FAQ: How to use the Africa sea-level tool
Table of contents
What time periods are shown in the historical view?
What scenarios are shown in the projection view?
What are sterodynamic and barystatic sea-level change?
Huh? How should I think about the difference?
Why does the “total” not always equal the sum of the different components?
What do the dots on the map represent?
What historical data does the tool use?
Why does sea-level rise differ from place to place?
What datasets and sources are used?
What does this tool show?
This tool shows two views of African coastal sea-level change.
The historical view is a reconstruction of relative sea-level change since 1900. The projected view shows relative sea-level change to the year 2100 under three different emissions scenarios.
Clicking a dot on the map shows results for a specific coastal segment. For each segment, the tool displays total relative sea-level change and the individual contributions from different physical processes.
What is relative sea level?
Relative sea level is the height of the sea relative to the land at a specific location.
This matters because sea level can change for two reasons: the ocean surface can rise or fall; or the land can move up or down. For coastal impacts, relative sea level is often the most useful measure because it reflects what communities, ecosystems, and infrastructure experience locally.
What time periods are shown in the historical view?
The historical view shows reconstructed relative sea-level change over three timespans: 1900–2021; 1940–2021; 1980–2021.
These periods allow users to compare long-term change with more recent trends.
What scenarios are shown in the projection view?
The projection view shows relative sea-level change through the year 2100 under three emissions pathways from the IPCC Sixth Assessment Report, or AR6.
These pathways – known as SSP1-2.6, SSP2-4.5, and SSP5-8.5 – represent different possible futures, from lower to higher greenhouse gas emissions. Higher-emissions scenarios generally lead to greater long-term sea-level rise because they increase ocean warming and ice loss.
What are sterodynamic and barystatic sea-level change?
Sterodynamic sea-level change comes from changes within the ocean. It includes changes in seawater density, temperature, salinity, and ocean circulation. For example, as the ocean warms, seawater expands. That thermal expansion raises sea level even if no additional water is added to the ocean. Sterodynamic change can vary strongly by region. Winds, currents, density gradients, and circulation shifts can also move water around the ocean, raising sea level in some places and lowering it in others.
Barystatic sea-level change comes from changes in the mass of the ocean. This often means water is added to the ocean from melting glaciers, the Greenland Ice Sheet, the Antarctic Ice Sheet, or changes in the amount of water stored on land. It can also include gravitational and rotational effects associated with the movement of mass from land ice into the ocean. Barystatic change affects global mean sea level, but its regional pattern is not uniform. For example, when a large ice sheet loses mass, nearby sea level can rise less than the global average, or even fall, because the ice sheet’s gravitational pull on surrounding ocean water weakens.
Huh? How should I think about the difference?
Sterodynamic change is about how ocean water behaves.
Barystatic change is about how much water is in the ocean.
What is vertical land motion?
For a variety of reasons, land moves up and down.
When land sinks, relative sea level rises faster. When land rises, relative sea level rises more slowly. One important source of vertical land motion is glacial isostatic adjustment (GIA), which is the long-term adjustment of Earth’s crust after the growth and retreat of large ice sheets. Yes, land is still adjusting after the end of last ice age nearly 12,000 years ago. Another important source of land subsidence is groundwater withdrawal for drinking or agriculture or industry. The tool includes vertical land motion because coastal risk depends on the movement of both the ocean and the land.
Why does the “total” not always equal the sum of the different components?
The total may not exactly equal the sum of the component medians (50th percentile) because totals are calculated from combined probability distributions.
Each component has uncertainty. When those uncertain components are combined, the median of the total distribution is not necessarily the same as adding together the medians of each individual component. This is a statistical issue, not an error in the tool.
What do the dots on the map represent?
There are 950 dots in the tool, each representing one coastal segment on the African continent and nearby islands.
The latitude and longitude shown for each dot mark the center of that segment. The coastal segmentation comes from the SLIIDERS dataset, which divides coastlines into standardized segments for analysis and comparison.
What historical data does the tool use?
The historical view uses a probabilistic reconstruction of sea-level change and its causes from Dangendorf et al. (2024). The reconstruction estimates both total relative sea-level change and the contributions from individual components (sterodynamic; barystatic, including Greenland, Antarctic, glaciers, and land water storage; and vertical land motion) over the historical periods shown in the tool. Each coastal segment was matched to the nearest historical reconstruction point using segment latitude and longitude.
Why are the barystatic components (e.g., glaciers, Greenland, Antarctica, and land-water storage) listed separately in the projections view?
These are all separate contributors to ocean mass change for which the IPCC AR6 projections provide individual assessments.
Glaciers, the Greenland Ice Sheet, and the Antarctic Ice Sheet each lose mass in different ways and at different rates. Land-water storage captures changes in water stored on land, including groundwater, reservoirs, lakes, and other hydrological changes. Listing these components separately helps users see which physical processes contribute most to projected relative sea-level change at a given coastal segment.
Why does sea-level rise differ from place to place?
Sea-level change is not globally uniform.
Several processes create regional differences. Ocean warming and circulation changes redistribute water. Ice loss changes Earth’s gravity field, rotation, and shape. Land can move up or down. These processes mean that two coastal locations can experience different amounts of relative sea-level change even under the same global scenario.
Who created the tool?
The tool was created by Gabriel Cederberg in the lab of Jerry Mitrovica at Harvard University’s Department of Earth and Planetary Sciences.
The data were accessed in March 2026. The site was updated in May 2026.
What datasets and sources are used?
The historical component estimates come from Dangendorf et al. (2024). The projected sea-level components come from the IPCC Sixth Assessment Report. Coastal segmentation is based on the SLIIDERS dataset. The interactive visualization was built with MapLibre GL JS and ECharts, and it draws methodological inspiration from NASA’s IPCC AR6 Sea Level Projection Tool.
References
Depsky, N., Bolliger, I., Allen, D., Choi, J. H., Delgado, M., Greenstone, M., Hamidi, A., Houser, T., Kopp, R. E., and Hsiang, S. DSCIM-Coastal v1.1: an open-source modeling platform for global impacts of sea level rise, Geosci. Model Dev., 16, 4331-4366. https://doi.org/10.5194/gmd-16-4331-2023, 2023. (coastal segmentation dataset)
Dangendorf, S., Sun, Q., Wahl, T., Thompson, P., Mitrovica, J. X., and Hamlington, B.: Probabilistic reconstruction of sea-level changes and their causes since 1900, Earth Syst. Sci. Data, 16, 3471-3494. https://doi.org/10.5194/essd-16-3471-2024, 2024. (regional historical component contributions)
IPCC. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. https://doi.org/10.1017/9781009157896, 2021. (regional projected component contributions)
NASA Sea Level Change Team. IPCC AR6 Sea Level Projection Tool. NASA Jet Propulsion Laboratory. https://sealevel.nasa.gov/ipcc-ar6-sea-level-projection-tool, 2023. (interactive visualization)