Les scénarios climatiques

"UVED presents its MOOC:

Climate change, impacts, mitigation and adaptation"

"Climate scenarios"

-To characterize future risks,

plan adaptation,

and to know which mitigation solutions,

those which limit future greenhouse gas emissions,

are acceptable for respecting the Paris Agreement,

we need to carry out what we call future projections.

For this, we use scenarios

which are a series of hypotheses, like, for example,

hypotheses on global population change.

These also include technological, socio-economic

and geopolitical hypotheses, for example.

In some, countries help each other,

and others feature increasing rivalry and nationalism.

Using these hypotheses,

economists can predict a change

in greenhouse gas emissions for each large region.

These greenhouse gas emissions and their future change

can be used in variable-complexity climate models

to project the potential change of physical parameters,

like the temperature, among others.

It can also allow these changes to be spatialized,

to know how these changes will happen, in time and space in the future.

From these physical changes, other models can be used

to characterize the changes in future impacts,

which we will see in a moment.

If we look at the different scenarios which have been used

in the IPCC's sixth assessment report,

we can see five major scenario types,

which are those widely used in climate models.

Here, we can see CO₂ emissions for each of these five scenarios.

Some of the scenarios project very strong sustainability,

with strong cooperation between countries

to deploy policies limiting greenhouse gas emissions.

This is shown in blue.

These scenarios feature zero or even negative emissions,

from 2050 or 2070, depending on the scenarios.

Then, there are intermediate scenarios.

The yellow scenario is,

in real terms, our current trajectory,

with a stagnation or even slight increase in emissions,

at least until 2050.

Then, there are upper scenarios, which may seem less plausible,

as we have already taken some political measures.

However, they are not completely physically impossible.

These include, for example, our development

being highly based on fossil fuels,

which is the highest scenario we can see shown here.

Even if the scenarios do not all have the same plausibility,

that is to say the same chance of happening,

including a wide range of possible futures

within our climate models

lets us see, for example, in the case of an upper scenario,

that we have no risk of amplified, runaway effects,

due to natural feedback from the system,

which is the reason we cover a wide range of possibilities.

Here, again, you have the CO₂ levels that I showed you.

It shows other greenhouse gas emissions, like methane and N₂O,

or atmospheric pollutants like SO₂,

which lead to the production of aerosols

that cause the atmosphere to cool.

All of these gases and compounds

are included in climate models,

but we can see they may not evolve in the same way,

in time and in space,

as they do not have the same emission sources

and do not evolve in the same way over time.

We use these emissions in climate models to project, for example,

the change in average global temperature,

which is represented here.

We see, when we analyze the results of these different scenarios,

an increase in global temperature over the next 20 years,

and in all the scenarios,

the 1.5 degree increase is almost certainly exceeded,

at the start of the 2030s.

This does not mean at all that reducing emissions has no effect,

but that as long as we have not reached zero CO₂ emissions,

and significantly lowered emissions of the other compounds,

this means we still emit, even if we emit less,

and are still contributing to global warming.

We can see this with the heating that will continue at least 20 years.

It is not linked to physical inertia,

but inertia in our economic systems,

to stop these carbon emissions.

In the second part of the century, emissions may be highly varied,

depending on different scenarios, where heating may stabilize,

or where it may continue to rise,

which may be around three degrees in the intermediate scenario.

It may even pass four degrees in the worst scenarios.

Outside of these temperature changes,

we can also measure the changes in different physical parameters.

We can see the change, for example,

in sea levels compared to 1900.

We can see that for these changes,

which are slow, due to their link with a slow oceanic system,

and which will slowly spread heat over the long term

across the climate system,

that these changes will continue throughout the century,

even if the temperature stabilizes.

In practical terms, regarding sea level rises,

we know that this happens over millennia.

However, we can see, by looking at the different scenarios,

that whether the scenario shows lowered or increased emissions,

this changes the speed in sea level rises,

which significantly changes the adaptation possibilities

for people living in littoral zones.

You can see a dotted line has appeared.

It represents an unlikely possibility, but with a serious impact.

In our climate models,

we may represent some things less well,

due to a lack of observation, or the processes being complex,

and non-linear.

So, we know that, potentially,

there could be poorly represented non-linearities,

like the instability of the ice caps,

which could melt more quickly than our models predict.

In this case, we would have a sea level rise

which would be much faster.

It is also key for the way the scenarios are used

to show dispersion, and things which are less likely,

but not impossible,

and must be considered in the engineering

for our solutions for coastal areas.

Climate models also allow us

to have spatial changes

where we see how they are shared out on the map

for different physical parameters.

This lets us, for example,

characterize what the changes will be, for different heating levels,

which is important for some parameters,

independently of when they happen.

What we have shown here

are the changes in average global temperature,

for different heating levels.

These are 1.5, 2, and 4 degrees.

We can already see

there is a significant gap between the oceans and continents,

with heating always being higher on the continents.

France, for example, has heated 1.8 degrees today,

while the average global increase has been 1.1 degrees.

We can see the reinforced heating

due to the feedback from ice melting in the Arctic zone.

When we look at the extreme changes,

we can see an important rise taking place.

If we look at the change in the hottest day of the year,

even if there is only a 0.5-degree change

between the 1.5 and 2 degree heating scenarios,

this does not mean the hottest day of the year

will only go up by 0.5 degrees,

but it may increase by up to one degree.

So, this rise of extremes is important,

for each level of heating we pass through.

Here you can see a representation with something I already mentioned,

but which has been made for a demographic

which is less receptive to line graph representations.

We call these climate stripes,

in which each vertical stripe represents the average temperature

with a color which becomes a deeper red for higher temperatures.

So, we can see, from the 1900 to 2020 period,

the increase in temperature observed,

and the five scenarios I already showed you,

being shown on the right.

This graphic shows the rise of heating over the next 20 years,

and very different futures which are open to us.

This also lets us see what this means on a lifetime scale,

the life of someone born in 2020,

and how the change in the climate over their life will differ for them

compared to the experiences of their parents or grandparents.

These climate projections also allow us to determine

the risks of future impacts.

We can see the example here of the risk to health,

which increases in the future for different heating levels,

and which is shown here

by the number of days per year where temperature and humidity

expose people to mortal danger.

This means conditions where working outside is very difficult,

like farming, building,

or simply travelling outside.

We can see, from the first range of heating levels,

which is from 1.7 to 2.3 degrees,

that there is a significant rise in the number of days

where people will be highly exposed, in terms of mortality,

to this excessive heat.

We can see, beyond 2.5 degrees,

conditions at the Equator become very difficult,

with over 250 days per year.

However, in North America, we see a large area

which has half the year

with conditions making outdoor working

and living difficult.

We can clearly see the extent to which climate change worsening

can impact health,

and how much these projections can be used to anticipate,

and in some cases design adaptation solutions,

for the parameters we can adapt to.

Of course, past a certain temperature level,

we know many adaptation solutions will reach their limits,

and we will not be able to fully protect people and ecosystems

from a rise significantly above 2 degrees.