Rising sea levels are likely to wreak havoc much sooner than expected

There is a group of white people in autumn clothes standing near a swamp with measuring tools.  It's a sunny day

Professor David Burdick and his students measure potential sea level changes.Bill Butcher/USFWS/Wikimedia Commons

This story was originally published by Hakai magazine and is reproduced here as part of the Climate window collaboration.

Around the world, communities are preparing for sea level rise: The Netherlands is stabilizing its dykes, Senegal is relocating neighborhoods, and Indonesia is moving its entire capital. These projects are heavy, expensive, and slow.

But they may 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 previously thought. It’s not that the water is rising any faster; it’s that the land was lower to begin with.

Calculating when a rising sea will inundate a place takes 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 land itself is going up or down, among a myriad of other factors. As with all science, the accuracy of these predictions is only as good as the data going into them.

The problem, according to the new study by Ronald Vernimmen and Aljosja Hooijer, two data analysts working on flood risk in Southeast Asia, is that time and time again, the coastal elevation measurements that scientists plug into their models are been extremely inaccurate. In tropical forests, Vernimmen says, these misinterpretations can be off by 20 meters or more. Obviously, you can’t use it, he says.

The problem stems from the limitations of the technology typically used to measure elevation: radar. Radar covers an area of ​​radio waves, then measures how long it takes for the waves to recover. But radar isn’t accurate enough to separate treetops from land, and a clump of pines or a group of apartment buildings can easily exaggerate the elevation. Many studies of sea level rise still use radar elevation data collected by the space shuttle 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 out up to a million pulses per second, firing lasers that can pinpoint gaps between buildings and trees to more accurately measure elevation of the underlying ground. Analysts still need algorithms to filter that barrage of information into a functional map, but the results are much more precise.

Vernimmen and Hooijer have spent the last few years filtering new satellite data for Earth’s immense coastline, comparing radar-collected elevation estimates with more recent lidar-based measurements. It wasn’t nice.

The scientists’ big finding is that forests and coastal buildings have distorted radar maps, presenting planners with inaccurate elevation data. Lidar has shown often lower coasts than initially realized. This has two major implications: The same amount of sea level rise will be able to reach much further inland, and it will happen much sooner than expected.

Scientists’ new lidar-based estimate predicts that about 482,000 square kilometers of land will be submerged with one meter of sea level rise, almost triple 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 under water by 2100 under a high-emissions scenario.

The risk is greatest for river deltas in tropical regions where the terrain is flat, the population is often large, and data tends to be out of date. With two meters of sea level rise, around the year 2150 under a high-emissions scenario, West Africa’s Niger Delta and Myanmar’s Irrawaddy Delta will have five times more land under water than suggested from old radar-based estimates. The same goes for the Chao Phraya delta, which extends into the metropolitan area of ​​Bangkok, the capital of Thailand with 11 million inhabitants.

For Vernimmen, the recalculation means that society needs to rethink some things. There are huge construction projects going on in areas that really shouldn’t be built on, she says.

The researchers have made their elevation dataset publicly available in hopes that governments will take note of the new timeline, adds Hooijer.

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 using aerial lidar, the type attached to drones or airplanes, rather than readings passive satellite-based. While more accurate, aerial lidar is also more expensive and requires pilots, aircraft, and planning.

Some rich countries and big cities have shelled out for aerial lidar surveys, but Matin says developing countries would also benefit. Rich countries, responsible for most of the global warming, could cover the costs, he says. Ultimately, climate change is a global phenomenon, Matin adds.

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