A real space the laser is sailing 300 miles above your head right now. Launched in 2018, NASA’s ICESat-2 satellite contains a lidar instrument, the same type of technology that enables self-driving cars to see in three dimensions by spraying lasers around them as they drive down the street and scanning the bouncing light. But instead of mapping a road, ICESat-2 measures the elevation of the Earth’s surface with pinpoint accuracy.
While this space laser does no harm to you, it portends disaster. Today in the review Nature Communication, scientists describe how they used new lidar data from ICESat-2 to map land on the planet less than 2 meters above sea level, making them vulnerable to elevation creep sea level. Combining these data with population figures, they calculated that 267 million people currently live in these at-risk areas. Assuming a sea level rise of 1 meter by the year 2100, they predict that 410 million people will eventually live in an affected area. Asian countries like Bangladesh and Indonesia are particularly vulnerable, but the United States and Europe will not be short of populations at risk either.
“We firmly believe that if the world is able to cope with sea level rise and conserve nature in coastal areas – this is an important aspect – the rise must be known”, states the Lead author of the study, Aljosja Hooijer, flood risk expert at the National University of Singapore and Deltares, a research institute in the Netherlands.
The document’s estimates, Hooijer points out, are cautious on several levels. On the one hand, they did without taking into account explosive population growth in cities around the world, due to uncertainties in calculating where people will ultimately move. Currently, 55% of the planet’s population lives in urban areas, which the United Nations predicts will increase to 68% by 2050. But this will not play out uniformly: the population of some cities may increase faster than others, or even decrease.
“The work fills a very big gap that we have right now,” says Manoochehr Shirzaei, a geophysicist at Arizona State University, who studies sea level rise but was not involved in the news. research. Scientists have well models of sea level rise, adds Shirzaei, “but when you want to quantify the risk of flooding, you also need to know the elevation. And that’s a big unknown.
Previously, researchers used satellite radar to map elevations. It works on the same principle as lidar, except that it bounces the radar off the ground instead of a laser. “The problem with radar is that it can’t penetrate vegetation, only a little,” says Hooijer. “It gets stuck somewhere between the canopy and the ground surface, and the elevation reading you get is somewhere in between.” Lasers, on the other hand, easily penetrate vegetation, giving a more accurate measurement. (You may have heard of how scientists are using lidar to see through the trees of the Amazon jungle and map the ancient ruins hidden below.)
Hooijer found that 72 percent of the population who will be at risk of flooding will live in the tropics. Tropical Asia alone will account for 59 percent of the area at risk, as the region is particularly low. “This is a huge problem for developed countries, for Europe and for states,” says Hooijer. “But if you look at the roadmap, who are the people who are going to suffer the most, and probably the earliest? They are poor people, most of whom live in underdeveloped areas. We don’t give it a lot of attention, it’s really the hot spot. And we were surprised at the numbers ourselves.
There is another problem: In addition to dealing with the encroachment of seawater on their shores, some cities are also sinking. Soil subsidence is a phenomenon in which the soil compacts, usually due to overuse of groundwater. Coastal cities are particularly prone to subsidence due to their geology, as urban centers have historically appeared where rivers meet the sea. Over the millennia, a river is said to have deposited layer upon layer of clay, and the city would have grown on it. But as the metropolis taps into the underlying aquifer, that clay crumbles like an empty water bottle, and the city can accept it. The more an urban center grows, the more people it needs to stay hydrated, which increases the rate and severity of subsidence.
Hooijer’s modeling takes subsidence into account, but it uses a uniform rate of elevation loss – half a centimeter per year – across the world instead of calculating the rate for each coastline alone. It would not be feasible. Yet researchers know that some areas are collapsing much faster than that: in parts of Jakarta, for example, the earth is sinking up to 10 inches a year. By 2050, 95 percent of northern Jakarta could be underwater, as the land’s elevation decreases while sea level rises. The problem is so serious that Indonesia is considering moving its capital out of the city.