Physics

1.10 - The Greenhouse Effect

4 min

note 1 of 4

The greenhouse effect keeps the Earth warm

Some gases in the atmosphere are greenhouse gases - the main ones are carbon dioxide (CO₂), methane, and water vapour.
These gases trap radiation in the atmosphere, keeping the Earth warm enough to support life.
The warming they cause is called the greenhouse effect.

How the greenhouse effect works

Radiation from the Sun reaches the Earth and warms its surface.
The Earth emits radiation of its own back towards space.
Greenhouse gases absorb some of this outgoing radiation and re-emit part of it back down towards the surface, rather than letting it all escape.

A sun shines on a globe labelled earth inside a blue dome labelled greenhouse gases; orange arrow 1 reaches the surface, arrow 2 leaves upwards, and arrow 3 turns back down.

note 2 of 4

How greenhouse gases trap heat

The Sun and the Earth emit radiation at different frequencies, and greenhouse gases absorb one but not the other.

The Earth emits lower-frequency radiation than the Sun

The frequency of radiation an object emits depends on how hot it is.
The Earth is much cooler than the Sun, so the radiation it emits is mostly at a lower frequency than the radiation arriving from the Sun.

Greenhouse gases absorb the Earth's radiation, not the Sun's

Greenhouse gases such as CO₂ and methane absorb lower-frequency radiation:

They absorb very little of the incoming radiation from the Sun, so it mostly passes straight through to warm the surface.
They absorb a large amount of the lower-frequency radiation the Earth emits on its way back out.

The absorbed radiation is re-emitted in all directions

Once greenhouse gases absorb radiation, they re-emit it in all directions - including back down towards the Earth's surface.
Energy that would otherwise have escaped into space is sent back, so the surface stays warmer.

note 3 of 4

Human activities increase greenhouse gases

Deforestation - Clearing forests leaves fewer trees to remove CO₂ from the atmosphere.
Burning fossil fuels - Releases locked-away carbon as CO₂.
Agriculture - Farm animals release methane during digestion.
Creating waste - Waste decaying in landfill sites and on farms releases both CO₂ and methane.

note 4 of 4

Rising carbon dioxide and climate change

Over the last 200 years, the proportion of CO₂ in the atmosphere has risen, mainly from burning fossil fuels and deforestation.
Most scientists agree this extra CO₂ is causing average global temperatures to rise (global warming).
This warming leads to climate change - long-term shifts in weather patterns around the world.

Why climate change is hard to predict

The Earth's climate is extremely complex with many variables, so it's difficult to build a model that isn't oversimplified.
As more data is gathered, computer models are refined to make better predictions.

key terms

TermDefinition

Greenhouse gasA gas in the atmosphere, such as carbon dioxide or methane, that absorbs radiation emitted by the Earth.

Greenhouse effectThe warming of the Earth caused by greenhouse gases absorbing and re-emitting radiation back towards the surface.

Global warmingThe ongoing rise in average global temperatures, largely driven by an increase in greenhouse gases.

Climate changeLong-term shifts in weather patterns around the world.

end of notes

Key points to remember

The greenhouse effect keeps the Earth warm

Some gases in the atmosphere are greenhouse gases - the main ones are carbon dioxide (CO₂), methane, and water vapour.

These gases trap radiation in the atmosphere, keeping the Earth warm enough to support life.

The warming they cause is called the greenhouse effect.

How the greenhouse effect works

Radiation from the Sun reaches the Earth and warms its surface.

The Earth emits radiation of its own back towards space.

Greenhouse gases absorb some of this outgoing radiation and re-emit part of it back down towards the surface, rather than letting it all escape.

How greenhouse gases trap heat

The Sun and the Earth emit radiation at different frequencies, and greenhouse gases absorb one but not the other.

The Earth emits lower-frequency radiation than the Sun

The frequency of radiation an object emits depends on how hot it is.

The Earth is much cooler than the Sun, so the radiation it emits is mostly at a lower frequency than the radiation arriving from the Sun.

Greenhouse gases absorb the Earth's radiation, not the Sun's

Greenhouse gases such as CO₂ and methane absorb lower-frequency radiation:

They absorb very little of the incoming radiation from the Sun, so it mostly passes straight through to warm the surface.

They absorb a large amount of the lower-frequency radiation the Earth emits on its way back out.

The absorbed radiation is re-emitted in all directions

Once greenhouse gases absorb radiation, they re-emit it in all directions - including back down towards the Earth's surface.

Energy that would otherwise have escaped into space is sent back, so the surface stays warmer.

Human activities increase greenhouse gases

Deforestation - Clearing forests leaves fewer trees to remove CO₂ from the atmosphere.

Burning fossil fuels - Releases locked-away carbon as CO₂.

Agriculture - Farm animals release methane during digestion.

Creating waste - Waste decaying in landfill sites and on farms releases both CO₂ and methane.

Rising carbon dioxide and climate change

Over the last 200 years, the proportion of CO₂ in the atmosphere has risen, mainly from burning fossil fuels and deforestation.

Most scientists agree this extra CO₂ is causing average global temperatures to rise (global warming).

This warming leads to climate change - long-term shifts in weather patterns around the world.

Why climate change is hard to predict

The Earth's climate is extremely complex with many variables, so it's difficult to build a model that isn't oversimplified.

As more data is gathered, computer models are refined to make better predictions.