So, let's end this sequence on the climate system with a short focus on extreme events.
Indeed much of what we'll be seeing in the rest of this MOOC might
be related in some ways to the behavior of extremes in a future climate.
Because extremes will be driving droughts,
they'll be driving floods,
they'll be driving elements that can disrupt hydro-power for example,
by increased sedimentation in rivers and so on.
So, let's have a quick look at what the extremes are.
There are many ways of viewing what an extreme event actually is.
I would suggest here's three different ways of viewing the very notion of an extreme.
The first one is in terms of its intensity,
how intense is an event and on this graph just as an example,
well we're seeing the course of temperatures during
the three months of the very extreme heat wave in Europe in particular here in Geneva.
And you'll see that if we take one particular thresholds,
the 30 degrees Celsius threshold,
then you'll see that this was exceeded
quite a number of times in June and particularly in August,
the hottest part of that particular heatwave.
And if we take another threshold,
the 35 degree threshold,
then you see that it's essentially in
the first half of August that this temperature threshold is exceeded.
So, 30 degrees has physiological meaning for humans.
It can be a temperature threshold in all latitudes at least where elderly people
or young children can start to have physiological stress because of the heat.
And beyond 35 degrees,
then we know that in many parts of Europe and
North America certain crops will experience irreversible damage.
So, these do have physiological meaning both for humans and for agriculture.
So, this is one form of viewing an extreme in terms of its intensity.
But then you can also look at extremes in terms of the frequency of occurrence.
On this particular graph,
we see the average summer temperatures in Geneva over the past 120 years or so.
And what you see is that if we take
one particular threshold say maybe four degrees above the mean,
which would be a very hot summer indeed,
then you see that this threshold has been exceeded
only three times in the past 120 years.
In other words, this is a rare event.
So, this is another way of viewing extremes.
The third way of viewing an extreme is to try and determine what are
the impacts an extreme event can generate.
Now, this is maybe a more difficult way of viewing extremes
because the same event might have very different consequences.
For example, a very strong windstorm might do a little bit of damage to buildings but
if they're well-constructed then the buildings should be able to resist the storm,
whereas a neighboring forest for example might actually be destroyed by the same storm.
So, when you're thinking about extremes in terms of the impacts they generate,
you need to think about the type of system that is being impacted,
the resilience of the system,
the vulnerability of that system.
So, this becomes a little bit more complicated.
Also what should be noted is that
some extreme impacts might not necessarily be generated by a particular extreme event.
One example of this would be for example,
sea level rise where
the economic and social impacts of rising ocean level would be immense.
But this is simply a long-term response of the ocean to long-term global warming.
It's not related to a short-lived extreme storm or
extreme heat wave or extreme precipitation event, for example.
So, where do extreme events actually come from?
If we look at what takes place in the mid latitudes,
these are amplifications of
small perturbations that are embedded within the mean atmospheric flows.
And under certain circumstances,
the amplification can be such that it develops into
a full storm that can bring heavy precipitation, strong winds.
It can also be related to the modulation of
certain weather types by events such as
El Nino which we talked about in the second module.
It can also be modulated by what we call
the North Atlantic Oscillation which can modulate climate on both sides of the Atlantic.
We can also have unusual storm tracks that might bring
extremely strong winds or strong precipitation to regions that are not
usually geared towards these types of events
and we've seen this occurring in the past in different parts of the world.
And finally, extreme events can also be generated
or are likely to be generated in coming decades in
a warming climate because warmer climates contains
more energy to drive many of these extreme systems.
And of course fortunately,
if heat in the system is one necessary condition to generate an extreme event,
it's by no means a sufficient condition.
So, in the final three of your graphs here,
I'll just show you three examples of
the possible evolution of extremes in Europe in coming decades.
The first one is extreme heat.
We've experienced in the past several years
or two or three decades some extreme heat waves.
And what we're seeing on this graph is on
average the location of very high temperatures beyond 40 degrees
Celsius in the reference climate of the 20th century which is the 1960s to 1990s period.
So, you see that very hot days
were at that period generally confined to the Mediterranean zone,
Southern Spain, North Africa of course,
Southern Italy, into Southern Turkey for example.
As we move to the middle of the century,
you see that based on climate model simulations,
we have this sort of northward shift of very hot days exceeding 40 degree threshold.
And by the end of the century,
you see that the number of very hot days could actually reach the Baltic Sea regions of
southern Scandinavia and the southern part of
the Baltic region which had never experienced such hot temperatures up to now.
So, basically in terms of extreme heat we can expect a sort of northward movement of
extreme hot events with anything up to
three or four days beyond 40 degrees in the Baltic by 2100,
but maybe 30 or 40 more days per year in places like Andalucía in southern Spain.
The second example of change as an extreme event is related to precipitation patterns.
Again, focusing on Europe,
what you see on the left-hand graphs that has just appeared on
the screen is the average change of summertime precipitation.
And you see that for much of Europe,
from the Atlantic to the Black Sea and mostly up to Southern Scandinavia,
you see that Europe would on average tend to dry out because
summertime precipitation is expected to
decline significantly according to most climate model simulations.
But at the same time,
as you see on the graph that just appeared on the right-hand part of this figure,
you see that the number of extreme precipitation events,
those that exceed 50 millimeters per day.
These are events that can cause local flooding.
That can cause landslides and mudslides in mountain regions, for example.
Well, those type of extremes could in
many parts of Europe actually increase in the future.
So, there's an apparent paradox here in the sense
that you have an average drying out of the continent,
but at the same time might have
actually more high levels of precipitation taking place in coming decades.
And one of the explanations for this is rather simple.
Imagine you have Europe which is warming up during the summer
and increasingly warming as we move into the 21st century.
So, you have a very hot continent and as soon as you
have some moisture coming in whether it's from the Atlantic or from the Mediterranean,
where you have the energy at the surface that is capable of moving these quantities of
moisture coming from the ocean or from the Mediterranean Sea to higher elevations.
So, this will generate much deeper clouds than today and if you have
deep clouds you have more rainfall coming out of these clouds.
So, this is why you can have at the same time a continent that dries out but on occasion,
short lived extremely heavy precipitation events that can increase into the future.
The final example of an extreme event is of course windstorms.
We've experienced several major windstorms over
the past two or three decades which have damaged buildings,
damaged forests, and so on with several billions of euros of damage in many instances.
And you see on this view graphs that already as in today's climate,
it's mostly Northern Europe from the north of
the British Isles into Scandinavia that will experience the largest change,
the largest increase in major windstorms.
Whereas, if you go south into the Mediterranean zone,
then we are likely to see a decline in the number of extreme storm events.
So, in a way this is fairly good news for parts of Europe,
maybe not so good news for the northern parts of Europe.
So, what we've been seeing in these three modules
in terms of climate is how the climate system functions,
why humans are part of the game in terms of the recent evolution of
climate and why they are likely to be a major part of the game in tomorrow's world.
With changes not only in mean temperatures and mean precipitation patterns,
we see that there will be
regional changes that will affect different parts of the globe and we
see that extreme events might actually
increase in intensity and possibly also in frequency.
And these changes that we've been exhibiting
throughout these three modules will obviously have
an incidence on the availability of
water emerging from many mountain regions of the world.
And this we'll be looking at in much more detail with
the following presentations to be made by many of our colleagues.
Thank you very much for your attention so far.
Martin BenistonHonorary Professor
Markus StoffelFull Professor
