooding in Chapter 4). Although New York City is not at immediate risk of extensive land inundation, the regions currently experiencing inundation provide a preview of potential permanent land loss due to sea level rise facing some New York City neighborhoods under the ARIM scenario in the later years of the 21st century (see Chapter 5, Fig. 5.1). The first areas that could be affected include low-lying city neighborhoods that will experience frequent tidal flooding and, in a few cases, permanent inundation by the 2050s and especially the 2080s (e.g., compare Fig. 5.1 with Figs. 5.2 and 4.4). (Note: Because the ARIM scenario shown in these figures is based on data with high associated uncertainties, it should be regarded as suggestive of areas that might become inundated and should therefore not be used for planning purposes. See further discussion and disclaimer in Chapter 4: Coastal Flooding, and Chapter 5: Mapping Climate Risk). Studies show that intertidal salt marshes and particularly their substrate play an important role in attenuating storm waves as they break on shore (Marsooli et al., 2017), although they may not lessen high storm water levels, or reduce flooding if deep shipping channels are present (Orton et al., 2015). Many New York City salt marshes, including in Jamaica Bay, have receded historically and have become increasingly ponded, with enlarging tidal inlets and pools (Hartig et al., 2002). In addition to historic sea level rise, other stressors have led to attrition of local salt marshes, such as channelization, shoreline development and armoring with engineered structures, excess nitrogen nutrient loading from nearby sewage treatment plants, and inadequate sediment supply (e.g., Hartig et al., 2002). As a result, salt marshes at the shoreline edge are converting to tidal mudflats. The National Park Service, in conjunction with the U.S. Army Corps of Engineers, is engaged in restoration efforts at several Jamaica Bay salt marshes (e.g., Elders Point Marsh, Yellow Bar Hassock, and Rulers Bar). Rising sea levels lead to longer periods of salt marsh submergence during high tides. Salt marsh vegetation zones can gradually shift landward, but may not find space, due to urban development (i.e., “coastal squeeze”) or too steep a rise in inland topography. Wetlands will drown in place wherever rates of accretion cannot keep pace with sea level rise, and/or if sediment supplies are insufficient. However, as noted above, sea level rise is just one of many environmental factors that contribute to New York City saltmarsh losses. Sea level rise, in addition to climate change, can alter the flow of saltwater and propagation of tide and storm surge up streams, in estuaries such as the Hudson River, and into coastal lagoons. The mean location of the salt front pushes upstream as a result. Hydroclimate also influences the position of the saltwater front in the Hudson River. A decrease in precipitation reduces streamflow, which allows the salt front to migrate further upstream (and vice-versa); higher temperatures increase evaporation and decrease freshwater runoff, also forcing an upstream migration of the salt front (Buonaiuto et al., 2011). Salt front migration up the Hudson River (and the Delaware River Basin; Chapter 2, Climate Science) during severe droughts and/or higher sea levels could adversely impact the emergency New York City drinking water supply from the Hudson River at the Chelsea Pumping Station. Sea level rise will also increase the salinity of brackish water in the estuary and lagoons, also affecting inflow of seawater to sewers and WWTPs located along the saltwater-dominated coastline and thereby lessen infiltration efficiency. In addition, higher water levels will reduce the capacity of WWTP effluents to drain by gravity and pumping (see also Chapter 4, Coastal Flooding). Although less urgent today, salinization accompanying sea level rise may become a major issue for drainage systems and warrants further investigation. Structures not designed for exposure to repetitive and lengthening saltwater exposure would also face more frequent and higher repair or replacement costs (Solecki et al., 2015). Sea level rise, in conjunction with higher waves and/or water levels during intense storms, such as Hurricane Sandy in 2012, is likely to exacerbate ongoing coastal erosion, particularly of exposed, ocean-facing shorelines. This can disrupt sediment transport and undermine natural landforms, like beaches and salt marshes offering protective features, with associated land loss and environmental degradation. In urban areas, coastal erosion and flooding can severely damage structures, and if unchecked, can undermine foundations, ultimately leading to building collapse, as shown during Hurricane Sandy for the New Jersey and New York regions (Hatzikyriakou et al., 2016; Hatzikyriakou and Lin, 2018). An integrated approach for managing high erosion risks includes upgrading major structural protections, such as seawalls, revetments, bulkheads, groins, etc., as well as implementing beach nourishment and living shorelines. Continual erosion of the city's sandy beaches requires periodic nourishment with sand dredged from offshore (New York City, 2014). Potential coastal restoration projects by the U.S. Army Corps of Engineers are in review for Coney Island and the Rockaways (USACE, 2016a, 2016b). Coastal erosion risks can also be mitigated by the limitation of high-density development in high-erosion hazard zones. Three current “erosion hotspot” neighborhoods (south shore of Staten Island, Coney Island, and Rockaway Peninsula) have been designated Coastal Erosion Hazard Areas (CEHA), for which new construction or land use change requires special permits from the New York State Department of Environmental Conservation (NYC, 2014). As atmospheric greenhouse gases continue to accumulate, and temperatures climb, sea level is expected to rise in the future at accelerating rates. Climate scientists look ahead by using computer-generated coupled global atmospheric-oceanographic models that are based on known laws of physics that govern our climate. Section 3.4.1 briefly reviews the sea level rise projections of the Intergovernmental Panel on Climate Change (IPCC, 2013), and several newer reports that suggest a higher future global sea level than that in the IPCC report. The sea level rise projections for New York City developed by NPCC (2015), which are reaffirmed for use as the basis of New York City resiliency planning, are described in Section 3.4.2. The IPCC AR5 (Church et al., 2013) projects future climate changes for a set of four representative concentration pathway (RCP) scenarios, which represent different trajectories of greenhouse gas emissions, aerosols, and land use/land cover (Moss et al., 2010). They range from a high greenhouse gas emission “business-as-usual” scenario (RCP8.5) to one involving strong mitigation efforts (RCP2.6). Driven by the RCPs, a suite of coupled atmospheric and oceanographic global climate models (AOGCMs) numerically simulate physical interactions between the atmosphere, ocean, continents, and sea ice, in order to project future trends in climate variables including temperature, precipitation, and sea level rise. AOGCMs directly compute changes in ocean density (temperature and salinity) and circulation patterns. Temperature and precipitation projections from AOGCMs are used to drive separate numerical models to estimate surface mass balance8 of glaciers and ice sheets. Models that include both dynamic ice flow and surface mass balance driven by climate projections (i.e., temperature, precipitation) estimate future changes in discharge of ice past the grounding line9 and calving rates of icebergs. The individual components are then summed to obtain global sea level. An alternative approach to projecting GMSLR, the semiempirical approach, makes projections of future sea level rise based on the assumption that the statistical relationship that existed between past temperatures and rates of sea level change will continue into the future. Thus, the future trajectory of sea level rise remains closely linked to that of increasing global temperature (e.g., Moore et al., 2013; Rahmstorf et al., 2012, Rahmstorf, S., 2007; Kopp et al., 2016). However, this assumption may no longer hold if processes that were minor contributors to past sea level change, such as ice sheet dynamics, become major contributors in the future. IPCC (2013) projects a “likely”10 GMSLR by 2100 of 0.9–2.0 ft for RCP2.6, 1.2–2.3 ft for RCP4.5, and 1.7–3.2 ft for RCP8.5 relative to a 1986–2005 baseline, and notes the potential for collapse of marine-based parts of the Antarctic Ice Sheet to contribute another several tenths of a meter (Church et al., 2013), IPCC, 2013, Chapter 13, Table 13.5). An earlier assessment (Pfeffer et al., 2008) suggested that 6.6 ft was a physically plausible upper bound to GMSLR, a level adopted by the Third National Climate Assessment for its highest sea level rise scenario (Parris et al., 2012). However, this upper bound was subsequently criticized (Miller et al., 2013) for failing to fully represent uncertainty regarding Antarctica (Bamber and Aspinall, 2013), thermal expansion (Sriver et al., 2012), and land water storage (IPCC, 2013). Since IPCC (2013), new observations from the Greenland and Antarctic Ice Sheets (e.g., Rignot et al., 2014), progress in ice sheet–ice shelf–ocean modeling (e.g., Joughin et al., 2014), and expert assessments (Horton et al., 2014; Bamber and Aspinall, 2013) have reaffirmed the physical plausibility of sea level rise well in excess of the IPCC (2013) “likely” range (Jevrejeva et al., 2014; Kopp et al., 2014; Slangen et al., 2017). Newly recognized mechanisms for ice-shelf instability further emphasize the plausibility of high-end outcomes, especially beyond 2100 in high-emission futures (Pollard et al., 2015; DeConto and Pollard, 2016; Kopp et al., 2017; Le Bars et al., 2017; Wong et al., 2017; see also, Section 3.4.2). Based on these findings, the Fourth National Climate Assessment recommended a suite of GMSLR scenarios for the period 2000–2100 that range between a “low” scenario of 1.0 ft to a physically plausible “extreme” scenario of 8.2 ft by 2100 (Sweet et al., 2017). Sweet et al. (2017) additionally describe methods for adapting these projections to regional scales, as illustrated for New York City in in Section 3.4.2. and Appendix 3.A. Although many future global sea level rise projections end in 2100, the longevity of atmospheric CO2 commits us to higher temperatures and sea level long after reduction of stabilization of greenhouse gas emissions. Ending further emissions by mid-century would allow some of the anthropogenic CO2 and temperature to slowly diminish after several decades, with gradual dissipation of the balance. It would take centuries to millennia to reach a new equilibrium state. In the interim, sea level will continue to rise well beyond 2100, because of the continued climate warming and slow heat penetration into the deep ocean (Clark et al., 2016; Mengel et al., 2016; Golledge et al., 2015). In its second report, the NPCC (2015) developed a multicomponent methodology for projecting future sea level rise for New York City (Horton et al., 2015a). Components include oceanographic changes (thermal expansion, dynamic ocean height), ice mass losses with associated gravitational and glacial isostatic adjustments, and anthropogenic land water storage change, for an ensemble of 24 CMIP global climate models and two climate change scenarios (RCP4.5, RCP8.5), as well as literature review and expert judgment. Sea level rise, relative to the 2000–2004 base period, was calculated for the 10th, 25th, 75th, and 90th percentiles from a model-based distribution and estimated ranges from the literature. NPCC (2015) assumed that all uncertainties were perfectly correlated so that, for example, the 90th percentile projection combined the 90th percentile values for each of the different terms. While this could lead to overly high estimates, NPCC (2015) offered some leeway in case the individual component projections—consistent with most sea level rise projections in recent decades—would later be found to underestimate the extreme tail of the distribution. NPCC (2015) projects a mid-range (25th–75th percentile) sea level rise of 11–21 in. (0.28–0.53 m) at The Battery by the 2050s and 18–39 in. (0.46–0.99 m) by the 2080s, relative to a 2000–2004 baseline. High-end estimates (90th percentile) reach 30 in. (0.76 m) by the 2050s, 58 in. (1.47 m) by the 2080s, and 75 in. (1.91 m) by 2100 (Table 3.1). Appendix 3.B illustrates how recent observed trends in sea level rise from 1900 to 2017 compare to these projected changes from NPCC (2015). The results of a similar study by Kopp et al. (2014), which did not assume perfect correlation of uncertainties, and applied a hybrid approach to ice sheets that blended NPCC (Horton et al., 2015a) and IPCC methodologies, are shown in Appendix Table 3.A.1. Results from a more recent study (Kopp et al., 2017), incorporating Antarctic ice-sheet projections from DeConto and Pollard (2016), and projections based on these studies, are also shown in Appendix Table 3.A.1. Appendix Table 3.A.2 places these projections in the context of the local sea level rise scenarios developed by Sweet et al. (2017) for the Fourth National Climate Assessment (see Appendix 3.A for more details). As mentioned in Section 3.1, sea level rise in New York City is expected to exceed global mean values (NPCC, 2015; Carson et al., 2016; Kopp et al., 2014; Love et al., 2016). This arises primarily because of GIA-related subsidence, far-field effects of Antarctic ice loss, and above-average ocean dynamic height due to projected slowdown of the AMOC with continued ocean freshening and Greenland ice losses (Yin and Goddard, 2013; Yin et al., 2010; 2009). Enhanced warming in the western Atlantic relative to the Pacific Ocean may also elevate steric sea level rise along the East Coast, particularly for high carbon emission scenarios (Krasting et al., 2016). Although gravitational effects associated with proximity to Greenland and northern hemisphere glaciers will partially reduce sea level rise, the combined effect of all contributing factors will result in higher than average sea level rise for New York City (Sweet et al., 2017; Love et al., 2016). It should be re-emphasized that the NPCC (2015) sea level rise projections represent the current scientific foundation for New York City decision making and planning. However, recent observed trends in land ice mass losses and advances in ice–ocean–atmosphere interactions raise the possibility of higher future sea levels than previously assumed (Section 3.5). Furthermore, NPCC (2015) sea level rise estimates lie within the 10–90% probability range. They do not provide sea level rise values with a lower than 10% probability of occurrence by 2100 (i.e., the very large sea level increases that lie in the upper 10% tail of the sea level rise probability distribution). Nevertheless, consideration of such high-end sea level rise outcomes is of great importance for effective long-term decision making. Focusing on the central range may lead to underestimation of the future risks, especially in the light of science that suggests that high-end scenarios may become more probable under high-emissions scenarios than thought a few years ago. A new upper-end, low-probability sea level rise scenario, introduced in Section 3.6, is designed to address the concerns of stakeholders interested in long-term planning, who may need to examine credible scenarios at the extreme upper tail of the distribution. The ARIM scenario provides one physically plausible, low-probability scenario (i.e., one with significantly less than 10% likelihood of occurrence by 2100) for considering the consequences of very unlikely, yet high-impact outcomes. For example, many public or private sector dec