This article was originally on Hakai magazine, an online publication about science and society in coastal ecosystems. Read more of these kinds of stories hakaimagazine. com.
Communities around the world are bracing for sea level rise: The Netherlands stabilizes its dikes, Senegal neighborhoods moved, Indonesia moves the entire capital. These projects are large, expensive and slow.
But maybe they need to pick up the pace. As new research shows, in many places, sea level rise will cause coastal flooding and other disruptions much sooner than anyone realized. It’s not that the water is rising any faster; it’s that the country was lower to begin with.
Calculating when a rising sea will flood a certain place requires a lot of math: you need to know the height of the water, the range of the tide, the elevation and slope of the land, the rate of sea level rise, and how much the land itself is rising or decreases, among numerous other factors. As with all science, the accuracy of these predictions is only as good as the data flowing into them.
The problem, according to the new study by Ronald Vernimmen and Aljosja Hooijer, two data analysts who work on flood risk in Southeast Asia, is that time and time again the measurements of coastal height that scientists feed into their models are wildly inaccurate. In tropical forests, says Vernimmen, these misinterpretations can be off by 20 meters or more. “Of course you can’t use it,” he says.
The problem stems from limitations in the technology typically used to measure altitude: radar. Radar covers an area with radio waves and then measures how long it takes for the waves to bounce back. But the radar isn’t precise enough to separate treetops from solid ground, and a stretch of pine or group of apartments can easily exaggerate the height. Many studies of sea level rise still use radar elevation data collected by the spaceship in 2000.
Lidar is very similar to radar, but uses lasers instead of radio waves. A lidar detector like the one on the ICESat-2 satellite, which NASA launched in 2018, can send up to a million pulses per second, firing lasers that can locate the gaps between buildings and trees to more accurately measure the height of the land below. to measure . Analysts still need algorithms to filter that barrage of information into a functional map, but the results are much more accurate.
Over the past few years, Vernimmen and Hooijer have been filtering the new satellite data for Earth’s immense coastline and comparing the radar’s height estimates with the newer lidar-based measurements. It wasn’t pretty.
The scientists’ big finding is that forests and buildings along the coast have skewed radar maps, giving planners inaccurate elevation data. Lidar showed coastlines that were often lower than previously realized. This has two important implications: the same amount of sea level rise will be able to travel much further inland, and it will happen much faster than expected.
The scientists’ new lidar-based estimate predicts that about 482,000 square kilometers of land will be submerged by one meter of sea level rise, nearly three times the 123,000 square kilometers predicted by radar-based projections. That’s an extra piece of land the size of Cameroon, currently home to about 132 million people, that will be flooded by 2100 under a high-emissions scenario.
The risk is greatest for river deltas in tropical regions where the land is flat, the population is often high and the data is often old. With a sea level rise of two metres, the Niger Delta in West Africa and the Irrawaddy Delta in Myanmar will have five times more land under water by the year 2150 under a high-emissions scenario than the older radar-based estimates suggested. The same goes for the Chao Phraya Delta, which extends across metropolitan Bangkok, the capital of Thailand with a population of 11 million.
For Vernimmen, the recalculation means that society has to reconsider a number of things. “There are huge construction projects going on in areas where building is not allowed,” he says.
The researchers made their altitude data set publicly available in the hope that governments take note of the new timeline, Hooijer added.
Mir Matin, a remote sensing expert at the United Nations University in Ontario who was not involved in the study, says these estimates could be made even more accurate by using airborne lidar – the type attached to drones or aircraft – instead of passive satellite measurements. . While airborne lidar is more accurate, it is also more expensive, requiring pilots, aircraft and planning. Some wealthy countries and major cities have embraced aerial lidar surveys, but Matin says developing countries would also benefit. Rich countries-responsible for most of global warming— could cover the costs, he says. “Ultimately, climate change is a global phenomenon,” adds Matin.
This article first appeared in Hakai magazine and is republished here with permission.