For generations, communities along the Gulf of Guinea have watched the coastline change. Beaches have narrowed, homes have flooded, freshwater has turned salty and the shores that support fishing, trade and some of West Africa’s oldest coastal cities have steadily retreated.
Long before scientists could quantify how rapidly sea levels were rising, many coastal residents already knew something fundamental was changing. They witnessed the sea inching inland, even if the reasons were not yet fully understood.
By reconstructing the causes of past sea-level rise locally – and measuring how much faster the rise is happening today than it was a few decades ago – scientists are helping policymakers confront excruciating choices about where to build protections and in some cases what land should be abandoned.
That is where a new interactive platform comes in.
Developed by scientists at Harvard University’s Salata Institute for Climate and Sustainability as part of a broader climate adaptation project run in collaboration with researchers at the University of Ghana, the Université Félix Houphouët-Boigny in Côte d’Ivoire and the University of Lagos, it reconstructs sea-level change at 950 locations around Africa’s coastline from 1900 and projects how it could change through to 2100 under different emissions scenarios.
Users can explore not only how much sea level has changed but also the physical processes driving those changes.
“Tide gauge records are sparse and discontinuous across most African coasts,” said Rebecca Berkoh-Oforiwaa, a post-doctoral fellow in atmospheric physics at Harvard.
“These gaps present a critical challenge for climate adaptation planning.”
“It’s a tool for the general public but it’s also a tool for people who are interested in adaptation policies,” said Jerry Mitrovica, Frank B. Baird Jr. professor of science at Harvard University. “This is the first time these results have been visualised at local scale in such an accessible way.”
Living with a shifting shore
One of the clearest examples lies along the Gulf of Guinea.
Since the mid-20th century, rates of sea level rise have tripled between Côte d’Ivoire and Nigeria, driven largely by melting ice sheets and the expansion of warming seawater.
The region is also experiencing some of the world’s most severe coastal erosion while rapid urbanisation is placing ever more people and infrastructure in harm’s way.
Professor Emmanuel Akyeampong, of Harvard University, where he is director of the Salata Institute project: Climate Adaptation in the Gulf of Guinea, noted that the Gulf of Guinea has seen coastal erosion going back to the very beginnings of the 20th century.
“Talk about climate change has been rising in recent years but even without that discourse, without that language, we’ve been experiencing the effects of climate change for several decades.”
Akyeampong describes the Gulf of Guinea as an example of what he describes as “slow violence” – environmental damage that accumulates gradually over decades rather than arriving as a single dramatic disaster.
“If you look at the Gulf of Guinea, most of the colonial capitals were situated on the coast, which means that many of the early infrastructure developments, such as ports, were also on the coast,” he said.
These coasts represent some of the earliest places where the capacity of humans to make technological interventions in hydrology took place.
“This combination of a place that has the Atlantic Ocean, has estuaries and has this creek barrier lagoon that connects it all makes it a very intricate sort of water system, a hydrological system. So it’s a good place to see what happens when humans make intrusions and to document what happens over more than a century of this interaction.”
The wider project is examining communities in Abidjan, Accra and Lagos, where rising seas are interacting with rapid urbanisation, flooding, saltwater intrusion and coastal erosion to threaten homes, infrastructure and livelihoods.
Researchers are evaluating adaptation options ranging from strengthening coastal defences and improving drainage to supporting planned relocation where communities become increasingly vulnerable. These gaps present a critical challenge for climate adaptation planning.
Reconstructing the past, projecting the future
Behind the platform is research led by Sönke Dangendorf of Tulane University. By combining multiple statistical approaches with the physics of how sea level changes across space and time, Dangendorf’s reconstruction enabled Mitrovica’s team to map sea level change around Africa’s coastline in unprecedented detail.
The reconstruction draws on long-running tide gauge records, some stretching back more than a century. But observations become increasingly sparse the farther back in time scientists look.
Processes such as melting ice sheets, warming oceans and shifting ocean circulation produce distinct geographical patterns in sea level. That means even scattered observations can help constrain what happened between measurement sites, allowing scientists to reconstruct changes where no direct measurements exist.
“Going back to 1900 is possible both because you have tide gauges and because you have these physical models of how sea level changes,” said Gabe Cederberg, of Harvard University’s department of earth and planetary sciences, who designed the tool.
The result is a reconstruction with the greatest uncertainty in the early 20th century, narrowing steadily as satellite observations and other modern datasets become available.
Along any stretch of coastline, sea level rise is shaped by a unique combination of overlapping forces: warming water that expands, melting ice that adds mass to the ocean and alters gravity, shifting currents and land that is itself moving up or down.
According to Mitrovica, treating sea level rise as a single regional or global number risks obscuring the information policymakers actually need.
“The online tool provides what you might call the ‘background sea level change’ that any site has experienced, or, in the case of projections, will experience. Extreme sea level events — which cause a lot of damage (for example, flooding) — occur when storm surges and high tides add to this background sea level change.
“If you want to prepare for the impact of storm surges on Gqeberha you have to know how the background sea level has changed at Gqeberha. Knowing how sea level has changed on average over a region (for example, the southern coastline) or even over the globe will be of limited use in preparing for the impacts of extreme events on that southern city.”
More than a rising ocean
Another insight emerging from the research is that it is not only the ocean that moves.
“One of the most underappreciated processes in African coastal planning is vertical land motion,” Berkoh-Oforiwaa said. “Sea-level rise is often viewed as the ocean rising but in some locations the land itself is also sinking or rising. This can significantly increase or reduce the sea-level change experienced by coastal communities.”
In some cases, she noted, human activities such as groundwater extraction can contribute to land subsidence, increasing the risk of coastal flooding.
She said the tool provides a “critical baseline for interpreting projected future changes”, particularly in coastal megacities such as Lagos, where rapid urbanisation and groundwater pumping appear to be contributing to sinking land and amplifying relative sea level rise.
Understanding these local processes allows scientists to distinguish between regional ocean trends and place-specific drivers of risk, helping policymakers design more targeted interventions — from improving water supply systems and reducing groundwater extraction to strengthening flood protection and in some cases planning for managed evacuations.
Although tide-gauge records remain limited across much of Africa, Berkoh-Oforiwaa noted that comparisons between the reconstruction and available observations show strong agreement.
“When communicating uncertainty to non-scientists, I emphasise that while the exact amount of sea level rise may vary, the overall trend is clear: sea level is rising along African coastlines. The reconstruction provides the best available estimate to support adaptation and planning decisions.”
The platform shows that sea level has already risen along South Africa’s coastline, she said. “Across the west, south and east coasts, sea level has risen since the mid-20th century, although the amount varies from place to place.”
She added that projections indicate continued rise throughout this century.
Under a moderate emissions scenario, the tool projects about another 0.576 metres of sea-level rise for Durban by 2100, 0.568 metres for East London, 0.599 metres for Cape Town and 0.602 metres for Gqeberha.
Turning science into action
For Akyeampong, the challenge is not simply generating more evidence but making that evidence usable in real-world decisions.
“In our policy dialogues with governments, we hear a need, indeed a hunger to find science-based evidence to shed light on the challenges but also to provide directions in terms of what can be done,” he said.
“There is no point telling someone in many elaborate ways that something bad is happening. You need to provide a way out of the problem.”
The sea level rise tool is designed with that in mind. It forms part of a broader “diagnostic kit”, that he noted, is “helping us understand the size of the problem”.
“Local communities understand the experiential aspect, they know the sea level is rising, they know that if they don’t tether their canoes, sometimes they get washed out to sea, because they’re experiencing this.
“To be able to confirm what they’re experiencing with the science is both affirming and revealing.