English subtitles for clip: File:Die wichtigsten Klimafaktoren (ZDF, Terra X) 720p 50FPS.webm

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[What determines the climate?]

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The formation of climate
is very complex and variable.

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What are the most important factors?

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[Climate factor 1: Sun]

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The causes of natural climate changes
are often cosmic in nature.

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As the earth moves
in an elliptical orbit around the sun,

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cyclical variations of the earth's climate
result according to the distance.

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Warm and cold periods

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have changed periodically
in the last few million years.

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The solar activity can also fluctuate
over longer periods of time.

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Furthermore, there is
the inclination of the earth's axis,

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causing that the illumination of the sun

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on the tumbling planet variates
over cycles of 41,000 years.

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"If we take a look at the sun,

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the solar radiation on earth
has become weaker in recent years.

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In other words, if the sun had really
determined today's climate change,

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we would assume
that it should have been colder,

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but that's not what we're observing.

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It is warming up right now,
which means that the sun

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cannot be the main driving force
behind today's climate change."

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[Climate factor 2: Carbon dioxide]

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A part of the sunlight - yellow -
is reflected directly,

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for example by ice surfaces.

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Another part is converted into
invisible thermal radiation - red here.

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This radiation cannot leave
the atmosphere unhindered.

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Greenhouse gases such as CO2
absorb them and partly throw them back.

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As a result, the earth gets warmer.

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Using an experiment,
atmospheric physicist Stephan Borrmann

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makes CO2's greenhouse effect visible.

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Also present: the student Svenja Kannt.

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The sun, in the model a lamp,
heats up the earth.

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Similar to the land and the oceans,
the black band absorbs the radiation

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and converts it into heat.

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Heat check: The thermometer
displays 80 degrees.

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Heat that is radiated again

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and recorded by a special infrared sensor.

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The object in between
is, so to speak, earth's atmosphere.

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"Now we can interrupt this radiation

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by simply holding a piece of paper
in between, for example."

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The heat radiation blocked by the paper
no longer reaches the measuring device.

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The pointer goes to zero.

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"Now we just fill this atmosphere
with a greenhouse gas, carbon dioxide,

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through a tube from the gas cylinder."

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For all sceptics: This is about
the principle of the greenhouse effect,

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not the simulation of real conditions,
which is impossible in the test tube.

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The experiment shows:

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The CO2 in the "atmosphere" 
absorbs the radiation.

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With the supercomputer "Mistral"
of the German Climate Computing Centre,

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the impact of greenhouse gases

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on the development of the global
mean temperature can be calculated.

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Its program is based on climate models
that are based on clear physical laws.

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But how do the researchers
know that the results,

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looking far into the future,
are actually true?

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The blue curve shows the change
in temperature based on measured data.

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If the industrial CO2 emissions
are omitted in the climate model,

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the values do not match -
as the yellow curve shows -

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to the real temperatures.

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It would be colder on earth.

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"If we take a climate model
and "tell" the climate model,

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so to speak, how much carbon dioxide

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we humans have emitted
in the last few decades, centuries,

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then this climate model is able,

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to simulate a temperature trend
over the last 150 years

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matching the observational data very well.

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But if we do not consider
the human factor,

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this man-made carbon dioxide,

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if we give a constant carbon dioxide
value into the climate models,

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then every climate model of the world
gives out a temperature development,

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no longer matching the observational data,
no longer matching reality.

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This is one of the arguments for us,

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one of the signs

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that we humans are really responsible
for most of today's global warming."

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[Climate factor 3: Ice]

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Nowhere else in the world

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global warming progresses
as rapidly as in the Arctic.

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A cause is the so-called albedo feedback.

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The white ice surfaces reflect
sunlight nearly completely.

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Unlike dark seawater and land areas.

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They absorb the radiation
and convert it into heat.

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A self-reinforcing process

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explaining the extreme
warming of the Arctic

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and melting the Greenland
ice sheet in the long run.

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Researchers are still unable
to completely predict

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every phenomenon in the climate system.

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Mostly the clouds give puzzles.

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In particular, the influence of water
is difficult to calculate.

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If evaporated, the water molecules act
similarly as a greenhouse gas like CO2.

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As clouds, however,
they also reflect light -

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like a white T-shirt in summer.

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In this way they cool the earth.

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Climate factor 4: Clouds

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Global warming is heating up
cloud formation -

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like a steam engine running ever faster.

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Overflowing clouds can rain away from
their place of origin in torrential rain.

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"From a research point of view,

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clouds are the biggest screw
in the climate system

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and one of the least known screws,

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because they are so highly variable,
because they are so locally different,

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occur at different heights,

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because they carry different
numbers of molecules and above all,

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because they can consist of
liquid water as well as solid ice.

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Because all these factors combined,

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it is so difficult to find
a mathematical description,

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which allows clouds to be built
into a computer model in general."

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Everything is connected in earth's gear.

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More clouds in the atmosphere

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influence rainfall and vegetation.

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As the ice melts at the poles,
sea levels rise.

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Also the temperature of the oceans,
the extent of forests,

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Volcanic eruptions
and the defrosting permafrost

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are determining factors
for the climate on earth.

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Even small changes
in this sensitive system

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can have unforeseeable consequences.