Documentation > Greenhouse effect > Physical aspects > What is the greenhouse effect ?
What
is the greenhouse effect ?
september
2003
website of the author
: www.manicore.com - contact the
author: jean-marc@manicore.com
What is a greenhouse? Everybody
knows that it is a small construction made of glass walls, which
leaves solar energy easily go inside - what increases the inner
temperature - but prevents the heat formed within the glass walls
to dissipate too quickly outwards. Two effects contribute to retain
the heat trapped inside:
a purely mechanical effect: because of the walls, that create
a solid barrier, the hot air just can't go anywhere else !
a "greenhouse effect" (additionnal to the previous
one): receiving energy (in the form of light) from the outside,
the inside of the greenhouse will heat and then emit infrared
radiations. It happens that glass is a material pretty opaque
to these infrared radiation emitted by the interior of the greenhouse.
That prevents the energy to dissipate easily outside and leads
to a temperature increase that would not happen if infrared could
freely escape from the greenhouse.
There are, in our atmosphere,
some gases (the "greenhouse gases"), present in (very)
small quantities, that act exactly the same way for our planet
than windows do for a greenhouse. They do not prevent sunrays
from reaching the ground, but, being opaque to the infrared radiation
emitted by earth's surface, they prevent the soil to cool too
quickly and lead to an additionnal temperature increase that would
not exist if they were not there. But the analogy with the greenhouse
does not apply to the "mechanical effect", as the greenhouse
gases do not confine "mechanically" the hot air near
the ground.
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What is a greenhouse
gas ?
Fundamentally, a greenhouse
gas is a gas which is partially opaque to the infrared radiation
emitted by earth's surface. Most of them are also transparent
to incoming solar radiation, but not all of them: ozone, in particular, is opaque to the
incoming ultraviolets. More details are available on the second
part of the page
that describe greenhouse gases in detail.
The greenhouse effect is
therefore nothing more than a difference in transparency of the
atmosphere depending on the radiation that tries to pass through.
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The two main greenhouse
gases (but there are many others) are perfectly natural and have
been present in our atmosphere for a very long time:
water vapour, which reprents roughly 0,3% of the atmosphère,
is as old in our atmosphere as water on earth (4 billion years),
carbon dioxide, or CO2, represents today 0,037% of the atmosphere,
but this proportion has greatly varied in the past.
"Trapping"
energy close to the ground (or actually preventing it to leave
easily the ground) leads to a temperature increase of the surface.
It is generally considered that the heating deriving from the
greenhouse effect leads to an increase of the average surface
temperature of 30 °C (in rough figures), which is enormous.
In other words, should our atmosphere be totally transparent to
the infrared radiation emitted by the ground, the average temperature
on earth would be around -18°C
rather than +15 °C, rending our planet all the most inhospitable
for the bipeds we are. In the first place, the greenhouse effect
is therefore a beneficial effect.
The problem that is sometimes
referred to by the term "greenhouse effect" is therefore
not the effect in itself, essential to life as we know it, but
the fact that we are quickly modifying it, what might generate
serious problems. So the proper
term for the process we have put in motion should rather be "climate
change", or "global warming", though, as we will
see later, the climate change to come can't
be reduced to just a temperature change, and by far.
***
A couple
of more technical explanations
Our star, the sun, sends us every
day a considerable amount of energy: the whole humanity's energy consumption
for one year represents only 3% of what the Sun delivers to the
surface of our planet every day.
This energy coming from the Sun
is delivered under the form of electromagnetic radiation.
Electromagnetic radiations
light,
infrared,
ultraviolets,
X-rays
radio waves,
micro-waves that are used in the ovens named after them,
and, for those that are interested in radioactivity, gamma rays
are radiations that are
all of the same nature: they constitute the vast family
of electromagnetic radiations. They "only" differ by
their wavelength and their energy.
Any object that has a temperature
above the absolute zero (that is minus 273,15 degrees Celsius)
emits some electromagnetic radiations in order to dissipate part
of its thermal energy:
If it is not very hot, it will only emit radio waves ; this is
the case, for example, of some "cold" celestial objects
(in outer space, it is not precisely hot: - 270 degrees Celsius
!),
getting hotter, it will emit infrared. For example, our body
is hot enough to emit infrared, even at night: it is this property
that enables to see us in the dark with "infrared cameras",
devices that can detect this radiation that our eye can't see,
if it is even hotter (starting from 700° C, for example a
piece of metal "red hot") it will also emit visible
light ; for example in our light bulbs all what we do is heating,
through the flow of an electric current, a filament of metal
to the temperature of 2 to 3.000 °C, and then it will radiate
all colors of the visible lignt (the result being white light),
even hotter, it will emit ultraviolets, that's the case of the
sun (and of many stars),
even hotter, it will emit X-rays: that's the case of some celestial
objects.
Our sun, which is reasonnably
hot (surface temperature of 6.000 °C), sends us a mixture
of various rays which is composed for (percentages represent
the proportion of the total energy):
10% of ultraviolets (a large part is intercepted by the famous
"ozone layer", what is fortunate for us because these
ultraviolets, which are "energetic" radiations, are
able to break down, in the cells of living beings, some essential
chemical bonds),
40% of visible light
50% of infrared.
Earth, which is not very
hot (average temperature: 15 ° C), emits only infrareds
(which are not the same than that of the sun).
It happens that a substance
can very well be transparent to one type of radiation but opaque
to another. Our soft tissues, for example, are pretty transparent
for X-rays (which explains why they are used in medical applications)
but totally opaque to visible light (otherwise we would never
say: move away, so that I can see !).
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When the incoming solar radiation
arrives at the top of the atmosphere, 30% is directely reflected
towards space, by clouds (20%), the various layers of the atmosphere
(6%), and the surface of the earth (4%), which includes some places
- for example the ice caps - that are particularly highly reflexive.
The remainder of this incoming
energy is absorbed by the various components of our planet (continents,
oceans, atmosphere), then re-emitted in the form of infrared energy.
In effect, just as our skin heats up when we sunbathe, the surface
of the earth and the atmosphere heat up when they absorb the incoming
solar radiation, and then disipate this energy by emitting infrared
radiation, as explained above.
The greenhouse gases, that had
let light pass through without being disturbed (almost), have
the ability to absorb infrareds. By doing so, they also pick up
the associated energy, and therefore also heat up. Just as earth's
surface, they will dissipate this energy by emitting infrareds,
part of which goes back to the ground, heating it up a second
time after the sun did the first part.
This whole process leads the greenhouse
gases, then the lower atmosphere, then the surface, to be hotter
than if these infrared could freely pass through the atmosphere.
Of course, the system always reachs
an equilibrium, but with a higher surface temperature than if
the greenhouse gases where not present.
Energy exchanges
in the atmosphere. The figures represent the mean value, over
time (for a full year), and over space (for the whole surface
of the planet), of each flux shown, in Watts per square meter.
The "global
warming" process can be roughly summarized the following
way: when the concentration of greenhouse gases increases, the
term B (infrared radiation emitted by the surface that directly
passes through the atmosphere without being intercepted by the
greenhouse gases) decreases. Correlatively the term C increases,
then the term D, what increases the amount of energy received
by the ground, and therefore its temperature.
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***
More
and more technical explanations: back to the electromagnetic
radiations ! (for those who wish it only)
Distribution of
the incoming solar radiation (6000 K) and the outcoming terrestrial
radiation (255 K) and simplified representation of the absorption
of these radiations by the greenhouse gases.
From Robert Sadourny,
le Climat de la Terre, Flammarion
Water Vapour (H2O) |
Carbon dioxide (CO2) |
 Ozone
(O3) |
 Methane (CH4) |
 Nitrous protoxyde (N2O) |
 Oxygen (O2) |
This little diagram comprises
two curves, that represent the energy carried by each wavelength
for the sun (6000 K, because the surface temperature of the sun
is 6000 Kelvin) and for earth (255 K).
Maybe it should be recalled
that kelvins equal Celsius degrees plus plus 273,15 (therefore
the absolute zero is either 0 kelvin or -273,15 ° Celsius).
Going from one unit to another is just a simple shift.
This two curves are normalized,
that is to say that their maximum has been put at the same level
(otherwise the curve for the sun would be a million times bigger
than that for earth !).
The colour below the curves
represents, for each wavelength, the proportion of the radiation
(of that wavelength) that is absorbed by a greenhouse gas (one
colour per gas). There are actually some overlappings (that is
radiations that can indifferently be absorbed by several greenhouse
gases), but they are not represented on the diagram in order
to simplify the presentation (this simplification is not scandalous
for a first look at the process).
When looking at this diagram,
it is easy to note that:
the peak of solar radiance is in visible light, as for the earth
it is in remote infrared (infrared is call "near" when
the wavelength is close to that of light, "remote"
in the other case),
solar infrareds are not the same -and much less intercepted by
the atmosphere - than those emitted by the earth,
almost all the ultra-violet coming from the sun is blocked by
stratospheric ozone (on the left), what allowed evolved life
to come out of the oceans
visible light is not much affected by greenhouse gases,
any radiation emitted by the earth is partially or totally intercepted
by a greenhouse gas (there is one colour or another under any
wavelength for the earth's curve), among which water vapour is
playing the leading role (light blue, on the far right)
It is clearly because the different greenhouse gases intercept
different wavelengthes that their effect is cumulative: if all
the greenhouse gases interfered with the same wavelengthes, we
would have very quickly a saturation phenomena for those precise
wavelengthes, but the remainder of the outcoming radiation would
be able to leave without problem no matter what proportion of
greenhouse gases in the air,
and, eventually, adding more greenhouse gases will have the greatest
impact where the proportion of the radiation already absorbed
by this gas is low: it means that we are far from saturation.
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