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La hausse du niveau des mers menace des millions d'habitants
We don’t really know how much rise in sea level will result from global warming. What should we expect in the coming years? What impact will it have on coastal populations? Marie-Noëlle Woillez, a specialist in climate issues at AFD, gives us some answers.
What are the current trends in sea levels?

The sea level rise is accelerating. Since the beginning of the 20th century, we’ve seen an overall rise of about 20 cm, at a speed that increased from an average of +1.4 mm per year between 1930 and 1972 to +3 mm per year from 1993 to 2015.

This rise isn’t regular everywhere around the world, mainly because water does not heat up in each place with the same intensity (the more water heats up, the more it expands and the more its level rises). Other factors can come into play, such as changes in dynamics that affect the how water bodies are distributed.

What should we expect in the coming years?

We don’t exactly know. The report published in 2013 by the Intergovernmental Panel on Climate Change (IPCC ) banked on a rise of 1 m maximum in 2100 if high emissions of greenhouse gas continue. The problem is that this projection doesn’t consider the possibility of ice caps melting rapidly.

Today we still don’t know how ice caps are going to keep reacting to ongoing climate change. Some scientists think we’ve already passed the tipping point, and that even if we were to stop emitting greenhouse gas tomorrow the ice in Greenland and Antarctica would continue to break up.

The IPCC forecasts will thus probably be revised upwards in the next report, expected in 2021. And some scientists think that if greenhouse gas emissions continue to be strong, the rise in ocean level won’t be 1 m but 2 m at the end of the century.

Especially because there are other factors linked to human activity – ones we don’t think about as much – that are intensifying the problem in various places around the world...

That’s right. In some places, humankind is degrading the earth, thereby exposing it to an even higher rise in sea level. The Nile Delta, for example, is losing ground to the Mediterranean Sea, especially because of upstream dams that reduce sediment in the delta. And, generally speaking, in many deltas the pumping of groundwater and the weight of new buildings are causing land to sink up to several centimeters per year. Elsewhere, it’s uncontrolled sand mining that’s pushing back beaches. This makes land and nearby cities vulnerable. 

What about the impact of this rise in sea level on people?

It’s difficult to say, because it’s currently impossible to obtain reliable forecasts at the local level for the entire world. What we do know is that 625 million people currently live in coastal zones less than 10 m above sea level and that the populations there are going to grow hugely given the current demographic curves and trends in urbanization.
The threat will differ depending on the number of people concerned, the geographical characteristics, and the local economic resources. It seems obvious to me that we won’t be able to build dikes everywhere. Everything will have to be evaluated case by case. And the sooner the better.

 

 

Further reading