WEBVTT

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We are at Mongstad - The
largest oil refinery in Norway.

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Mongstad symbols an important
driver of Norwegian economy

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- the oil.

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But it is also the largest
emitter of CO2 in Norway,

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emitting the equivalent of
1/2 a million cars every year.

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But what are the main human
influences on climate?

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What are the mechanisms
that make them important?

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And can we make some
simple physical reasoning

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to try to estimate the
effect they have on climate?

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That is some of the
questions I am going to try

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to answer in this lecture.

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Proxy records of
temperature indicate

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that that climate has
been rather stable

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the last 2000 years,
- until 150 years ago

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when a rapid temperature
increase started.

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This rapid rise coincide with
a period when world population

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have risen from just above
1 billion to 7 billions --

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and every day the population
rises with another 200 000.

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In the same time
economic wealth have

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increased by a factor of 10.

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As world population has grown
and we have become richer,

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more energy is needed to
sustain the world economy.

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We are today using more than
30 times the amount of energy

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we used 150 years ago.

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Currently almost
80% of that energy

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comes from the burning
of fossil fuels.

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We are now emitting over
1000 ton of CO2 every second.

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What is it that makes
it so convenient

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for us to use fossil fuels?

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It is cheap, it's plentiful,
and it's safe to use.

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But maybe more important:
it is energy dense.

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My energy source today
is this sandwich.

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If this was made of oil instead
of wheat it would be 5 times

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as energy rich.

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And this large energy
density is what

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made oil the preferred
energy source already

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a hundred years ago.

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It's effect on climate
was discovered much later.

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In 1957 the young scientist
Charles David Keeling,

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began observing CO2
at the South Pole

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and the summit of
Mauna Loa in Hawaii.

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And by 1961 it was established
that atmospheric CO2

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was steadily rising.

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We now know from air bubbles in
the Antarctic and Greenland ice

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sheets that the increase
Keeling documented

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started at the same time as
humans started its intense use

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of fossil fuels.

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This and several other
scientific findings the last 60

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years makes us sure that it
is the use of fossil fuels

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that are the main reason
for the large changes we

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have seen in the atmospheric
content of CO2 the last 150

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years.

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As more greenhouse
gasses are emitted

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into the atmosphere more
of the radiation that

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is trying to escape
to space will

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be absorbed and re-emitted.

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Some will be emitted to space
and some to the surface.

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This additional energy will
make the surface warmer.

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And it is this
additional energy that we

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call the radiative
forcing which is formally

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defined as the difference in
radianting energy received

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by the Earth and
energy radiated back

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to space before the surface
temperature has responded

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to the change in radiation.

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If the difference is zero the
earth is in radiative balance

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and we will have
a stable climate.

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If the value is positive
the earth will warm

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and if it is negative
we will have a cooling.

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The emissions of burning
fossil fuels here at Mongstad

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do not only contain
greenhouse gasses.

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It's a source of aerosols,
small particles containing

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soot and sulfate.

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Unlike CO2, which have an
atmospheric lifetime of 50-100

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years, these small aerosols
only stay in the atmosphere

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for a few weeks.

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But despite their
short lifetime they

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play an important role in
human induced climate change.

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Aerosols is the
second biggest effect

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that humans have on climate.

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Different aerosols
scatter or absorb

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sunlight to a varying
degree, depending

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on their physical properties.

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These scattering and
absorbing properties

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are denoted as the
"direct effect"

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of aerosols on the
Earth's radiation balance.

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As an example, pure
sulfate aerosols

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reflect nearly all radiation.

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An increase in these particles
will cool the earth --

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in contrast, soot
absorbs radiation,

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so increasing the amount of
soot in the atmosphere will have

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a warming effect.

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Thus the climatic effects
of aerosols are complex

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- some may cool the earth while
others may induce a warming.

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To further complicate
things they

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can also alter the climate by
changing the optical properties

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of clouds.

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These effects are often
called the aerosol's

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"indirect effects".

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As mentioned previous
a convenient index

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for measuring the importance
of different factors on climate

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is the radiative forcing.

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However, this is a measure that
cannot be directly observed,

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but since we know the changes in
greenhouse gasses and aerosols

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fairly well, we can estimate
the radiative forcing by using

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climate models.

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Using state of the
art climate models

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shows that due to
changes in greenhouse

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gasses the climate
system now receives over

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3 W/m2 more of energy
than it did 150 years ago.

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This is partly counteracted
by the cooling effect

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of the increased amount of
aerosols giving a net change

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in energy of around 2.5 W/m2.

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This increase in
radiative forcing

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is a fundamental finding
in climate science

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- it is this forcing
that will drive

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the changes we've
seen in temperature

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and other climatic variables.

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We now have an estimate of
the main human induced changes

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in radiative forcing -
so let's go on and try

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to connect these changes
to temperature changes.

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Our first attempt will
be a simple model where

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we allow the surface
temperature to respond

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to the additional
energy provided

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by the radiative
forcing, but we allow

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no other climatic
parameters to change.

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If you are interested you can
find the mathematical details

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of the model in the
reading material.

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For simplicity we consider
a doubling of CO2.

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The response of the model is a
temperature change of 1.2 ºC.

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Comparing this number with
estimates from proper climate

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models we find that our
estimation is a factor 2 or 3

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too low.

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So why do we get
this underestimation?

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- what effects is it that
our simple model is not

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taking into account?

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- that is going to be the
topic of next week's lecture.

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