Expert Commentary

Global warming, rising seas and coastal cities: Trends, impacts and adaptation strategies

Updated roundup of research on climate-change risks and the regions and groups most threatened by them, attempts to mitigate these risks, and adaptive efforts for coastal regions.

(Pixabay/public domain)

Rising seas are one of the central impacts of global warming, and they’re not some abstract challenge for a future day: Areas of the United States now routinely have “sunny-day flooding,” with salt water pushing up through drains even in the absence of storms. When London built the Thames Barrier in 1982, it was expected to be used two to three times a year at most, but has since been employed at twice that rate, a pace that is expected to accelerate. U.S. Army facilities in coastal Virginia already see “recurrent flooding,” according to the Department of Defense. And longer range, things get even more challenging: For example, because of a sea-level “hotspot” on the Northeastern U.S. coast, tides could rise as much as 7.5 feet by 2100 in cities such as Boston. Proposals for a “Venice by the Charles River” are anything but far-fetched.

Yet even as the seas are rising, coastal areas are booming: From 1970 to 2010, the population in the coastal United States grew 39 percent, according to the National Oceanic and Atmospheric Administration, which expects the population in these areas to increase another 8 percent between 2010 and 2020. As of 2010, 123 million Americans lived in coastal counties, at population densities more than four times higher than those of the country as a whole. The same pattern holds around the globe: 60 percent of cities with populations over 5 million are within 60 miles of the sea, and they’re growing rapidly. This rush to the shore puts more lives, wealth and infrastructure in harm’s way, increasing losses when storms inevitably hit. A 2013 study in Global Environmental Change estimates that by 2100 sea-level rise could put up to 7.4 million U.S. residents at risk — many of already disadvantaged — and cut the country’s GDP by as much as $289 billion.

While climate change remains a politically charged issue in the U.S. despite the overwhelming evidence, efforts are underway to better understand the risks, prepare for the future and increase community resiliency. The Department of Defense, per its “Climate Change Adaptation Roadmap” of October 2014, has sought to adapt its facilities to a projected sea-level rise of 1.5 feet as early as 2034, though congressional Republicans have blocked efforts to fund the research. The landmark Paris Climate Accord, negotiated under the the U.N.’s Framework Convention on Climate Change (UNFCCC), was endorsed by 174 nations in April 2016. That notably included both the United States and China, though the U.S. has not yet ratified it. The pact lays out ways to limit or reverse harmful trends in greenhouse-gas emissions, with many suggestions that are “actionable” by state and local governments, businesses and individuals. Resources like FloodTools and the National Flood Insurance Program’s FloodSmart website aim to educate citizens about flood risks and preparedness measures, while the Georgetown Climate Center has page on state and local adaptation plans. And such adaptations can be effective: A 2014 study in the Proceedings of the National Academy of Sciences found that large-scale urban adaptation strategies have the potential to counteract some of the effects of long-term global climate change.

Below is a series of studies examining climate-change related risks and the regions and demographic groups most threatened by them; efficacy of attempts thus far to mitigate these risks; and adaptive solutions for coastal regions. Many recent studies focus on particular communities facing inundation around the world.


“Trapped in Place? Segmented Resilience to Hurricanes in the Gulf Coast, 1970–2005”
Logan, John R.; Issar, Sukriti; Xu, Zengwang. Demography, 2016. doi:10.1007/s13524-016-0496-4.

Abstract: “Hurricanes pose a continuing hazard to populations in coastal regions. This study estimates the impact of hurricanes on population change in the years 1970–2005 in the U.S. Gulf Coast region. Geophysical models are used to construct a unique data set that simulates the spatial extent and intensity of wind damage and storm surge from the 32 hurricanes that struck the region in this period. Multivariate spatial time-series models are used to estimate the impacts of hurricanes on population change. Population growth is found to be reduced significantly for up to three successive years after counties experience wind damage, particularly at higher levels of damage. Storm surge is associated with reduced population growth in the year after the hurricane. Model extensions show that change in the white and young adult population is more immediately and strongly affected than is change for blacks and elderly residents. Negative effects on population are stronger in counties with lower poverty rates. The differentiated impact of hurricanes on different population groups is interpreted as segmented withdrawal—a form of segmented resilience in which advantaged population groups are more likely to move out of or avoid moving into harm’s way while socially vulnerable groups have fewer choices.”


“A Comprehensive Review of Climate Adaptation in the United States: More Than Before, but Less than Needed”
Bierbaum, Rosina; et al. Mitigation and Adaptation Strategies for Global Change, March 2013, Vol. 18, Issue 3, 361-406. doi: 10.1007/s11027-012-9423-1.

Abstract: “We reviewed existing and planned adaptation activities of federal, tribal, state, and local governments and the private sector in the United States to understand what types of adaptation activities are underway across different sectors and scales throughout the country. Primary sources of review included material officially submitted for consideration in the upcoming 2013 U.S. National Climate Assessment and supplemental peer-reviewed and grey literature [working papers]. Although substantial adaptation planning is occurring in various sectors, levels of government, and the private sector, few measures have been implemented and even fewer have been evaluated. Most adaptation actions to date appear to be incremental changes, not the transformational changes that may be needed in certain cases to adapt to significant changes in climate. While there appear to be no one-size-fits-all adaptations, there are similarities in approaches across scales and sectors, including mainstreaming climate considerations into existing policies and plans, and pursuing no- and low-regrets strategies. Despite the positive momentum in recent years, barriers to implementation still impede action in all sectors and across scales. The most significant barriers include lack of funding, policy and institutional constraints, and difficulty in anticipating climate change given the current state of information on change. However, the practice of adaptation can advance through learning by doing, stakeholder engagements (including “listening sessions”), and sharing of best practices.”


“Future Flood Losses in Major Coastal Cities”
Hallegatte, Stephane; Green, Colin; Nicholls, Robert J.; Corfee-Morlot, Jan. Nature Climate Change, August 2013, 3:802-806. doi: 10.1038/nclimate1979.

Abstract: “Flood exposure is increasing in coastal cities owing to growing populations and assets, the changing climate, and subsidence. Here we provide a quantification of present and future flood losses in the 136 largest coastal cities. Using a new database of urban protection and different assumptions on adaptation, we account for existing and future flood defenses. Average global flood losses in 2005 are estimated to be approximately U.S. $6 billion per year, increasing to U.S. $52 billion by 2050 with projected socio-economic change alone. With climate change and subsidence, present protection will need to be upgraded to avoid unacceptable losses of U.S.$1 trillion or more per year. Even if adaptation investments maintain constant flood probability, subsidence and sea-level rise will increase global flood losses to U.S.$60–63 billion per year in 2050. To maintain present flood risk, adaptation will need to reduce flood probabilities below present values. In this case, the magnitude of losses when floods do occur would increase, often by more than 50%, making it critical to also prepare for larger disasters than we experience today. The analysis identifies the cities that seem most vulnerable to these trends, that is, where the largest increase in losses can be expected.”


“Increasing risk of compound flooding from storm surge and rainfall for major U.S. cities”
Wahl, Thomas; et al. Nature Climate Change, 2015. doi:10.1038/nclimate2736.

Abstract: “When storm surge and heavy precipitation co-occur, the potential for flooding in low-lying coastal areas is often much greater than from either in isolation. Knowing the probability of these compound events and understanding the processes driving them is essential to mitigate the associated high-impact risks. Here we determine the likelihood of joint occurrence of these two phenomena for the contiguous United States (US) and show that the risk of compound flooding is higher for the Atlantic/Gulf coast relative to the Pacific coast. We also provide evidence that the number of compound events has increased significantly over the past century at many of the major coastal cities. Long-term sea-level rise is the main driver for accelerated flooding along the US coastline; however, under otherwise stationary conditions (no trends in individual records), changes in the joint distributions of storm surge and precipitation associated with climate variability and change also augment flood potential. For New York City (NYC)—as an example—the observed increase in compound events is attributed to a shift towards storm surge weather patterns that also favour high precipitation. Our results demonstrate the importance of assessing compound flooding in a non-stationary framework and its linkages to weather and climate.”


“Relative Sea-level Rise and the Conterminous United States: Consequences of Potential Land Inundation in Terms of Population at Risk and GDP Loss”
Haer, Toon; Kalnay, Eugenia; Kearney, Michael; Moll, Henk. Global Environmental Change, September 2013, 23:1627-1636. doi: 10.1016/j.gloenvcha.2013.09.005

Abstract: “Global sea-level rise poses a significant threat not only for coastal communities as development continues but also for national economies. This paper presents estimates of how future changes in relative sea-level rise puts coastal populations at risk, as well as affect overall GDP in the conterminous United States. We use four different sea-level rise scenarios for 2010–2100: a low-end scenario (Extended Linear Trend) a second low-end scenario based on a strong mitigative global warming pathway (Global Warming Coupling 2.6), a high-end scenario based on rising radiative forcing (Global Warming Coupling 8.5) and a plausible very high-end scenario, including accelerated ice cap melting (Global Warming Coupling 8.5+). Relative sea-level rise trends for each U.S. state are employed to obtain more reasonable rates for these areas, as long-term rates vary considerably between the U.S. Atlantic, Gulf and Pacific coasts because of the Glacial Isostatic Adjustment, local subsidence and sediment compaction, and other vertical land movement. Using these trends for the four scenarios reveals that the relative sea levels predicted by century’s end could range — averaged over all states — from 0.2 to 2.0 m above present levels. The estimates for the amount of land inundated vary from 26,000 to 76,000 km2. Upwards of 1.8 to 7.4 million people could be at risk, and GDP could potentially decline by USD 70–289 billion…. Even the most conservative scenario shows a significant impact for the U.S., emphasizing the importance of adaptation and mitigation.”


“Risks of Sea Level Rise to Disadvantaged Communities in the United States”
Martinich, Jeremy; Neumann, James; Ludwig, Lindsay; Jantarasami, Lesley. Mitigation and Adaptation Strategies for Global Change, February 2013, Vol. 18, Issue 2, 169-185. doi: 10.1007/s11027-011-9356-0.

Abstract: “Climate change and sea level rise (SLR) pose risks to coastal communities around the world, but societal understanding of the distributional and equity implications of SLR impacts and adaptation actions remains limited. Here, we apply a new analytic tool to identify geographic areas in the contiguous United States that may be more likely to experience disproportionate impacts of SLR, and to determine if and where socially vulnerable populations would bear disproportionate costs of adaptation. We use the Social Vulnerability Index (SoVI) to identify socially vulnerable coastal communities, and combine this with output from a SLR coastal property model that evaluates threats of inundation and the economic efficiency of adaptation approaches to respond to those threats. Results show that under the mid-SLR scenario (66.9 cm by 2100), approximately 1,630,000 people are potentially affected by SLR. Of these, 332,000 (∼20%) are among the most socially vulnerable. The analysis also finds that areas of higher social vulnerability are much more likely to be abandoned than protected in response to SLR. This finding is particularly true in the Gulf region of the United States, where over 99% of the most socially vulnerable people live in areas unlikely to be protected from inundation, in stark contrast to the least socially vulnerable group, where only 8% live in areas unlikely to be protected. Our results demonstrate the importance of considering the equity and environmental justice implications of SLR in climate change policy analysis and coastal adaptation planning.”


“Coastal Flood Damage and Adaptation Costs under 21st Century Sea-level Rise”
Hinke, Jochen; et al. Proceedings of the National Academy of Sciences, March 2014, Vol. 111, No. 9, 3292-3297. doi: 10.1073/pnas.1222469111.

Abstract: “Coastal flood damage and adaptation costs under 21st century sea-level rise are assessed on a global scale taking into account a wide range of uncertainties in continental topography data, population data, protection strategies, socioeconomic development and sea-level rise. Uncertainty in global mean and regional sea level was derived from four different climate models from the Coupled Model Intercomparison Project Phase 5, each combined with three land-ice scenarios based on the published range of contributions from ice sheets and glaciers. Without adaptation, 0.2–4.6% of global population is expected to be flooded annually in 2100 under 25–123 cm of global mean sea-level rise, with expected annual losses of 0.3–9.3% of global gross domestic product. Damages of this magnitude are very unlikely to be tolerated by society and adaptation will be widespread. The global costs of protecting the coast with dikes are significant with annual investment and maintenance costs of US$12–71 billion in 2100, but much smaller than the global cost of avoided damages even without accounting for indirect costs of damage to regional production supply. Flood damages by the end of this century are much more sensitive to the applied protection strategy than to variations in climate and socioeconomic scenarios as well as in physical data sources (topography and climate model). Our results emphasize the central role of long-term coastal adaptation strategies. These should also take into account that protecting large parts of the developed coast increases the risk of catastrophic consequences in the case of defense failure.”


“Climate Change Risks to U.S. Infrastructure: Impacts on Roads, Bridges, Coastal Development and Urban Drainage”
Neumann, James E.; et al. Climatic Change, January 2014. doi: 10.1007/s10584-013-1037-4.

Abstract: “Changes in temperature, precipitation, sea level, and coastal storms will likely increase the vulnerability of infrastructure across the United States. Using four models that analyze vulnerability, impacts, and adaptation, this paper estimates impacts to roads, bridges, coastal properties, and urban drainage infrastructure and investigates sensitivity to varying greenhouse gas emission scenarios, climate sensitivities, and global climate models. The results suggest that the impacts of climate change in this sector could be large, especially in the second half of the 21st century as sea-level rises, temperature increases, and precipitation patterns become more extreme and affect the sustainability of long-lived infrastructure. Further, when considering sea-level rise, scenarios which incorporate dynamic ice sheet melting yield impact model results in coastal areas that are roughly 70% to 80% higher than results that do not incorporate dynamic ice sheet melting. The potential for substantial economic impacts across all infrastructure sectors modeled, however, can be reduced by cost-effective adaptation measures. Mitigation policies also show potential to reduce impacts in the infrastructure sector — a more aggressive mitigation policy reduces impacts by 25% to 35%, and a somewhat less aggressive policy reduces impacts by 19% to 30%. The existing suite of models suitable for estimating these damages nonetheless covers only a small portion of expected infrastructure sector effects from climate change, so much work remains to better understand impacts on electric and telecommunications networks, rail, and air transportation systems.”


“Increased threat of tropical cyclones and coastal flooding to New York City during the anthropogenic era”
Reed, A.J.; et al., Proceedings of the National Academy of Sciences, 2015. doi: 10.1073/pnas.1513127112.

Abstract: “In a changing climate, future inundation of the United States’ Atlantic coast will depend on both storm surges during tropical cyclones and the rising relative sea levels on which those surges occur. However, the observational record of tropical cyclones in the North Atlantic basin is too short (A.D. 1851 to present) to accurately assess long-term trends in storm activity. To overcome this limitation, we use proxy sea level records, and downscale three CMIP5 models to generate large synthetic tropical cyclone data sets for the North Atlantic basin; driving climate conditions span from A.D. 850 to A.D. 2005. We compare pre-anthropogenic era (A.D. 850–1800) and anthropogenic era (A.D.1970–2005) storm surge model results for New York City, exposing links between increased rates of sea level rise and storm flood heights. We find that mean flood heights increased by ∼1.24 m (due mainly to sea level rise) from ∼A.D. 850 to the anthropogenic era, a result that is significant at the 99% confidence level. Additionally, changes in tropical cyclone characteristics have led to increases in the extremes of the types of storms that create the largest storm surges for New York City. As a result, flood risk has greatly increased for the region; for example, the 500-y return period for a ∼2.25-m flood height during the preanthropogenic era has decreased to ∼24.4 y in the anthropogenic era. Our results indicate the impacts of climate change on coastal inundation, and call for advanced risk management strategies.”


“Reducing Coastal Risks on the East and Gulf Coasts”
Committee on U.S. Army Corps of Engineers Water Resources Science, Engineering, and Planning: Coastal Risk Reduction; Water Science and Technology Board; Ocean Studies Board; Division on Earth and Life Studies; National Research Council. 2014, the National Academies Press.

Summary: “Hurricane- and coastal-storm-related economic losses have increased substantially over the past century, largely due to expanding population and development in the most susceptible coastal areas… This report calls for the development of a national vision for managing risks from coastal storms (hereafter, termed “coastal risk”) that includes a long-term view, regional solutions, and recognition of the full array of economic, social, environmental, and life-safety benefits that come from risk reduction efforts. To support this vision, a national coastal risk assessment is needed to identify those areas with the greatest risks that are high priorities for risk reduction efforts. Benefit-cost analysis, constrained by other important environmental, social, and life- safety factors, provides a reasonable framework for evaluating national investments in coastal risk reduction. However, extensive collaboration and additional policy changes will be necessary to fully embrace this vision and move from a nation that is primarily reactive to coastal disasters to one that invests wisely in coastal risk reduction and builds resilience among coastal communities.”


“The Role of Ecosystems in Coastal Protection: Adapting to Climate Change and Coastal Hazards”
Spalding, Mark D.; Ruffo, Susan; Lacambra, Carmen; Meliane, Imen; Hale, Lynne Zeitlin; Shephard, Christine C.; Beck, Michael W. Ocean and Coastal Management, March 2014, 90:50-57.

Abstract: “Coastal ecosystems, particularly intertidal wetlands and reefs (coral and shellfish), can play a critical role in reducing the vulnerability of coastal communities to rising seas and coastal hazards, through their multiple roles in wave attenuation, sediment capture, vertical accretion, erosion reduction and the mitigation of storm surge and debris movement. There is growing understanding of the array of factors that affect the strength or efficacy of these ecosystem services in different locations, as well as management interventions which may restore or enhance such values. Improved understanding and application of such knowledge will form a critical part of coastal adaptation planning, likely reducing the need for expensive engineering options in some locations, and providing a complementary tool in hybrid engineering design. Irrespective of future climate change, coastal hazards already impact countless communities and the appropriate use of ecosystem-based adaptation strategies offers a valuable and effective tool for present-day management. Maintaining and enhancing coastal systems will also support the continued provision of other coastal services, including the provision of food and maintenance of coastal resource dependent livelihoods.”


“Sea Level and Global Ice Volumes from the Last Glacial Maximum to the Holocene”
Lambeck, Kurt; Rouby, Hélène; Purcell, Anthony; Sun, Yiying; Sambridge, Malcolm, Proceedings of the National Academies of Science, September 2014. doi: 10.1073/pnas.1411762111.

Abstract: “Several areas of earth science require knowledge of the fluctuations in sea level and ice volume through glacial cycles. These include understanding past ice sheets and providing boundary conditions for paleoclimate models, calibrating marine-sediment isotopic records, and providing the background signal for evaluating anthropogenic contributions to sea level. From ~1,000 observations of sea level, allowing for isostatic and tectonic contributions, we have quantified the rise and fall in global ocean and ice volumes for the past 35,000 years. Of particular note is that during the ~6,000 years up to the start of the recent rise ~100−150 years ago, there is no evidence for global oscillations in sea level on time scales exceeding ~200-year duration or 15−20 cm amplitude.”


“Sea-level rise due to polar ice-sheet mass loss during past warm periods”
Dutton, A; et al. Science, 2015. doi: 10.1126/science.aaa4019.

Abstract: “Interdisciplinary studies of geologic archives have ushered in a new era of deciphering magnitudes, rates, and sources of sea-level rise from polar ice-sheet loss during past warm periods. Accounting for glacial isostatic processes helps to reconcile spatial variability in peak sea level during marine isotope stages 5e and 11, when the global mean reached 6 to 9 meters and 6 to 13 meters higher than present, respectively. Dynamic topography introduces large uncertainties on longer time scales, precluding robust sea-level estimates for intervals such as the Pliocene. Present climate is warming to a level associated with significant polar ice-sheet loss in the past. Here, we outline advances and challenges involved in constraining ice-sheet sensitivity to climate change with use of paleo–sea level records.”


“The Multimillennial Sea-level Commitment of Global Warming”
Levermann, Anders; et al. Proceedings of the National Academies of Science, June 2013. Vol. 110, No. 34. doi: 10.1073/pnas.1219414110.

Abstract: “Global mean sea level has been steadily rising over the last century, is projected to increase by the end of this century, and will continue to rise beyond the year 2100 unless the current global mean temperature trend is reversed. Inertia in the climate and global carbon system, however, causes the global mean temperature to decline slowly even after greenhouse gas emissions have ceased, raising the question of how much sea-level commitment is expected for different levels of global mean temperature increase above preindustrial levels. Although sea-level rise over the last century has been dominated by ocean warming and loss of glaciers, the sensitivity suggested from records of past sea levels indicates important contributions should also be expected from the Greenland and Antarctic Ice Sheets…. Oceanic thermal expansion and the Antarctic Ice Sheet contribute quasi-linearly, with 0.4 m °C−1 and 1.2 m °C−1 of warming, respectively. The saturation of the contribution from glaciers is overcompensated by the nonlinear response of the Greenland Ice Sheet. As a consequence we are committed to a sea-level rise of approximately 2.3 m °C−1 within the next 2,000 years. Considering the lifetime of anthropogenic greenhouse gases, this imposes the need for fundamental adaptation strategies on multicentennial time scales.”


“From the extreme to the mean: Acceleration and tipping points of coastal inundation from sea level rise”
Sweet, William V.; Park, Joseph. Earth’s Future, 2014. doi: 10.1002/2014EF000272.

Abstract: “Relative sea level rise (RSLR) has driven large increases in annual water level exceedances (duration and frequency) above minor (nuisance level) coastal flooding elevation thresholds established by the National Weather Service (NWS) at U.S. tide gauges over the last half-century. For threshold levels below 0.5 m above high tide, the rates of annual exceedances are accelerating along the U.S. East and Gulf Coasts, primarily from evolution of tidal water level distributions to higher elevations impinging on the flood threshold. These accelerations are quantified in terms of the local RSLR rate and tidal range through multiple regression analysis. Along the U.S. West Coast, annual exceedance rates are linearly increasing, complicated by sharp punctuations in RSLR anomalies during El Niño Southern Oscillation (ENSO) phases, and we account for annual exceedance variability along the U.S. West and East Coasts from ENSO forcing. Projections of annual exceedances above local NWS nuisance levels at U.S. tide gauges are estimated by shifting probability estimates of daily maximum water levels over a contemporary 5-year period following probabilistic RSLR projections of Kopp et al. (2014) for representative concentration pathways (RCP) 2.6, 4.5, and 8.5. We suggest a tipping point for coastal inundation (30 days/per year with a threshold exceedance) based on the evolution of exceedance probabilities. Under forcing associated with the local-median projections of RSLR, the majority of locations surpass the tipping point over the next several decades regardless of specific RCP.”

Keywords: research roundup, Katrina, Sandy, preparedness, global warming, water, oceans, sea level rise

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