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Absorption/emission of EM radiation: 4. Global warming and climate change - the greenhouse effect of planet Earth from the point of view of absorbed and emitted infrared radiation

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EMR shorthand for electromagnetic radiation

INDEX of physics notes: Absorption & emission of EM radiation by materials - temperature and surface factors

4. Global warming & climate change - the greenhouse effect of planet Earth from the point of view of comparing absorbed and emitted infrared radiation

The ideas about surfaces from above in section (c), and those introduced about wavelength/frequency in section (a) can now be applied to considering the Earth's temperature.

The overall temperature of the Earth depends on three factors relating to EM radiation

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.

Graph 1

Graph 1 shows the recent global warming compared to most of the last 1500 years.



Graph 2


Graph 2: The rise in use of fossil fuel use in terms of CO2 emission 1850-2003

Graph 3

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


INDEX physics notes: Absorption and emission of EM radiation by materials

Keywords, phrases and learning objectives for absorption and emission of radiation

Know and explain that global warming, causing climate change, is due to the greenhouse effect of planet Earth, which is increasing with rising carbon dioxide levels, which leads to more infrared radiation being absorbed by the Earth's atmosphere.


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INDEX physics notes: Absorption and emission of EM radiation by materials