The Nile Delta makes up just 2% of Egypt’s total area, but it’s home to 41% of its population—roughly 95 million people. These communities are under threat, however; much of the northern delta is gradually sinking into seawater, drowning rich agricultural land and communities. But just how quickly is it going under? A new study based on satellite data provides an estimate: If sea level rise, oil and gas drilling, and groundwater pumping continue unchecked, nearly 3,000 square kilometers of the delta will sink by 2100.
The delta’s subsidence can be traced to many factors. One key contributor is the upstream Aswan High Dam, built in the 1960s, which has reduced the amount of sediment that reaches the delta by more than 98%. Unable to replenish sediment lost to erosion, the delta has been gradually starved of its fertile mud. Simultaneously, over the past 30 years, Egypt has been pumping groundwater for agricultural, industrial, and urban use at an exponential rate, causing large areas to subside. In addition, Egypt has rapidly become Africa’s second largest producer of natural gas, extracting much of that fuel from the thick layers of sand and shale underlying the delta and exacerbating subsidence.
Although scientists have long known that large swathes of the delta are caving in and will be flooded by seawater, other regions appear to be uplifting because of flex in the sedimentary basin’s geology. To track the region’s deformation, Gebremichael et al. decided to take a bird’s-eye—or, rather, a satellite’s— view of the entire >40,000-square-kilometer delta and surroundings. They obtained a series of 84 highly detailed images taken between 2004 and 2010 and used a technology called persistent scatterer interferometry to reveal subtle changes in its topography.
The analysis revealed which regions of the delta are sinking and which are gradually lifting upward. Between the northern and southern parts of the delta, the team found an east–west zone of uplift roughly 40 kilometers across at its widest point, slowly rising at rates of up to 7 millimeters per year. The highest rates of subsidence occurred in the northern delta and in regions where natural gas and groundwater extraction is booming, such as the Menoufia Governorate and the Abu Madi gas field. Overall, the team concluded, 2,660 square kilometers of the northern delta will be flooded by seawater by 2100 if the current rate of topographical deformation continues. This calculation assumes a climate scenario in which atmospheric carbon dioxide levels remain below 500 parts per million and global sea levels rise 0.44 meter. Under that scenario, the loss of land in the delta would displace or otherwise affect nearly 5.7 million people, the scientists report. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB015084, 2018)
—Emily Underwood, Freelance Writer