 recent decades, and how much more space the water takes up. We have satellites and other tools that have measured that,” said Ben Hamlington, the current lead of NASA’s sea level change team. “The same thing is true for several of other factors that influence sea level, such as the mass of the ocean, the salinity, and how much water is stored on land.”
That growing knowledge base is why scientific organizations like the Intergovernmental Panel on Climate Change (IPCC) are publishing sea level rise projections with increasing levels of confidence. In its 2019 report, the IPCC projected (chart above) 0.6 to 1.1 meters (1 to 3 feet) of global sea level rise by 2100 (or about 15 millimeters per year) if greenhouse gas emissions remain at high rates (RCP8.5). By 2300, seas could stand as much as 5 meters higher under the worst-case scenario. If countries do cut their emissions significantly (RCP2.6), the IPCC expects 0.3 to 0.6 meters of sea level rise by 2100.
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A host of competing factors will influence how global sea changes translate to regional and local scales. Among them: the rising or falling of the land surface due to plate tectonics and human activity; gravity anomalies that can create regional bulges and dips in sea surface height; variations in the temperature and salinity of seawater; changes in the amount of water stored on land in reservoirs; isostatic adjustment due to the addition, loss, and movement of land ice; and changes in erosion and how much sediment rivers carry to coastal areas.
Sorting out how river deltas will respond is a particularly thorny and consequential issue. Tens of millions of people live on river deltas around the world (such as India’s Krishna Delta, shown above), and many of them are subsiding (sinking), often at twice the mean rate of sea level rise. The subsidence is due to a combination of factors like the natural settling of sediments, groundwater and oil extraction, and the extra weight of buildings. Inland dam construction and land management practices can also starve deltas of the raw material needed to replenish and build coastal land.
But it is hard to predict human settlement patterns—and the subsidence it causes—decades or centuries from now. Many IPCC projections do not even attempt to incorporate estimates of this subsidence partly because of the uncertainties in future land use and human behavior and because there is a lack of readily available, large-scale data on vertical land motion to feed into models of sea level rise.
The current shortage of land motion data is poised to become an abundance with the launch of the NASA-ISRO Synthetic Aperture Radar (NISAR) mission in 2022. The radar will make daily, global measurements of land motion that sea level experts like Manoochehr Shirzaei of Virginia Tech say will lead to major improvements in regional sea level rise projections.
Likewise, teams of scientists have been surveying the fast-sinking Mississippi River Delta to get a better understanding of how changes in sediment and vegetation affect the delta. Scientists participating in NASA’s Delta-X campaign have collected several types of data to develop and calibrate a model of how the delta might respond to rising sea levels in the next century.
“The combination of anthropogenic subsidence and increasing rates of sea level rise is a five-alarm fire for many delta cities,” said Shirzaei. “Places like New Orleans, Kolkata, Yangon, Bangkok, Ho Chi Min City, and Jakarta will undoubtedly face increasing pressures from flooding and saltwater intrusion.”
Still, the long-term picture—hundreds of years into the future—is unlikely to be perfectly clear. “When you think about future impacts of sea level rise, you also have to consider what people might do in response,” said Hamlington. Some countries—like The Netherlands and the United States—have already built elaborate sea walls and water-control systems that protect vulnerable deltas like the Rhine and the Sacramento-San Joaquin. They will likely continue reinforcing these systems as sea levels rise. In others del