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What is the greenhouse effect ?
We have been talking a lot about greenhouse effect in recent years,
but it was already being used by the Romans, 2000 years ago,
in order to enjoy certain vegetables all along the year.
How is a greenhouse made?
The principle is simple:
the Sun light is used to heat
but a protective cover is made to keep the heat.
The heat escapes in the form of infrared radiation,
So, the protection has to let the visible light
that heats the greenhouse pass
but it also has to reflect the infrared light
that would cool it down.
Glass and some transparent plastics
has precisely this property.
So, in a greenhouse,
the Sun light goes through the glass walls
to heat the greenhouse,
but the thermal infrared radiation transporting the heat
is reflected by the glass
and is trapped in the greenhouse.
In fact,
there is only a part of the thermal radiation
that is reflected by the glass,
and the temperature within the greenhouse
won't rise endlessly.
Regardless,
it will be much more warmer in the greenhouse
than outside.
The greenhouse effect can be observed in other situations.
For instance, in summer,
a car might be overheated
because of the greenhouse effect.
The greenhouse effect is used as well
to heat bioclimatic houses
and intervene in the functioning of thermal solar panels.
The Earth also benefits from the greenhouse effect.
Without it,
the Earth surface would be 30° colder and so,
much less hospitable.
But let's go back to the example of the greenhouse in a garden,
what plays the glass role around the Earth?
In the air,
there are gases that, as the glass,
let the visible light pass
and reflect the infrared light.
The most important of these gases
is water vapour.
Besides, we know it well:
in winter,
a night without clouds
is much more colder
than a night with a covered sky.
But even when the sky is clear,
there is water vapour.
It represents actually 0.3% of our atmosphere
and still fulfils its part of greenhouse effect
at the scale of the planet.
Water vapour contributes to the greenhouse effect on Earth
only up to three quarters.
There is a second gas that plays an important role:
the carbon dioxide (CO2).
There is ten times less CO2 in water
than in the Earth atmosphere,
but as CO2 is far more efficient than water
to keep heat from escaping,
it can contributes for 25% to the greenhouse effect.
Besides,
the quantity of CO2 increases in the atmosphere.
Its concentration has increased of more than a third
during the last 250 years,
which implies an increase of the greenhouse effect,
and so, of the Earth surface temperature:
that's what we call global warming.
Production: Unisciel/ University of Lille 1
Conception/Production: Maxime Beaugeois, Damien Deltombe and Daniel Hennequin
Editing/Special effects: Damien Deltombe
Music: Sébastien Ride, « Thunder Chacha » (SR Music)
Presentation: Maxime and Nina Beaugeois