Absorption - how much of the
incoming EM radiation from the Sun is absorbed by land and water (seas,
oceans, lakes etc)
Reflection - how much of the
incoming radiation is reflected by the clouds, land or water.
Emission - how much of the absorbed
radiation is re-emitted.
Diagram of some of the possible
absorptions and emissions of the Earth's surface and atmosphere
(the outcomes of the incoming FM
radiation from the Sun, but in no particular order)
1. Reflection of the Sun's EM
radiation from clouds.
2. Absorption of the Sun's EM
radiation by clouds.
3. Re-radiated EM radiation totally
escaping from the Earth's surface.
4. Direct absorption of the Sun's EM
radiation by the Earth's surface - short wavelength as wells visible
light and a little uv radiation.
5. Reflection of incoming sunlight EM
radiation by the atmosphere
6. Re-radiated EM radiation
from the Earth's surface - scattered by the clouds and atmosphere and
eventually re-absorbed by the atmosphere, including greenhouse gases
like carbon dioxide and methane.
7. Direct reflection back into space
of incoming EM radiation from the Sun.
Land coated in 'shiny' ice will
act as a good reflector, so if it melts, more radiation will be
absorbed.
8. Direct absorption by the
atmosphere of the incoming Sun's EM radiation.
During in daytime a huge amount of EM radiation is transferred to the
Earth's surface and atmosphere.
Some radiation is absorbed by the
atmosphere but a lot passes through and absorbed by the Earth's surface.
This warms up the surface and increases the temperature, particularly
areas in bright sunlight.
Overall more EM radiation is absorbed
than is emitted, so the temperature rises in daylight.
The clearer the sky, the less
sunlight energy is reflected back into space, the higher the maximum
temperature reached since more infrared radiation reaches, and is
absorbed by, the Earth's surface.
The reverse is true at night, when
more FM radiation is emitted than is absorbed.
The lack of sunlight causes a decrease in
temperature, and the heat loss increases if the sky is clear, because some of the
re-radiated EM
radiation is absorbed or reflected back off clouds.
It is also absorbed
by greenhouse gases like carbon dioxide and methane, which overall adds
to a reduction in the temperature fall of the earth's surface.
The greatest fall in temperature
occurs when the nighttime sky is clear and the emitted infrared is not
reflected or absorbed by clouds.
Conversely, with cloudy nights, some
of the emitted radiation from the Earth's surface is reflected back off
the clouds or absorbed by them, so the nighttime temperature fall is not
as great..
In terms of the amount of radiation that
the Earth absorbs, emits and reflects, the net result is fairly constant
temperature.
It is neither too hot or too cold
for many forms of life to survive.
Global warming
However, any significant changes in the
Earth's atmosphere may cause the average temperature to change and currently
it is believed (consensus scientific view) that global warming is taking
place due to the increase in carbon dioxide levels due to fossil fuel
burning.
The greenhouse gases, principally,
carbon dioxide (CO2), water vapour (H2O)
and methane (CH4) absorb radiation in the Earth's
atmosphere which allows the Earth to warm up. The relatively small
concentrations of carbon dioxide and methane have a potentially a large
and disproportionate effect on the Earth's average temperature - they
are really good greenhouse gases!
There is a net transfer of heat
energy from the much hotter Sun to the much cooler Earth.
In terms of frequency, the principal
frequency of EM radiation from the Sun is much greater than that of the
Earth.
Although all particles
(atoms/molecules) absorb particular frequencies of radiation from the
Sun, the higher frequency radiation (particularly the infrared, IR) gets
through to the Earth's surface (see diagram above). The higher frequency
IR is not as readily absorbed by most molecules in the atmosphere and
reaches the Earth's surface.
The re-emitted infrared radiation
from the Earth's surface is of lower frequency (longer wavelength) than
the incoming IR radiation. It is the likes of carbon dioxide and methane
(and other human-made molecules) that readily absorb the lower frequency
radiation keeping the Earth warmer than if this radiation escaped.
The more greenhouse gases in the
atmosphere, the greater the absorption of the re-emitted IR radiation
and the warmer the Earth gets and it is believed that human activity is
contributing to this!
This increases the temperature of the Earth
compared to what it would be without the atmosphere - this is one
reason why organic based life exists on Earth - not to cold - Mars
(little atmosphere) and our moon (no atmosphere) are much colder.
Certain gases in the atmosphere are more
effective than others in absorbing the re-radiated energy - water
vapor, carbon dioxide, methane, nitrous oxide and ozone all occur
naturally,
but we have added other greenhouse
gases like chlorofluorocarbons (CFCs) and
hydrofluorocarbons (includes HCFCs and HFCs), albeit in very tiny
concentrations as well as significant extra carbon dioxide in
the atmosphere from fossil fuel burning - both quantities are
still n the increase!
The steady rise in carbon dioxide
concentration means more re-radiated infrared radiation is being
absorbed by the Earth's atmosphere.
The result is that the Earth is warming
up a bit more than might have been expected and the average
temperature is rising.
In particular, it is the rising level
of carbon dioxide from fossil fuel burning that is the most worrying and
major contributor to global warming above what we might expect without
burning fossil fuels.
Graphs 2 and 3 show the steady rise in
carbon dioxide concentration as result of the increasing use of fossil
fuels.
You can have cooling effects!
Huge volcanic eruptions transfer
enormous quantities of fie particles into the atmosphere.
These particles scatter sunlight and
decrease the amount of the Sun's infrared radiation that reaches the
Earth's surface.
This causes a cooling effect, and the
Earth's temperature can be significantly lowered.
This is sometimes called a 'volcanic
winter' effect e.g.
The 1815 eruption of Mount Tambora, a
massive volcano in Indonesia caused what came to be known as the "Year
Without a Summer" of 1816. Europe, still recovering from the Napoleonic
Wars, suffered from food shortages. There were large scale crop failures
from the ensuing reduction in both global temperatures and intensity of
sunlight.
I'm not writing any more on this here,
because I've already written a lot in my GCSE chemistry notes on the
evidence and possible consequences of rising carbon dioxide levels and
global warming on ...
Global warming, climate change,
reducing our carbon footprint from fossil fuel burning
I've included everything mentioned in
any GCSE chemistry or physics syllabus relating to the 'Greenhouse
Effect'.
See also Biodiversity, land management,
waste management, maintaining ecosystems - conservation gcse
biology
