As the world warms, the sea swells. Water molecules expand with rising temperatures, and melting glaciers unleash gushes of fresh water, adding to the sea’s volume.
One element of sea level rise is not always fully accounted for in climate change predictions, however: the rate at which land is rising or sinking. Localized vertical land motion can amplify or dampen the rising water levels caused by climate change.
Focusing on Africa’s 20 most populous coastal cities, researchers recently combined vertical land motion data captured by satellites with predictions of climate-driven sea level changes to produce more accurate—and alarming—estimates of flood risk. The team’s work is the first to focus on relative sea level rise in Africa, meaning it considers vertical land motion when predicting future patterns of inundation. The team will present its research at AGU’s Annual Meeting 2023 in San Francisco.
Earth’s seemingly rigid surface is actually malleable, and while some pockets of the surface are rising, many coastal areas—especially fast-growing, highly populated ones such as Lagos, Nigeria, whose urban population is projected to reach 20.6 million by 2030—are sinking. This phenomenon is called subsidence.
One of the main causes of land subsidence is groundwater extraction. To meet the needs of rapidly expanding populations, water is often pumped out faster than it can refill. “It’s like wringing out a sponge,” said Leonard Ohenhen, a geosciences doctoral student at Virginia Polytechnic Institute and State University (Virginia Tech) and one of the presenters at the AGU event.
Ohenhen and his fellow researchers used a satellite technology called interferometric synthetic aperture radar, or InSAR, to calculate subsidence rates. “Imagine you throw a ball against the wall and it comes back to you, and you keep throwing it precisely,” Ohenhen explained. “If you move closer to the wall…you would notice that the ball will come to you quicker,” and if you move farther from the wall, the ball will return more slowly. InSAR technology works the same way, with satellites pinging signals to Earth and measuring the time it takes for the signals to return. By compiling several of the resulting images, researchers can estimate land’s changing elevation over time.
The team was surprised to learn just how rapidly some African coastal cities are sinking. Overall, they found high rates of subsidence—greater than 10 millimeters per year—in most of the cities they studied, including Lagos; Alexandria, Egypt; and Douala, Cameroon. The global average rate of sea level rise is currently around 4.4 millimeters per year.
“Ten millimeters [of subsidence] per year in African cities is big.…That’s fast. If you’re living in these cities…this is something that’s going to have impacts,” said Robert Nicholls, director of the Tyndall Center for Climate Change Research. Nicholls was not involved with the study but has collaborated with the paper’s researchers in the past.
Sinking land and rising water can combine with other factors to increase risk to communities. “The flooding in Libya and the extent of the loss of life and property that occurred there was a combination of sea level rise, extreme weather events, land subsidence, and then, importantly, weak infrastructures,” said Seyi Dasho, lead author of the research and a geosciences doctoral student at Virginia Tech.
Both Dasho and Ohenhen are Nigerian. “Coming from Africa ourselves,” Ohenhen said, they recognize the significance of the subsidence. “You’re able to see how this is affecting people right now,” he said.
Dasho, Ohenhen, and their team combined subsidence rates gleaned from InSAR data with five sea level change scenarios published by the Intergovernmental Panel on Climate Change.
“These are quite important observations,” said Nicholls, who has himself studied sea level rise in African cities but did not account for subsidence. He explained how the team’s InSAR data allow for “high-resolution, high-accuracy assessment” to help vulnerable cities predict future flood risk and plan accordingly. “Recognizing the problem is the first step,” he said. The next challenge, he said, is deciding what to do about it.
—Leigh Dorsey, Science Writer