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"Turn Down the Heat:
Why a 4-Degrees Warmer World Must be Avoided."
In order to understand that,
we have to look at some major advances,
which provide us with insights about our climate system,
how it behaved in the past,
what we see currently,
and how it might behave, very likely actually, in the future,
due to human interference.
Well, since our empirical understanding
of the change in the climate system,
what we often call global warming,
is really related to this very mountain,
the Mauna Loa in Hawaii, which is a tremendous mountain,
altogether, about 17,000 meters high,
and in the late 1950's,
Charles Keeling started to measure carbon dioxide
in the atmosphere.
And amazingly, he saw two things, actually.
One is that there is a seasonal variation.
In the fall,
a lot of CO2 is released back into the atmosphere,
while in the spring, the CO2 is sucked up by the trees.
But there is a long-term trend.
It's climbing and climbing and climbing.
And you see that a red line was transgressed in 2013
that is 400 parts per million CO2 in the atmosphere.
Let us put it in historical perspective,
and, for that, another major triumph of modern science
is ice core drilling.
An ice core, miles deep really, was drilled in Antarctica,
and to reconstruct from these ice layers
really how the ancient atmosphere looked like.
It's an amazing achievement really.
Now, if we reconstruct CO2 concentration in the atmosphere,
over the last, sort of 800,000 years,
you see this up and down,
but then something very strange happens.
There is a huge spike over the last 100 years.
This will actually have a major impact
on what we call the paradise of the Holocene.
This is now a chart
which tells you how the global mean temperature looked like
over the last 100,000 years,
and you see it during the ice age over major fluctuations.
So the temperature dropped by 10 degrees or something.
And, finally, around 11,000 years ago,
it became very, very stable, almost no fluctuations anymore.
And this actually gave birth
to what we call the Neolithic Revolution
because *** sapiens were settling down,
which was only possible because it was a very stable climate.
In an unstable climate,
you have to move on all day, all week, all month, all year,
so to speak.
The Neolithic Revolution happened,
so agriculture was invented,
and this was the basis, actually,
for the second big revolution in the history of civilization,
namely the Industrial Revolution.
So, you see, in the 19th century,
factories were being created.
Now, it all has to do with the burning of fossil fuels.
In the beginning, coal, later on, oil, natural gas.
And you see still a little bit of biomass,
a little bit of nuclear energy.
Then the renewables are coming along right now,
but still the bulk is based on fossil fuels.
But fossil fuels have two major drawbacks.
One is, they are limited --
they cannot go on forever --
and the other drawback is that the emissions coming
from burning of fossil fuels are changing the atmosphere.
This is a wonderful reconstruction
of global mean surface temperature,
and you see, towards the end of the Holocene, a huge spike,
but it's in temperature, not in CO2.
Now, if you look at just the last hundred years or so,
this just demonstrates that each of the last decades was warmer
than the previous one, and there's quite a difference.
So, clearly, the planet is warming,
and it is due to human interference.
And the IPCC also tries
to allocate or assign probabilities,
but human interference is the dominant cause.
Now we have this famous statement
that human interference is,
with a probability of more than 95%,
the dominant cause of this change
in the behavior of the atmosphere.
It's all based on what we call the greenhouse effect.
We receive our energy mainly from the sun.
Now, if the sunlight gets absorbed
by the surface of the planet,
it is being transformed and re-emitted as infrared light,
and that infrared light cannot pass the atmosphere
as easily as the visible light, actually.
So it's scattered back by the greenhouse gases,
in particular, like CO2.
Now, the amazing thing is that the net input every second,
through enhanced greenhouse effect, is four Hiroshima bombs.
It makes 125 million Hiroshima bombs
of energy imploded every year.
Where does all this excess energy go to?
Well, contrary to common belief,
it's not going into the atmosphere --
just about 2% --
most of it is going into the oceans.
So, the oceans have to heat up, and we have to measure that.
The ocean, between the surface and 2,000 meters deep,
is heating up tremendously.
And, since most of the energy is really contained in the ocean,
that is what matters when it comes to global warming.
Just to give you the scope of the heat contained --
if the extra heat absorbed by the ocean since the 1950's
would be released in one instant,
the atmosphere would warm by 36 degrees Centigrade,
instantly.
So, it's a tremendous time bomb slumbering in the ocean.
There are two scenarios for the future, very important.
One is where we do just everything
in order to contain greenhouse gas emissions.
It will be a major transformation
of our industrial metabolism,
but if we stay the business-as-usual course,
which is unfortunately more likely right now,
global warming will be 3 or 4 degrees
by the end of the century.
But it will go on and it will not stop there
because there's so much heat in the ocean.
For example,
it will reach 6 degrees, 7 degrees, 8 degrees by 2300.
That would be a completely different world.
So, discussing what will happen in a 4 degrees warmer world,
maybe already by the end of this century,
is clearly a question we have to address.
Now, you do not have to wait until 2100
to see already the impacts of global warming.
The planet has warmed so far by 0.8, 0.9 degrees,
and you see the impacts already everywhere.
But this is nothing compared to what will happen
in a 4 degrees warmer world,
and that's what the report is all about.