Risk assessement for sea level rise due to climate change

A recent study published in Nature Communications (https://www.nature.com/articles/s41467-019-12808-z) provides an update on the question, how many people will be affected by rising sea level due to climate change. In this new study, it is stated that up to 190 Million people, a plus of 80 Million worldwide, live in land areas below the projected sea level in 2100. This is three times the value used in other studies.

Where do the new data come from?

As the first guess, one would expect, that new climate change projections or new models have increased the expected sea level rise and thus result in a larger amount of people affect by flooding.

But the climate projection data have been unchanged. The new values come from the other side of the risk assessment, touching the question how many land areas are actually below the projected sea level rise and how many people are living in these areas.

Almost all studies rely on the globally available NASA SRTM data to obtain the height of land (DEM). These data are available with a resolution of approximately 30 x 30 m. In ENVI-met MONDE you can easily access these data using the “Get Topography..” feature.

However, these data have a certain level of uncertainty concerning the height data as they are captured using satellites and these satellites also see vegetation (forests) and urban areas(buildings) and it is hard to difference between the level of the ground surface and for example the top of a tree canopy. So, the NASA data tend to overestimate the height of the terrain with uncertainties around +2 m, depending on the region on earth.   

Unfortunately, this error is critical in regions on earth which are close to the sea level where being 2 m lower or higher decides if an area is flooded or not.

So what is the news in the study?

Instead of using the NASA SRTM the authors used a new terrain model for the costal regions called CostalDEM.  For many regions on earth, this new terrain model shows lower land heights than the SRTM, hence the chance of these areas to be flooded increase.

What are the consequences?

First of all, being below the sea level does not necessarily mean that the land will be flooded. As the authors also state, there are many regions on earth that are below or at sea level and not flooded as they are protected through different technical or natural means. This is the case for example in the Netherlands, Shanghai or New Orleans just to name a few. While the updated data, on a global scale, show an impressive increase of land and people affected, the situation locally can differ much.

It is well known and part of practice, that sheltering structures need to be increased and elevated to protect the populated areas behind it. This of course demands time and a lot of money and hence will not be an option everywhere on earth. So new study even enforces the message to act now on all frontiers, from cutting down emissions up to land protection measures.