NEED may seem an overwhelming and unrealistic solution at first. Regardless, we here present preliminary quantification and thoughts on the financial feasibility, social–political considerations, environmental impacts, and technological challenges of NEED compared to that of alternative solutions, such as (managed) migrations and that of country-by-country protection efforts. In doing so, we arrive at an alarming conclusion: a solution as considerable as NEED might be a viable and cost-effective protection measure for even a few meters of SLR. For northern Europe, NEED may therefore be preferred over the alternative solutions. This conclusion reflects the magnitude of the threat that society is facing as a result of global-mean SLR. We here do not and cannot conclusively determine if NEED could and should be constructed. Yet, we do emphasize that the conclusions of our preliminary findings advocate for immediate action to intensify and further climate mitigation efforts, so that solutions with a scale and impact such as NEED are not going to be required.

The proposed location of the Northern European Enclosure Dam (NEED; thick black lines) superimposed on the topography ( Smith and Sandwell 1997 ), combined with areas where the population density exceeds 200 persons per square kilometer in the year 2020 (pink dots; CIESIN 2017 ). We use the combination of these data to provide the number of people within the enclosure that will be submerged for a certain amount of sea level rise (see inset). NEED-south runs from France (Ploudalmézeau, ∼25 km north from Brest) to England (the Lizard Heritage Coast, ∼100 km west from Plymouth), measures 161 km in length, and has an average ocean depth of about 85 m and a maximum depth of 102 m. NEED-north runs from northern Scotland (John o’ Groats, ∼200 km north of Aberdeen) via the Orkney Islands to the Isle of Noss (part of the Shetlands Islands) from where it crosses the North Sea to Bergen in Norway. Making use of the islands, the part from Scotland to the Isle of Noss is only 145 km in length and averages 49 m in depth. The crossing from the Isle of Noss to Norway measures 331 km in length and has an average depth of 161 m, with a maximum depth of 321 m in the Norwegian Trench. The construction of NEED would protect coastal communities of 15 countries, namely, Belgium, Denmark, England, Estonia, Finland, France, Germany, Latvia, Lithuania, Poland, Netherlands, Norway, Russia, Scotland, and Sweden. This includes the capital cities of Amsterdam, Copenhagen, Edinburgh, Helsinki, London, Oslo, Riga, Stockholm, and Tallin and major cities such as Bremen, Hamburg, Rotterdam, St. Petersburg, and The Hague.

The proposed location of the Northern European Enclosure Dam (NEED; thick black lines) superimposed on the topography ( Smith and Sandwell 1997 ), combined with areas where the population density exceeds 200 persons per square kilometer in the year 2020 (pink dots; CIESIN 2017 ). We use the combination of these data to provide the number of people within the enclosure that will be submerged for a certain amount of sea level rise (see inset). NEED-south runs from France (Ploudalmézeau, ∼25 km north from Brest) to England (the Lizard Heritage Coast, ∼100 km west from Plymouth), measures 161 km in length, and has an average ocean depth of about 85 m and a maximum depth of 102 m. NEED-north runs from northern Scotland (John o’ Groats, ∼200 km north of Aberdeen) via the Orkney Islands to the Isle of Noss (part of the Shetlands Islands) from where it crosses the North Sea to Bergen in Norway. Making use of the islands, the part from Scotland to the Isle of Noss is only 145 km in length and averages 49 m in depth. The crossing from the Isle of Noss to Norway measures 331 km in length and has an average depth of 161 m, with a maximum depth of 321 m in the Norwegian Trench. The construction of NEED would protect coastal communities of 15 countries, namely, Belgium, Denmark, England, Estonia, Finland, France, Germany, Latvia, Lithuania, Poland, Netherlands, Norway, Russia, Scotland, and Sweden. This includes the capital cities of Amsterdam, Copenhagen, Edinburgh, Helsinki, London, Oslo, Riga, Stockholm, and Tallin and major cities such as Bremen, Hamburg, Rotterdam, St. Petersburg, and The Hague.

The magnitude of the threat that SLR may pose demands a response with a solution that reflects the scale of the problem. On these grounds, we propose the construction of the Northern European Enclosure Dam (NEED) that disconnects the North and Baltic Seas from the Atlantic Ocean, to protect 15 northern European countries from global-mean SLR. This can be achieved by constructing two enclosure dams ( Fig. 1 ). The southern part of NEED connects France (near Brest) to the southwest coast of England and measures 161 km in length with an average depth of about 85 m and a maximum depth of 102 m. The northern part of NEED extends from the northeast tip of Scotland, via the Orkney and Shetland Islands to Bergen in Norway. The northern part has a total length of 476 km and average depth of 127 m with a maximum of 321 m in the Norwegian Trench. The two components together are referred to as NEED and have a total length of 637 km. The construction of NEED would protect coastal communities that under current population density consist of about 25 million people below 2-m SLR while 55 million live below 15-m SLR (inset Fig. 1 ). If constructed, NEED would be one of the largest civil-engineering challenges ever faced. Alternative configurations of NEED are considered less effective ( appendix A ).

It might be impossible to truly fathom the magnitude of the threat that global-mean sea level rise poses. However, conceptualizing the scale of the solutions required to protect ourselves against global-mean sea level rise aids in our ability to acknowledge and understand that threat. On these grounds, we here discuss a means to protect over 25 million people and important economical regions in northern Europe against sea level rise. We propose the construction of a Northern European Enclosure Dam (NEED) that stretches between France, the United Kingdom, and Norway. NEED may seem an overwhelming and unrealistic solution at first. However, our preliminary study suggests that NEED is potentially favorable financially, but also in scale, impacts, and challenges compared to that of alternative solutions, such as (managed) migrations and that of country-by-country protection efforts. The mere realization that a solution as considerable as NEED might be a viable and cost-effective protection measure is illustrative of the extraordinary global threat of global-mean sea level rise that we are facing. As such, the concept of constructing NEED showcases the extent of protection efforts that are required if mitigation efforts fail to limit sea level rise.

From the forgoing discussion we conclude that to understand how NEED compares to other solutions, we only have to compare NEED against national-based protection, as that seems to be the most viable ongoing measure. In what follows we will therefore provide a preliminary discussion on the technical challenges; financial feasibility; and environmental, social, and political impacts of NEED, with respect to that of national-based protection. To our surprise, NEED can sometimes be conceived as a better solution than continuing future upgrades of ongoing efforts. That a solution as radical as NEED has the potential to be preferred over ongoing protection measures is a direct reflection of the magnitude of the threat that SLR poses.

If we accept the reasoning provided above, we are left with protection as the most realistic solution. With economic and population growth in coastal areas, protection also becomes increasingly more worthwhile, while encouraging a proactive rather than reactive attitude to the threat of global-mean SLR ( Nicholls 2011 ). Current protection measures are implemented on a country-by-country (national) basis. Instead, NEED could offer a concerted effort to address protection of coastal zones across Europe against SLR ( Tol et al. 2008 ).

Managed retreat could potentially be less expensive than protection in certain locations ( Diaz 2016 ) and may theoretically be a good solution when implemented over long periods of time, well before a potential disaster occurs ( Nicholls and Klein 2005 ; Dronkers et al. 1990a ). In the case of SLR this requires immediate implementation. However, managed retreat leads to intangible costs such as large social and psychological difficulties in displacing people from their homes as well as cultural heritage loss. Related migration can lead to national and international social–political instability, forcing decision-makers to shy away from spurring processes to facilitate managed retreat ( Hino et al. 2017 ). Consequently, managed retreat is currently not widely implemented and arguably not a viable solution to timely address the threat of SLR.

There is substantial expertise available with regard to engineering of dikes, enclosure dams, and land reclamation projects. The largest constructed enclosure dams to date are the Afsluitdijk (Netherlands) and the Saemangeum Seawall (South Korea, Fig. 2 ). The Afsluitdijk 1 is 32 km long, about 11 m in height, and 90 m wide. The Saemangeum Seawall 2 is 33 km long, 36 m in height on average (maximum of 54 m), and 290 m wide. These dimensions are not far off those required for the construction of NEED-south and NEED-north near the Orkney and Shetland Islands. However, we expect a substantial but surmountable technological challenge for the part of NEED-north that crosses the Norwegian Trench with depths over 300 m. Fixed oil rigs are feasible in depths over 500 m, while moored oil rigs operate in waters with depths over 2000 m, indicating that having fixed constructions over 300-m depths is possible. Although dams have different requirements than oil rigs, this is encouraging for the possibility of constructing NEED.

Enclosing the North and Baltic Seas will yield a net freshwater discharge of 40,000 m 3 s −1 into the basin ( appendix B ). The discharge would lead to a SLR of 0.9 m yr −1 within the enclosure and must therefore be pumped out into the Atlantic Ocean ( appendix B ). Recently, a pumping station with a capacity of 550 m 3 s −1 was taken in operation in New Orleans ( USACE 2015 ), while the Dutch Afsluitdijk will install two new pumping stations with a capacity of 400 m 3 s −1 each. 3 As such, the discharge can be accounted for with less than 100 of such pumping stations, while we may expect more efficient and higher-capacity pumps likely to become available in the future. The discharge will also lead to freshening of the basin and reduce the salinity by a factor of 10 in about 100 years ( appendix B ). The freshening is expected to affect ecosystems, biodiversity, and the fishing industry (discussed later).

Regardless, the effect of NEED on the maritime industry will remain uncertain, both economically and technically. However, it is certain that without the construction of NEED, the maritime industry will also be economically affected and technically challenged as SLR will force ports to relocate or to adopt and continuously upgrade their protection measures.

The construction of NEED would significantly impact the maritime industry. The busiest trading ports in Europe (Rotterdam, Antwerp, Hamburg, Bremerhaven) lie within the enclosure. Without a proper solution to reduce the impact of NEED on the maritime industry, NEED would be a less viable solution for protection against SLR. Solutions are available, as NEED could for example incorporate sluice gates to allow for a continuation of ongoing shipping traffic. Sluice gates allowing for some of the largest ships in the world are already operational in the Netherlands and Belgium. Alternatively, harbors could be built on the ocean side of NEED from where goods could be transferred to trains or to vessels operating within the enclosure.

Assuming a dam with a middle width of 50 m, two sloping sides with a 1:2 (height:width) ratio ( Jonkman et al. 2013 ), and adding 20 m to the ocean depth to take into account future global-mean SLR, the volume of NEED-south and NEED-north are 4.6 and 31.6 km 3 , respectively. With most dams made out of a sand- or clay-like material, building NEED would require about 51 billion tons of sand (using a density for sand of 1400 kg m −3 ), which is equal to about one year’s worth of global sand use ( Peduzzi 2014 ). With sand becoming an increasingly scarcer material ( Torres et al. 2017 ), the availability, sourcing, and transport of building material and related energy cost to build and maintain the enclosure could pose limitations on the ability to construct NEED. However, we argue that constructing new coastal defenses and maintaining, upgrading, and expanding the thousands of kilometers of coastal defense that are already in place to protect northern European coastal communities will also provide complex technological challenges that may not be easy to overcome. As such, these challenges could well exceed those that arise when constructing NEED, showcasing that no solution will be straightforward when dealing problems as complex as SLR. As such, solutions of the extent as NEED are to be considered in shaping our future protections measures against global SLR.

Combining all the above, we estimate the total costs to be roughly 250–550 billion euros. When assuming a 20-yr construction time over which to spread the costs, this gives an annual expense of 0.07%–0.16% of the combined gross domestic product (GDP) of the 15 involved countries. 6 The United Kingdom, the Netherlands, Germany, Belgium, and Denmark would likely drive the construction of NEED because of their awareness of SLR, their vulnerability, or both ( Tol et al. 2008 ). For these five countries alone, the total expenses would amount to 0.15%–0.32% of their GDP, annually for 20 years. These numbers are achievable and pose no financial limitation, even when fewer countries contribute.

In addition to the construction of the dam itself, several discharge pumping stations must be included. When considering the total discharge scaled with the cost and capacity of the pumps of either the Afsluitdijk (200 million euros) or New Orleans (500 million euros), this would add an additional 20–33 billion euros. If the construction of sluices is required, this would add additional costs.

The most recent and comparable construction to NEED is that of the 33.9-km-long Saemangeum Seawall. 4 Using the ratio of the volume of the Saemangeum Seawall (0.34 km 3 ) and of NEED (36.1 km 3 ), multiplied by the cost of the Saemangeum Seawall (1.83 billion euros, 2018 value), we find an estimate of 192 billion euros to construct NEED. The Maasvlakte 2 is a 20-km 2 extension of the Rotterdam harbor that includes hard and soft flood protection and basic infrastructure such as quays, rail track, and roads. 5 Land was reclaimed from 17-m depth to 5 m above sea level, using 0.24 km 3 of sand at a total cost of 3.38 billion euros (2018 value). When we use volume to scale up the total cost (including infrastructure), we estimate a cost of 508 billion euros to construct NEED. Finally, by assuming that dike height and construction costs scale linearly and with an upper estimate of 42 million euros per kilometer for an enclosure dam at depths of 10 m ( Dronkers et al. 1990b ; Jonkman et al. 2013 ), we estimate 313 billion euros for the construction of NEED.

For countries other than the Netherlands, there are fewer estimates available that detail costs of protection against SLR, but we here discuss a few. In Germany, an SLR of 1 m would put more than 300,000 people at risk in the coastal cities and communities, and economic values endangered by flooding and erosion would amount to more than 270 billion euros ( Sterr 2008 ). With 3,700 km of German coast, ongoing improvements of SLR protection measures may become too costly and alternative measure may have to be found ( Sterr 2008 ). In 1990 it was estimated that 80 billion U.S. dollars (140 billion euros in 2018 values) of protection cost were needed to protect western and northern Europe and the Baltic coast against a 1-m SLR ( Dronkers et al. 1990b ). As SLR may already reach 1 m in 2100, actual costs are likely to quickly become much higher.

A low-end estimate suggests Dutch protection will increase from 0.35 to possibly 1.5 billion euros per year in 2200 for 2-m SLR, with a total cost of over 100 billion euros ( Kok et al. 2008 ). Other estimates suggest that continuing ongoing protection measures in the Netherlands for SLR up to 1.5 m in 2100, the cost range from 1.6 to 3.1 billion euros per year until 2050 with an integrated costs of 32–140 billion euros in 2100 (0.1%–0.5% of the GDP annually) ( Aerts et al. 2008 ; Kabat et al. 2009 ; Hinkel et al. 2018 ). In short, for only 1.5-m SLR, protecting the Netherlands is about one-third of the costs of NEED. For more SLR, protection quickly becomes technically and financially challenging and possibly unsustainable. Therefore, we argue that integrated over the next 100–200 years, even for the Netherlands alone, NEED may both technically and financially be a better solution than scaling up existing protection measures.

Based on the discussion above, we conjecture that protection of other coastal areas and cities against SLR exceeding 2 m, will quickly become multibillion euro investments. For SLR of even a few meters, we expect that the integrated cost of individual protection of all 15 countries together far exceeds the costs of constructing NEED. For protection against long-term SLR projection (>10 m), NEED is almost certainly the least costly option.

We do so by focusing on the impact of NEED on ocean dynamics, which describes fundamental changes that feed into higher-order changes such as that of biodiversity. To remain within the scope of this study, we only roughly extrapolate the dynamical results to gain some preliminary insight of other major impacts that may be expected.

Technical and financial consideration on constructing NEED have so far not excluded NEED as a possible solution to address the threat of SLR to northern Europe. Regardless of the initial reluctance to construct NEED, this motivates to further progress the debate and provide our preliminary view on the possible impact of NEED on the environment, society and politics.

The impact of NEED on ocean circulation is quantified using simulations with a version of the numerical ocean model NEMO that explicitly resolves tides (details in appendix C). The computational domain covers the northeast Atlantic including the North Sea. The results are shown for simulations with and without NEED constructed (Fig. 3).

F ig . 3. View largeDownload slide The circulation and sea surface height (SSH) for a scenario (a) without and (b) with NEED. The SSH is assessed by taking the 95th percentile of the hourly SSH output from the numerical model run and is given by the blue colors ( appendix C). Orange contours show the 0.2-, 2-, and 20-Sv contours of the barotropic streamfunction (1 Sv = 106 m3 s‒1). Solid contours indicate anticlockwise flow. (c) The bathymetry of the cross section traversed by NEED, split into its southern and northern components. Superimposed is the mean velocity as computed by the numerical simulation (described in appendix C). Note that tidal velocities are averaged out, but could locally reach values over 1 m s‒1. F ig . 3. View largeDownload slide The circulation and sea surface height (SSH) for a scenario (a) without and (b) with NEED. The SSH is assessed by taking the 95th percentile of the hourly SSH output from the numerical model run and is given by the blue colors ( appendix C). Orange contours show the 0.2-, 2-, and 20-Sv contours of the barotropic streamfunction (1 Sv = 106 m3 s‒1). Solid contours indicate anticlockwise flow. (c) The bathymetry of the cross section traversed by NEED, split into its southern and northern components. Superimposed is the mean velocity as computed by the numerical simulation (described in appendix C). Note that tidal velocities are averaged out, but could locally reach values over 1 m s‒1.

Under current circumstances, a tidal Kelvin wave propagates around the North Sea basin in an anticlockwise manner, leading to large tidal amplitudes (>1 m) and velocities (>2 m s‒1; Fig. 3a) (Otto et al. 1990). This sets up a circulation in which water is entering the North Sea between the Orkney and Shetland Islands and exiting along the Norwegian coast. Due to the construction of NEED, the Kelvin wave is obstructed from entering the basin and the tidal amplitude inside the basin becomes very small (Figs. 3b and C3). Instead, the new geometry causes the tidal amplitude to increase by about 0.7 m along the coasts of southwestern England and Wales and about 0.4 m for northwestern England (Figs. 3 and A1). With NEED constructed, an anticlockwise circulation is set up inside the North Sea basin that is driven by wind, baroclinic circulation from freshwater discharge, and very small tidal motions excited within the basin itself. Furthermore, with the changes in tides and circulation due to the construction of NEED, there should also be an associated change in the location of tidal energy dissipation and mixing. This could, for example, influence overturning circulation outside of the basin.

As such, constructing NEED will unquestionably have a large impact on the circulation and exchange of nutrients, sediment, and small marine life within the enclosure and possibly outside of the enclosure in the Atlantic Ocean and along the European Shelf. Changes in atmospheric circulation and rain patterns could also occur. Finally, we note that it remains unclear if the freshening of the enclosed basin can be compensated for. Such compensation could require hundreds of desalination plants and/or a drainage system. The latter would require additional pumps. Without compensation for the freshening, however, wholesale ecosystem changes will occur.