Coastal Risks in South Asia - SAR-CLIMATE

Re-evaluating Coastal Risks: Land Elevation Data and Climate Change

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Dr. Benjamin Strauss (Photo by ADPC)

Dr. Benjamin Strauss is Chief Executive Officer and Chief Scientist at Climate Central based in New York, USA.

While most of the scientific community researches disasters and climate change, we’ve focused 99% of our energy on how far and how quickly the sea-level will rise or how high storms can raise waters when they come in.

But when we’ve looked at the moving ball, it turns out that we don’t know a lot about the elevation of the land in many parts of the world.

Asia mostly uses satellite-based elevation data, and unfortunately, measuring from a satellite can produce much less accurate data when compared to airborne lidar data.

The image below compares airborne-lidar and satellite-based data in the same area, and as you can see, it shows a much lesser threat.

A comparison of airborne-lidar and satellite-based observations in the same area (Photo by Climate Central)

When you’re sending signals down from a satellite to measure elevation on land, by the time the beam reaches the surface, it’s pretty broad and when it comes down, it’s going to hit things like rooftops and treetops.

The satellite signal generally can’t tell the difference between a rooftop and a treetop, at least the legacy signals that were used in the development of all of the major global elevation datasets used by the research community today.

When rooftops and treetops blend in with ground elevation, it makes the land look higher than it is and this is a real problem when you’re trying to understand flood risk and sea-level rise risks. However, a new dataset that a colleague and I developed is using machine learning to take those satellite data sets and is trying to eliminate the effect of rooftops and treetops.

In September 2021, we also published the second version of Coastal DEM v2.1, which is a high-accuracy and high-resolution global coastal elevation model. When I talk about its performance, the bars below show the error at validated elevation data compared to the best we have for ground truth.

A comparison of vertical bias Digital Elevation Monitoring (DEM) and Ice, Cloud and land Elevation Satellite (ICESat-2) Satellite Lidar in meters (Graphic by Climate Central).

The research community has principally used the data set called MERIT and its predecessor, which doesn’t perform as well, called SRTM. MERIT comes from scientific researchers, AWD3D30 is developed by the Japanese government, TanDEM-X by the German Aerospace Agency and NASADEM is NASA’s development in the United States.

On average, all of these elevation data sets think that the land is one and a half meters higher than its actual height (or more), so when we’re projecting only less than a meter of sea-level rise by the end of this century, that’s a big problem.

Both of our Coastal DEM versions try to reduce this error – the first one we released did underestimate elevation by almost half a meter in a global average, but the newest version gets essentially to zero.

It turns out that we don’t know a lot about the elevation of the land in many parts of the world!

Here’s Kolkata, India, which is under great threat of increased flooding due to rising sea levels. We’ll look at the sea-level rise just by itself as projected through the 2050s.

Here are areas below the one-year flood line, which is much more extensive, and it has turned Kolkata into a small dry island.

So there’s a possibility that as much work as we’ve done to improve our data, it may not yet be doing the full appropriate correction for the most densely developed downtowns, where you have the most buildings and density.

There’s something very profound to understand about sea level rise and climate change, which is that ice takes time to melt.

We know it’s going to melt and the same is roughly true for sea-level projections in the scientific community. It is a great struggle to understand exactly how quickly sea levels will rise in the coming decades or this century, but we have more confidence in our long-term projections of how much ice will melt and how much water will expand based on the warming that we cause.

Despite the more than one degree Celsius of warming that we’ve already experienced on the planet, the ice sheets have fortunately not yet come close to catching up with that level.

However, they’re still melting because of that, and we can expect there to be around two meters of sea-level rise from the carbon pollution that we’ve already put in the atmosphere and a lot more from higher levels of warming that we could cause in the future.

Dr. Benjamin Strauss spoke with ADPC during its Climate Talks panel discussion on sea-level rise.

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homolata

homolata.borah@adpc.net

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