2020 Festival of Science

Welcome to this first encounter which will plunge us into an ocean which keeps rising all across the world, by submerging islands, coasts, and even our shores. Added to that, sadly, as we have seen, are increasingly violent storms and cyclones. All due, of course, to anthropogenic global warming. Which means caused by human activity. I'm delighted to introduce Gonéri Le Cozannet.

Hello.

You're with the BRGM: the Geological & Mining Research Bureau. You're a researcher in the Risk & Prevention Department. Is that right?

Exactly.

Let's dive right in. You agree, no doubt about it, the oceans are rising?

No doubt, the oceans are rising. They have risen 15 to 20 cm since the mid-19th century. You brought along a diagram for us to see.

Here it is.

Exactly. It shows observations between 1870 and 2000. The 15 to 20 cm I mentioned. And the predictions of two scenarios. In the first, we severely cut greenhouse-gas emissions.

In the second, we do nothing.

We continue emitting greenhouse gases. That's the red.

And that would be totally catastrophic. The oceans are rising, and there's often talk of 2100. As if they'll stop rising in 2100 if we do nothing!

We know they're rising, and we know why. Due to the thermal expansion of the oceans, the melting glaciers...

Whoa there! We understand the melting glaciers, but you used a long term before that.

I'm sorry. The thermal expansion of the oceans.

And what's that?

The oceans are heating up and dilating. That explains half of what we saw in the 20th century.

The oceans are rising because temperatures are higher. The ocean surface is notably much warmer and so it's swelling. Like when you boil water.

Climate change is an energy imbalance in the earth's system. 93% of additional warmth due to climate change goes into the oceans, which absorb it and swell.

OK, but if we look way back into the past... You brought another diagram on this. We know the oceans have risen, because a long time ago, they were very low. Then things stabilized, before starting to rise again.

21,000 years ago, there was an ice age. There were four major ice sheets. One over Scandinavia, one over Canada, one over Greenland, and one over Antarctica. These ice sheets melted, and 6,000 years ago, the ocean level reached its current height. The ice sheets were 120 metres thick, so you could walk from France to England! Or 200 km from Nantes before you saw the sea! Currently, we're in a fairly warm interglacial period. Since 1870, after being stable for 6,000 years, the ocean level has been rising due to climate change linked to the greenhouse effect.

Clearly due to emissions of greenhouse gases.

No doubt at all. There's scientific consensus on the rising ocean level. It's one of the most integrated ways of observing climate change. The earth's energy imbalance is difficult to observe. But the rising ocean level is a variable which you can observe and which embodies this imbalance.

Something that allows us...

To observe easily and reconstruct this imbalance.

Absolutely. So it's fact. To sum up, we have the dilatation of water, melting glaciers, along with the melting of the poles... Added to that, sadly, as we've seen in recent years, depressions bringing monster storms which in turn bring more and more water to our shores.

Two things make us think there are more storms. Firstly, we see them more easily, because there are more assets along the coasts. We know there are more storms because there's more damage. The second thing is the rising sea level, which has caused an increase in extreme episodes. Added to storm cycles, which haven't changed much in 100 years, is the rising sea level. We characterize storms by their return periods, like centennial storms which occur every 100 years, decennial storms, every 10 years... There's generally only a 40-cm difference between them.

At the BRGM, you model these storms to see how far inland they go. We have a very interesting animation to watch.

We all recall 2010 and Cyclone Xynthia, and the ensuing disaster. But this is an animation of Storm Johanna. It was in March 2008. It submerged the Gâvres Peninsula in the Morbihan in Brittany.

That's right. Here you can see the incoming tide. The tide is oncoming, the storm is causing a fall in atmospheric pressure, the sea level is rising, and the wind is forcing water sideways.

All very quickly?

Yes. We modelled each wave and we see a complex flooding process.

It's just starting...

It's not a static water level going over the defences. Each wave is going over them. So for the overall model, we had to model each wave. And there, very quickly, the peninsula is flooded. Evidence of this remains with water-level marks on the walls. There were no victims, but lots of damage. So the traces on the walls validate our model. It allows us to formulate coastal risk-prevention plans so we can reduce the risks across the area.

That's the aim of the work in your department at BRGM: to establish prevention plans. We just saw, very clearly, how France's coasts can be suddenly assailed by the ocean and be totally flooded. What are the most vulnerable areas along the coasts of mainland France?

It's the very low coastal areas that are floodable. Along with erodible areas. For example, in Aquitaine, some sandy beaches are eroding by 1 metre a year. If there are homes and businesses behind the beach, we're gradually reaching a situation where we either move the buildings or protect them with breakwaters or riprap, for example. Other low coastal areas in France are Languedoc-Roussillon, Poitou-Charentes... All over, really.

More than we imagine.

All along the coasts, not only in big regions, but well identified, are flood-risk zones. They're in Brittany, everywhere.

The Camargue is often mentioned, but there's Aquitaine, Brittany, Charente...

The Giens Peninsula...

So global warming and the rising ocean level are of great concern to us. A final word on French overseas territories. Notably Guadeloupe, which is a very fragile zone.

The problem with Guadeloupe is a low area called Jarry, where 40,000 people work every day.

Near Pointe-à-Pitre.

Exactly. It's a crucial area for Guadeloupe's economy, and it's only 40 cm above sea level. So imagine if the ocean were to rise 40 cm. Well, this is expected to happen by the second half of the 21st century.

So there will be problems.

Big problems. Problems, too, for French Polynesia, and the Tuamotu Islands. So the harsh realities are being placed in front of our eyes through your studies.

We're at a moment where the sea level is starting to accelerate. The question is: how fast? Today, it's 3 mm a year. 1 cm a year is possible during the 21st century. It depends on our greenhouse gas emissions. Thank you, Gonéri.