The densely populated and lowly elevated coastline of the Gulf of Guina is particularly exposed to coastal erosion and sea-level rise. The entire coastline is currently experiencing alarming erosion rates, ranging from 1 to 15 meter per year between Côte d’Ivoire and Nigeria, mostly because of human activities. The problem is well studied and various programs aim to put in place coastal protection measures. By contrast mid- and long-term vulnerability to sea-level rise triggered by climate change has received less attention in this region. 

In addition, to assess the various risks induced by sea-level rise, it is also necessary to consider land surface movements. The gradual lowering of the land surface in some places is called subsidence. The phenomenon is widespread at the global scale, especially in coastal megacities and deltas: 51 to 71% of present-day relative sea-level rise experienced by people worldwide is actually caused by subsidence. The rate can reach several centimeters per year in some places, however coastal subsidence is globally critically under-quantified, and the Gulf of Guinea region is no exception to this.

This research program aims to fill the lacune in knowledge on coastal land subsidence and relative sea-level rise along the Gulf of Guinea. It will provide useful space-borne quantifications of current coastal subsidence rates, assess the driving forces of land subsidence and provide first projections of potential future subsidence using numerical models. 

Impact will further be created by collaborating with local institutes, connecting to other projects and disseminating findings, data and tools to local stakeholders. 

One of the long-term objectives is also to create awareness and agenda setting of this issue of relative sea-level rise in the region, in order to support sustainable development or adaptation projects in coastal areas and policymaking in the region. 

The first working group will assess current subsidence rates for the whole coastline, using satellite data to monitor vertical land movement over several years. The results will be combined to satellite elevation data to provide a first assessment of the areas that could be flooded in future decades. 

The second working group will focus on the city of Lagos and surroundings, to provide an in-depth analysis of subsidence and to understand drivers and processes. The research team will build a process-based numerical model of subsidence to provide projections of future subsidence rates according to different scenarios.

Then, the methods developed for Lagos will also be applied to 3 or 4 other areas, coastal megacities or deltas, identified as “hotspots” for relative sea-level rise by the first working group. 

Knowledge exchange events (seminars and workshops) will also be regularly organized, to disseminate research findings and build a regional community of interest on subsidence and relative sea-level rise. 

Results will be shared through scientific papers, technical reports, on-line databases and policy briefs. They will provide valuable information for policymakers and coastal (infrastructural) development projects potentially impacted by relative sea-level rise. In particular, results will help to better calibrate coastal adaptation measures and may also identify areas where the efficiency of natural coastal defences could fail in the future. Understanding the drivers of subsidence will also help to identify potential mitigation measures. The project will also foster public policy dialogue on coastal vulnerability, to favor adaptation projects.

• Marie-Noëlle Woillez, research officer on climate change impacts, AFD
• Marine Canesi, research officer, AFD