Gases in the atmosphere
The Earth is surrounded by an atmosphere made up of different gases. The radiation from the Sun that can pass through the atmosphere is infrared radiation, light and ultraviolet radiation. Infrared and ultraviolet radiation are close to light in energy and frequency.
The radiation from the Sun provides energy for photosynthesis and warms the Earth’s surface.
The Earth emits infrared radiation, but this radiation is at a low energy (low frequency). These energies are absorbed by some gases in the atmosphere such as carbon dioxide, water vapour and methane. This keeps the Earth warm.
It is called the greenhouse effect. Without the greenhouse effect the Earth would be much colder – probably too cold for some species to survive.
In the last 200 years, the amount of carbon dioxide in the atmosphere has steadily increased. This is partly because we burn so much fossil fuel. We have also cleared many forests so that fewer trees are using the carbon dioxide for photosynthesis. This means that more of the infrared radiation is being absorbed and the Earth is warming up. This global warming causes climate change, which, scientists agree, is already taking place. There is less agreement about how much change is likely and what effect it will have. It could result in:
- extreme weather conditions in some regions
- rising sea levels due to melting ice and expansion of water in oceans, which may flood low-lying land
- some regions no longer able to grow food crops.
High-energy (high-frequency) infrared radiation from the Sun can pass through the glass into a greenhouse. Low-energy (low-frequency) infrared radiation from the plants cannot pass out through the glass. This keeps the greenhouse warm – the greenhouse effect.
Dust in the atmosphere
When a volcano erupts it produces a lot of gases and dust, which spread around the atmosphere. These reflect the radiation from the Sun and cause the Earth to cool.
If factories in cities produce large amounts of smoke and dust, these can reflect the radiation emitted from the city and keep it warmer.