How to prepare coastlines for sea level rise

• Sea and ocean levels will continue to rise long after global temperatures stabilize, according to our partner The Conversation.

• It is therefore necessary to prepare for a generalization and intensification of submersion at high tide, during storms or cyclones, for the salinization of estuaries and coastal aquifers and the permanent submersion or erosion of low or sandy coasts.

• This analysis was conducted by Gonéri Le Cozannet, geoscience researcher at the Bureau of Geological and Mining Research (BRGM).

Sea level rise has a special place in the second part of the IPCC's 6th assessment report published in February 2022. Unlike other climate variables such as temperature and precipitation, sea level will continue to rise long after global temperatures stabilize, as mountain glaciers and ice caps in Antarctica and Greenland will take centuries to adjust to new temperatures.

Thus, if it remains possible today to limit the rate of sea level rise to around 4 mm/year, it is no longer possible to stabilize the sea level itself.

The report therefore warns of the unique nature of adaptation to sea level rise. While we are just beginning to perceive the increase in the frequency of chronic flooding in certain sites such as Venice or the east coast of the United States, we already know the following stages: a generalization and intensification of submersion at high tide, during storms or cyclones, the salinization of estuaries and coastal aquifers, and finally, the permanent submersion or erosion of low or sandy shores.

These issues motivated the drafting, within the February 2022 report, of a summary on the risks associated with sea level rise.

This summary clearly states that our ability to adapt to current and future coastal risks will depend on two immediate actions: compliance with the Paris agreements in order to stabilize global warming below 2°C above pre-industrial periods;

adaptation to the inevitable effects of sea level rise.

​Stabilize the climate below 2°C to allow time for adaptation

Observations from tide gauges and altimetry satellites show that sea level rise is accelerating.

From 1.4 mm/year in the 20th century, it is today around 4 mm/year.

Limiting global warming, by reducing greenhouse gas emissions, allows more time for adaptation.

Beyond 2°C, the rate of sea level rise could reach 1 cm/year on a global average after 2050, and perhaps more in the case of a rapid melting of the ice sheets in Greenland , especially in Antarctica.

Such a scenario unfortunately cannot be completely ruled out today.

It could lead us to 1.7m of global sea level rise in 2100, 4 or 5 meters in 2150 and 15 meters in 2300. Even if this scenario does not materialize, projections for 2300 range from 3 to 7 meters for a scenario with high greenhouse gas emissions, enough to make it very difficult to protect many coastlines around the world, in a context where access to energy and materials will be very different from the current situation.

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Sea level rise varies from one region to another, but only deviates from the global value by ±20% for most of the inhabited coasts.

Thus, between 2°C and 2.5°C of global warming, the coasts will be reshaped for centuries and millennia, threatening submersion of the coastal areas in which live between 0.6 and 1.3 billion people today. .

The benefits of a policy of reducing greenhouse gas emissions and stabilizing climate change are therefore obvious for the coasts.

Adapt to sea level rise

Adapting is now a matter of urgency.

Admittedly, today we are seeing progress in terms of adaptation.

This has been the case, for example, in France for at least 25 years with the progressive evolution of coastal risk prevention policies or the coastline management strategy.

However, whatever the country considered, the adaptation is never complete.

In France, it focuses on the 2100 deadline, when flooding at high tide will occur long before, and current developments may have longer-term consequences, and ultimately lead to dead ends.

Moreover, coastal adaptation takes a long time, sometimes several decades.

In Venice, it took more than 40 years to implement the MOSE system to prevent the

increasingly frequent

This is one of the important results of the latest work by the IPCC: adaptation to sea level rise takes a very long time to set up, sometimes several decades.

From the point of view of adaptation, the risk is therefore to be overtaken, and to no longer have the time to organize the protection or the relocation of issues.

​Adaptation to the detriment of coastal ecosystems?

Of all the adaptation measures currently available, most have additional benefits, particularly for the quality of life or ecosystems.

This is the case, for example, of the greening of cities in order to mitigate heat waves.

Nevertheless, for coastal ecosystems, adaptation presents a risk if it is mainly based on engineering solutions such as the construction of dykes, riprap or estuarine barriers.

The risk is that this adaptation will take place to the detriment of coastal ecosystems such as marshes or mangroves.

So-called "nature-based" solutions can consist of leaving space for sediments and coastal ecosystems such as dunes or marshes, in order to attenuate water level peaks during storms and limit the risks to human life and infrastructure during storms.

It is then a question of removing the exposed stakes from maritime hazards, while using the space made free to restore and healthy ecosystems and thus contribute to limiting the loss of biodiversity.

This approach has its limits.

We don't always have the space to implement nature-based solutions.

Moreover, coastal ecosystems are often themselves vulnerable to climate change.

This is particularly the case for corals which are subject to increasingly frequent bleaching due to the warming of surface waters.

For corals that harbor 25% of marine biodiversity, dissipate wave energy and supply sediment to beaches, irreversible impacts are projected beyond 1.5° of global warming.

What coastlines do we want for tomorrow?

The work of the IPCC shows that it is possible to adapt coastlines to sea level rise while preserving coastal ecosystems.

The report also highlights that adaptation takes place more effectively when it is accompanied by a process of inclusive engagement of the communities concerned, taking into account their socio-cultural values ​​and their development priorities.

Dossier “GLOBAL WARMING”

The issue of adaptation to sea level rise is therefore not limited to technical measures such as coastal protection, the relocation of stakes, or the construction of houses on stilts.

It invites us to think about the coastlines we want for tomorrow.

While the transformations described in the IPCC report are extremely important, they come with many benefits far beyond the single issue of sea level rise. Not using these solutions proposed by science would is compromising the future.

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This analysis was written by Gonéri Le Cozannet, geoscience researcher at the Bureau of Geological and Mining Research (BRGM).

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• Planet

• Video

• The Conversation

• Global warming

• Sea

• IPCC

• Flood

• Venice