1.14: Past, Present and Future Climates III – Extreme climate events (Prof. M. Beniston – UNIGE)

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From the course by University of Geneva

Climate Change and Water in Mountains: A Global Concern

19 ratings

University of Geneva

Climate Change and Water in Mountains: A Global Concern

19 ratings

What is climate change ? How are mountain regions affected by the evolution of water resources and their uses ? What kind of risks need to be considered ? Mountains are recognized as particularly sensitive physical environments where intense and rapid changes have in the past, and may increasingly in the future, place pressure on their resource base. In this context, a team of roughly 100 experts worked from 2008 to 2013 for the European ACQWA project (www.acqwa.ch) which was coordinated by the University of Geneva. The primary objectives of the project were to assess the impacts of a changing climate on the quantity and quality of water originating in mountain regions, particularly where snow- and ice melt represent a large, sometimes the largest, streamflow component. A further objective of the project was to determine the potential disruptions to water-dependent economic activities related to the climate impacts on hydrological systems, and to propose a portfolio of possible adaptation strategies. This particular MOOC is inspired by the ACQWA Project and offers a better understanding of climate change, its impacts on the quality and quantity of water in mountain regions and the risks related to changing water resources. From an interdisciplinary perspective, the participation of twenty-five instructors from five different countries (Switzerland, England, South Korea, India and Nepal) and fourteen institutions (UNIGE, RTS, UNIFR, UZH, ETHZ, Meteodat GmbH, WGMS, Imperial College London, Agroscope, République et Canton de Genève, Yonsei University, IHCAP, ICIMOD, SDC, FOEN) highlights the diversity of both theoretical and practical viewpoints related to these issues. By the end of this course, you will be able : - to define the general concept of climate change in mountain regions - to understand the concepts associated with climate change such as adaptation and water governance strategies - to consider the impacts of climate change on water resources in mountain regions - to identify the impacts of climate change on hydropower, agriculture, aquatic ecosystems and health - to enumerate risks that can occur in mountain areas and lead to disruptions in water availability and use. Your acquired knowledge will be evaluated through multiple-choice quizzes at the end of each unit of the course. This MOOC on “Climate Change and Water in Mountain Regions : A Global Concern” was initiated and financed by the University of Geneva, through its Institute for Environmental Sciences. We look forward to you joining us !

From the lesson

Unit 1: Introduction to climate change

In this module, you will learn about the functioning of the climate system, in particular the numerous interacting components such as the atmosphere, the oceans, the cryosphere and the biosphere. Physical mechanisms that can disrupt climate on different time and space scales will be outlined, and the concepts underlying anthropogenic climate change will be introduced. Following a brief overview of how climate simulation models function, you will be able to see what may be the evolution of climate over coming decades in response to increases in greenhouse-gases in the atmosphere, and what are the limits and uncertainties to these model projections.

1.12: Past, Present and Future Climates I – 20th-century observed change (Prof. M. Beniston – UNIGE)4:08

1.13: Past, Present and Future Climates II – 21st-century climate futures (Prof. M. Beniston – UNIGE)6:55

1.14: Past, Present and Future Climates III – Extreme climate events (Prof. M. Beniston – UNIGE)11:20

1.15: Forecast in Switzerland in 50 years – In summer (Mr. P. Jeanneret – RTS)3:02

Meet the Instructors

Martin Beniston
Honorary Professor
Institute for Environmental Sciences
Markus Stoffel
Full Professor
Institute for Environmental Sciences

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.

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