The Sun and global warming

The Earth’s ozone layer absorbs ultraviolet light from the Sun and prevents it from reaching the surface. This protects living things from its harmful effects.

Greenhouse gases such as methane, water vapour and carbon dioxide absorb infrared radiation emitted from the Earth. This increases the temperature of the planet. The concentration of greenhouse gases is increasing, leading to global warming.

Carbon dioxide is removed from the atmosphere by photosynthesis, but returned by respiration and the combustion of fuels. Processes like these form the carbon cycle.

The ozone layer

The Earth is surrounded by a deep layer of gas called the atmosphere. The atmosphere is a mixture of gases, including nitrogen, oxygen, argon and carbon dioxide.

Oxygen

Plants use the energy in sunlight to make their own food by photosynthesis:

carbon dioxide + water → glucose + oxygen

Photosynthesis releases oxygen into the atmosphere, and removes carbon dioxide. This reverses the effect of respiration.

Ozone

Ultraviolet light from the Sun provides the energy needed to make ozone, O₃, from atmospheric oxygen, O₂. Ozone absorbs ultraviolet radiation. This prevents it from reaching the ground and harming living organisms, especially animals.

Ozone layer

The ozone layer is the part of the atmosphere where most ozone is found - between about 15km and 40km above the Earth’s surface. The concentration of ozone there is still only around eight parts per million.

Absorbing ultraviolet light - higher

Ozone molecule formation

The greenhouse effect

Greenhouse gases such as methane, water vapour and carbon dioxide absorb infrared radiation emitted from the Earth. This is called the greenhouse effect keeps the Earth warmer than it would otherwise be.

Greenhouse effect

1. Sun’s rays enter the Earth’s atmosphere.
2. Heat is emitted back from the Earth’s surface
3. Some heat passes back out into space.
4. But some heat is absorbed by carbon dioxide (greenhouse gas) and becomes trapped within the Earth’s atmosphere. The Earth becomes hotter as a result.

Remember the Moon and Earth are the same distance from the Sun. The Moon has no atmosphere and an average surface temperature of -18ºC, while the Earth has an average surface temperature of 14°C. So you can see that greenhouse gases are not a bad thing in themselves.

Global warming

The concentration of greenhouse gases is increasing. This is increasing the greenhouse effect and is leading to global warming.

The carbon cycle

Carbon dioxide is one of the greenhouse gases. It is present in the atmosphere in small concentrations, about 0.038%. Carbon dioxide is recycled constantly through various processes that form the carbon cycle.

The slideshow should help you to understand how the carbon cycle works.

Global warming

For thousands of years, the concentration of carbon dioxide in the atmosphere remained much the same. But during the past 200 years it has increased steadily.

The rise in atmospheric carbon dioxide has been caused largely by:

• burning increasing amounts of fossil fuels as an energy source
• burning forests to clear land

Burning forests has two unfortunate effects:

• the burning wood releases carbon dioxide
• there are fewer trees left to photosynthesise and remove carbon dioxide from the air

As the concentration of carbon dioxide in the atmosphere has increased, so have mean global temperatures. This is called global warming.

Effects of global warming

Global warming could cause:

• climate change
• extreme weather conditions in some areas
• rising sea levels

Climate change may make it impossible to grow certain food crops in some regions. Melting polar ice and the thermal expansion of seawater could cause rising sea levels and the flooding of low-lying land.