"The ocean at the heart of humanity"
"The ocean and the climate"
-I am going to talk to you about the role the ocean plays
in our climate system.
When you think about the climate, you think about the weather,
about the atmosphere.
But our climate system is much more complex than that.
It is made of sub-systems
that you can see on this picture taken from space.
The atmosphere is transparent, so what you see is clouds.
But you can see the cryosphere, which is made of ice,
you can see the continents and the biosphere,
that is to say life on land and in the sea,
and you can see the ocean, which covers most of the surface.
It covers two thirds of our planet.
It is a layer of water, which is very important,
because it gives it specific physical properties.
Its thermal capacity is huge.
It is a thousand times bigger than the atmosphere's.
So it is able to absorb large amounts of heat.
The presence of water in the atmosphere is also mostly due to the ocean.
The ocean's water evaporates,
and as water is transferred, heat is also transferred
through a process of latent evaporation heat transfer.
Water also has huge dissolving properties,
so the ocean dissolves salts,
but also gases, such as CO₂ and oxygen,
which largely impacts marine life.
So the characteristics of our climate system
depend, in many respects,
on the presence of the ocean on our planet.
When talking about the climate system and about climate change,
one usually has in mind the increase in greenhouse gases
and the rise in the atmosphere's temperature.
Actually, climate change is due to an increase
in greenhouse gases within the atmosphere
which is itself due to human activities.
And because of this increase in greenhouse gases,
our climate system stores more energy,
which induces a rise in the atmosphere's temperature,
and hence global warming.
But is the rise in the atmosphere's temperature
the only cause of global warming?
If you look at climate change,
you need to take into account the amounts of energy
and the changes going on within our climate system.
And if you measure them,
which you can do by observing the different climate subsystems,
as it has been possible for a few decades,
you can see where energy goes.
If you look at these graphs
showing the results of these observations,
you will see that the atmosphere has absorbed only 1% of this energy.
Let us look at the ice.
It has absorbed only 4% of this energy.
6% have been transferred to the continents,
and 90%, almost all of it, have been transferred to the ocean.
So when we say that the planet is warming,
it is actually the ocean.
What are the consequences of this rise in the ocean's temperature?
Let us look at the rise in sea levels.
According to the measurements
we have been taking from space for 25 years,
the rise in sea levels is not the same everywhere.
On this picture, you can see the acceleration of this rise.
Sea levels rise more quickly in the regions in red
than in the regions in gray or in yellow.
This is a consequence of ocean currents,
because of which this rise is concentrated in some regions.
But when thinking about the rise in sea levels,
one usually has in mind the melting of continental ice.
When ice melts, its volume ends up in the sea,
so the volume of the sea increases and sea levels rise.
And this accounts for 50% of this rise.
However, the ocean is warming, and when water warms up, it expands.
So 30% of the rise in sea levels
are due to the ocean's thermal expansion.
The warming of the ocean
also has consequences on the hydrologic cycle,
that is to say the cycle of water
which evaporates from the ocean into the atmosphere,
then falls on land,
and finally ends up in rivers and goes back to the sea.
So within this hydrologic cycle,
if the ocean is warmer, the atmosphere is warmer.
And a warmer atmosphere can contain more particles of water vapor.
So evaporation increases, and so does the hydrologic cycle.
And that is how heat exchanges
between the ocean and the atmosphere increase.
This increase in precipitation water
and in energy in the atmosphere has consequences,
because it leads to more extreme climate events.
Climate change is not just an increase in temperature.
It is also an increase in extreme climate events.
Over the course of your life,
you will probably be faced with heat waves,
floods and violent storms.
But in the ocean, there are also extreme events
and heat waves.
Some parts of the ocean abnormally warm up
for a few weeks, sometimes a few months,
and gain up to two, three or four degrees.
And this phenomenon has become increasingly frequent
over the last decade.
And this abnormal warming of the ocean has consequences on marine life.
For example, corals cannot withstand significant changes in temperature,
so they end up dying or bleaching.
In the future, because of climate change,
the ocean's temperature will rise
and those extreme climate events will increase.
They will not only be more intense, as you can see on the right.
They will also be more frequent.
And all forecasts agree on that.
So extreme events such as heat waves
are increasingly frequent and intense.
Another example of extreme events is coastal floods,
which are the consequence of two factors.
The first one is the rise in sea levels,
and the second one is the intensification of weather events.
They are more intense and more frequent.
And with the rise in sea levels,
floods will therefore become more intense and more frequent.
Here, you can see the flood forecasts
for the middle of the century, so around 2050,
and for the end of the century.
At the middle of the century,
floods will already be significantly more frequent.
In comparison with today,
in the regions in red, floods will be 100 times more frequent,
and 1 000 times in the regions in dark red.
These results have a major impact on the way we manage coastal regions,
in terms of infrastructures and human presence.
Today, we are already faced with erosion and flood disasters,
and we must be aware that they will only get worse
to make the right decisions and adapt to these changes.