Our results provide what is, to our knowledge, the first large-scale assessment of the vulnerability of individually mapped buildings to inundation by large magnitudes of SLR. We find that the exposure of building stock varies widely between coastal countries, depending on the relationship between topography and building distributions. There is relatively little inundation for magnitudes of SLR that have been commonly considered (0.5–1 m), but the rate of inundation increases rapidly throughout much of the study area between 2 and 4 m of LSLR. Our results therefore highlight the large difference in building inundation likely to occur under low vs. high-emissions scenarios.
Previous research has estimated that 0.7–0.9 m global mean SLR is likely to occur by 2100, and 2.2–2.5 m by 2300, if the objectives of the Paris Agreement are met13. This magnitude of LSLR across coastlines in the study area alone would inundate on the order of 5 million existing buildings at high tide by the end of this century, approaching 20 million buildings by 2300. Importantly, this simple estimate does not account for geophysical and oceanographic effects (e.g. the effects of gravity, Earth rotation and deformation initiated by land-ice loss) that will cause LSLR to exceed the global mean across the majority of locations within the study area7,26. Furthermore, non-climatic local factors such as erosion and subsidence will also increase LSLR in many places during this time frame. Exceeding temperature targets set by the Paris Agreement would only worsen this, having significant impacts on the resiliency of coastal infrastructure.
Combining our findings with sea level projections may provide an opportunity for policymakers and urban developers to anticipate future changes and plan accordingly. Depending on the extent of sea water inundation, there is potential for adaptation in some locations through the use of flood protection or land reclamation27,28, which would decrease the amount of infrastructure at risk of inundation from LSLR. However, the long-term feasibility and costs of such measures are strongly dependent on the ultimate magnitude of SLR, which depends on the emissions trajectory29. Our results may help to inform decisions between adaptation and retreat. We provide an interactive map of our results on Google Earth as a part of this publication to facilitate this (see Data and Code Availability). Note that while we do not consider extreme SLR events directly in this study, our gridded infrastructure-elevation mapping results could be modified to assess vulnerability of infrastructure to projected short-term flooding as well.
Multiple datasets of varying resolutions were used in our work, each with their own uncertainty, including the tidal range estimates, building location and floor area, and topographic elevation11,12. In terms of tidal ranges, differences between tide gauge measurements and the FES14 tidal model predictions we adopt are less than 7 cm for all wave types, which is not significant on the multi-meter scale addressed here30–32. To validate our use of high tide, an assessment of the extent of areas identified prematurely as flooded at 0 m LSLR, and was found to be > > 1%, meaning this assumption is likely not the cause of any errors in our results (details can be found in Supplemental SIV). Additionally, Tidal ranges are dynamic in nature and there is evidence to support that regional tidal variations will intensify with LSLR in some places and diminish in others17. Therefore, changes in the height of high tide should be considered when combining our results with LSLR projections. In terms of topography, the mean error for FABDEM is reported to be 0.1–0.5 m depending upon landscape, with highest errors occurring where there is high density canopy cover. Additionally there is limitations for FABDEM in areas without forest that have steep slopes, for which the reanalysis FABDEM conducted on the original COPDEM30 model will not apply. Finally, the existence, location, and size of the buildings contributes to uncertainty. Sirko et al. (2021)21 reports 95% precision for the Open Buildings Polygon Dataset, however due to it’s novelty, to our knowledge there is currently no peer-reviewed third-party comparison of this dataset. In order to validate our findings, we conducted a preliminary accuracy assessment (see Supplemental SIII for more details) and found the accuracy to be much higher than reported by Sirko et al.
At large scales it is likely that these primary sources of error are correlated and more likely to occur in similar locations, which should minimize their impact on aggregate measures. However, the uncertainties will have a larger effect on smaller scale, local results, particularly in areas with highly variable tides and regions where building roofs are made of visually similar material to the ground21, to a degree which is difficult to assess at present.
In this study, we adopt the simple approach of considering exposure as a function of LSLR rather than as a function of time to avoid tying our results to uncertain climate projections. However, it is important to emphasize that our metric does not estimate vulnerability to levels of global mean SLR, as LSLR and flood risks are spatiotemporally variable. For example, loss of ice from the Greenland and Antarctic ice sheets in the coming centuries will result in gravitational, Earth effects that will produce a non-uniform pattern of sea level change, with mid and low latitude regions far from the polar ice sheets expected to experience the greatest rise relative to the mean global value7,33,34. As a result, the exposure for each country by meter of LSLR (Fig. 3) will not occur simultaneously and will accentuate the already present issue of inequality between mid and high-latitude impacts evident in Fig. 326.
Given that SLR can amplify the impacts of other processes such as high tide extent, subsidence, and storm surges35, damage to coastal infrastructure may occur well before the area is under water at high tide. Subsidence, erosion, and tidal intensification will likely lead to the destruction of more buildings than our metric predicts. Future work could incorporate spatiotemporally variable sea level projections and extreme events to produce global maps that would provide a more accurate assessment of impact on buildings for a given scenario. We also note that, although we have focused on buildings, inundation of other forms of infrastructure may threaten the functionality of coastal communities, and their roles within the broader economy. These risks could be assessed by comparing our results with data on waste containment, transportation infrastructure, energy infrastructure, cultural monuments, or other demographic data at local and regional scales where available.
SLR will pose a major challenge to coastal societies throughout the coming centuries, impacting hundreds of millions of currently existing buildings and the lives and livelihoods of their residents. Our results provide a novel perspective on the impact of SLR scenarios across spatial scales, by mapping building inundation at 30 m resolution across hundreds of thousands of kilometers of coastline36. Due to the high resolution of the datasets and the broad geographical scope, the results can be assessed from an area spanning multiple countries, to a single building on a city block, making our findings potentially useful for informing large and small-scale efforts for building resiliency against climate change impacts. As our results were not tied to any specific Representative Concentration Pathway (RCP) / Shared Socioeconomic Pathway (SSP) climate warming scenarios, our findings will remain relevant even as projections change with improved data and techniques37,38.