In three decades, the African coastlines have seen the reference level climb by more than eleven centimeters. The readings indicate not only a continuous progression, but also a measurable acceleration, driven by the thermal expansion and the increased supply of water from the continental ice.
The seas are rising all over the planet, under the effect of global warming and melting ice. This rise changes from one year to the next, but it accumulates and transforms the coasts. The rise of the oceans does not progress at the same pace according to the region, and around Africa, measurements show that it is now reaching a critical threshold.
What thirty years of satellite measurements reveal
In the early 1990s, altimetric satellites began to measure with millimeter accuracy the distance between them and the surface of the ocean. This method offers a stable reference, independent of tides or storms, and allows you to follow the actual evolution of sea level. Between 1993 and 2024, the waters surrounding Africa rose by about 11.26 centimeters, a figure higher than the world average.
The annual rate reaches 3.54 millimeters per year around the continent, compared to 3.45 millimeters on a global scale. The gap seems modest, but it produces significant cumulative effects over the years. The analyses reported by Phys.org from the work of oceanographers also show a measurable acceleration. The level does not increase at a constant speed, it increases faster than before.
The 2023-2024 period marks a particularly marked episode. The instruments recorded an anomaly of up to 27 millimeters, the strongest ever observed in African waters. In just two years, an additional 2.34 centimeters were added, or almost 19% of the total elevation measured since 1993. This recent jump is in addition to an already upward trend.
Why the rise of the oceans is accelerating around the African continent
Two processes explain the rise in sea level. The water expands when it heats up, which increases its volume. The melting of the glaciers and caps of Greenland and Antarctica adds water to the ocean basins. These phenomena result from global warming linked to greenhouse gas emissions.
The El Niño episode of 2023-2024 reinforced these effects in some already abnormally hot African areas. Surface temperatures in the western Indian Ocean and the tropical Atlantic were above their usual averages. Unusual winds have limited upwelling, preventing the rise of colder deep waters and retaining heat on the surface. Thermal expansion would have accounted for more than 70 percent of the exceptional increase observed during this event.
The researchers studied temperature and salinity up to 300 meters deep to distinguish the share related to the expansion from that from the additional water supply. Their results, published in Communications Earth and Environment, show that the thermal content of the ocean has quadrupled compared to the El Niño episode of 2015-2016. The current acceleration is based on a continuous accumulation of heat and mass in the ocean.
African regions more vulnerable than others
The rise in sea level does not affect all African coasts in the same way. In the western Indian Ocean, around Mozambique, Madagascar and the Comoros, the rate reaches about 3.88 millimeters per year, with an estimated acceleration of 0.16 millimeters per year squared. Currents, the configuration of the seabed and the structure of the coastline contribute to this increased sensitivity.
The central eastern Atlantic, including the Gulf of Guinea and the coasts of Senegal, Ghana, Nigeria or Cameroon, recorded an increase of close to 3.90 millimeters per year. More than 200 million people live near the African shores. Each additional centimeter raises the base level on which tides and storms are superimposed, increasing the risk of flooding, erosion and soil salinization. Some areas are thus becoming particularly exposed to a sustainable transformation of their coastal balances.
source : science et vie
