Several past efforts have characterized or quantified the effects of SLR on US coastal resources (see CCSP 2009 for a summary), but only a few of the models that have been applied are tractable for economic analyses at a national scale (Neumann et al. 2010a), and only local or global-scale models have considered the role of mitigation policies in reducing effects of SLR (Nicholls et al. 2011; Yohe et al. 2011). We rely on US Environmental Protection Agency’s (USEPA) National Coastal Property Model (NCPM), which comprehensively examines the contiguous US coast at a detailed 150 m × 150 m grid level; incorporates site-specific elevation, land subsidence, and property value data; estimates cost-effective responses to the threat of inundation; and provides economic impact results for three categories of response: shoreline armoring, beach nourishment, and property abandonment (Neumann et al. 2010b – note that inland, riparian flooding effects are addressed in a companion paper in this special issue, see Strzepek et al., Submitted for publication in this issue). Additional methodological details for this application of the NCPM are described in Online Resource 2.
The scenarios used here reflect the IGSM results for global SLR through 2100 (see Paltsev et al. 2013), but also incorporate adjustments to account for the omitted effect of dynamic ice-sheet melting, a potentially important factor for SLR projections (Meier et al. 2007). Dynamic ice-sheet melting scenarios incorporate estimates from the empirical model of Vermeer and Rahmstorf (2009), and use as inputs the decadal trajectory of global average air temperature results from the IGSM results. The results of this adjustment are shown in Fig. 2 of Online Resource 2 – the adjustment increases SLR results that derive directly from the IGSM model by as much as a factor of 2.5 in 2100, yielding SLR estimates of about 1.4 m by 2100, but the effect of incorporating estimates of dynamic ice-sheet melting is much stronger at the end of the 21st century than in the early and mid-21st-century periods.
The cumulative undiscounted results of NCPM economic modeling over the 21st century for the scenarios that incorporate dynamic ice sheet melting are presented in Table 1 of Online Resource 2. Dynamic ice sheet melting scenario economic impact results are roughly 70 to 80 % higher than results that do not incorporate dynamic ice sheet melting. Discounting at 3 % the annual trajectory of results reduces all estimates by a factor of approximately 3 to 4 – discounting has a substantial effect because of the upward sloping trajectory of SLR scenarios, with impacts evident throughout the century but growing larger at the end of the century.
Economic analysis results for these scenarios are also presented in Fig. 1, with the height of the bars illustrating the effects of different climate sensitivities and mitigation policies. The results show protection (as opposed to abandonment) of coastal property is the most prevalent economic response to SLR, with shoreline armoring making up a larger share of economic impacts than beach nourishment. Note that the prevalence of protection strategies may create new issues, as protection can induce additional development, which in turn can increase future vulnerability – these dynamic land development effects are well –documented (CCSP 2009) but have not been addressed in this analysis.
As expected, higher climate sensitivities, on the left side of Fig. 1, yield higher impact estimates. Mitigation Policy 3.7 reduces damages in both the 3 and 2 °C climate sensitivity runs by $68 billion ($6.2 billion discounted at 3 %), but with climate sensitivity of 6 °C the benefits of this policy increase to $87 billion ($8.1 billion at 3 %). Results for the 3˚C climate sensitivity runs show that most of the benefits of mitigation policy $57 billion ($5.3 billion at 3 %) compared to $68 billion ($6.2 billion at 3 %) can be realized through Policy 4.5.
These results reflect the existing NCPM’s capability to analyze threats of gradual inundation from SLR. A growing body of literature suggests, however, that the combined effect of SLR and storm surge on coastal properties may be critically important (Tebaldi et al. 2012, Lin et al. 2012). The effects of climate change on storm surge are two-fold: 1) changing storm frequency and severity in a given location; and 2) SLR providing a higher “launch point” for surge even if storm frequency and severity remain constant. Both effects have been demonstrated in prior work for non-US sites (see Neumann et al. 2012 for application in Vietnam), applying a cyclone simulation model (Emanuel et al. 2008), a storm surge estimation model (NOAA’s SLOSH model) and local elevation and property value data. The international applications, however, often suffer from poor elevation and property value data, limiting the usefulness of the approach to estimate economic impacts. These data limitations are greatly reduced at US sites.
Preliminary results of combining the cyclone simulation model used in Neumann et al. (2012) with the elevation and property value estimates in the NCPM are available for two US sites: Tampa, Florida and New York City. Incorporating storm surge also requires modifying the NCPM in three ways: 1) Estimating a cumulative distribution function for location-specific storm surge; 2) Estimating a cumulative distribution function for economic damages (similar to the approach applied in Kirshen et al. 2012); and 3) Adding another response option (property elevation) that represents a cost-effective alternative in areas subject to episodic flooding but which are not permanently inundated.
Figure 2 presents the results of incorporating storm surge at the Tampa site. The left panels illustrate the cost-effective adaptation response to SLR risks, with red areas indicating abandonment, black areas lines of armoring defense, yellow areas beach nourishment, and brown areas structure elevation. The incremental effect of dynamic ice sheet melting is shown in the bottom panels – as expected, the area of influence of SLR grows larger with dynamic ice sheet melting, and as a consequence red areas in particular are larger, but black, brown, and yellow areas also expand. A larger difference is evident when comparing the left and right panels, with the right column showing the cost effective response when storm surge is considered. In the right panels, the area of influence of the coastal threat from climate change is much larger than on the corresponding left panels, with red areas in the low elevation east bay showing great sensitivity to storm surge, and a much expanded area of armoring (in black) being justified by the potential economic damages. A similar map for New York City, shown in Figure 3 of Online Resource 2, shows less abandonment and more protection and elevation in response to risks of episodic flooding, owing to higher property values in New York City’s vulnerable areas. Estimates of economic damage for the Tampa site are 15 to 20 % larger when storm surge is incorporated, and for New York City 40 to 50 % larger. As a result, the value of mitigation policy generally increases – Policy 3.7 yields estimates of avoided costs in New York City that are roughly 6 times greater than the SLR-only NCPM. New research is therefore focused on estimating the effect of storm surge on economic impacts and estimates of the value of mitigation in reducing economic impacts.
A parallel effort within the CIRA program examined the effects of SLR on socially vulnerable populations in the US (Martinich et al. 2012). The result is that areas which the NCPM anticipates to be abandoned have a higher percentage of socially vulnerable populations than areas likely to be protected. Further, moving from that study’s high scenario (similar to the REF scenario) to that study’s mid scenario (similar to Policy 3.7) substantially reduces the risk of SLR to the socially vulnerable population, and reduces areas likely to be abandoned. This work suggests that mitigation policies, such as those considered here, also have potential to enhance environmental justice objectives.