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# Greenhouse effect

## Carbon cycle

Industries release a lot of carbon dioxide from burning of fuels.

The amount of carbon dioxide ($\ce{CO2}$) in air is controlled by the carbon cycle.

The combustion of fossil fuels (such as methane, $\ce{CH4}$) releases carbon dioxide into the air.

Humans and other living organisms also release carbon dioxide into air because of cellular respiration.

In respiration, glucose ($\ce{C6H12O6}$) reacts with oxygen gas to produce carbon dioxide. This reaction releases energy that cells use for work.

$$\ce{C6H12O6 {(s)} + 6O2 {(g)} -> 6CO2 {(g)} + 6H2O {(g)}}$$

Carbon dioxide is removed from air through photosynthesis. In photosynthesis, plants use carbon dioxide to produce glucose.

$$\ce{6CO2 {(g)} + 6H2O {(g)} -> C6H12O6 {(s)} + O2 {(g)}}$$

Photosynthesis helps to stabilise the amount of carbon dioxide in air.

## Greenhouse gases and the greenhouse effect

When the sun's infrared radiation reaches the Earth's surface, some of the radiation is trapped in Earth's atmosphere by greenhouse gases.

Greenhouse gases are gases that absorb and emit infrared radiation from sunlight in the Earth's atmosphere.

The trapping of heat in the Earth's atmosphere due to greenhouse gases is called the greenhouse effect.

This process is called the greenhouse effect because the way the heat is trapped in the atmosphere is analogous to the way a greenhouse traps heat.

Without the greenhouse effect, the Earth would be $33^{\circ}\text{C}$ colder than the current average temperature.

Major contributors to the greenhouse effect are methane and carbon dioxide. These occur both naturally and as a result of human activity.

## Natural and anthropogenic sources of greenhouse gases

Greenhouse gases can come from natural or anthropogenic sources.

Natural processes such as respiration and decomposition emit carbon dioxide. Methane can also be released from decomposition and other natural sources, such as livestock (mainly cattle).

UN studies show that rearing of cattle generates more greenhouse gases than transportation does.

Livestock release methane as part of their digestion. This accounts for a large amount of methane emitted into the atmosphere.

Anthropogenic greenhouse gases are released due to human activity.

Large amounts of carbon dioxide released in recent years are attributed to human activities, namely burning of fossil fuels.

Methane is also released from the processing of fossil fuels and from other human activities such as rice farming and creation of landfill sites.

The enhanced greenhouse effect describes the enhancing of the greenhouse effect due to human activity. This is also linked to the great elevation of global temperatures.

## Global warming

Polar ice caps have been melting due to global warming.

Global warming is the rise in global temperatures. This occurrence is due to increasing amounts of greenhouse gases in the air.

The concentration of greenhouse gases in the atmosphere has increased significantly since the start of the Industrial Revolution because of the burning of fossil fuels to produce energy.

Excess heat is being trapped in the atmosphere by the higher concentrations of greenhouse gases, causing global temperatures to rise.

Global warming is melting the polar ice caps, causing a rise in sea levels around the world. This results in flooding in coastal regions.

Global warming could also significantly change weather patterns on Earth in the future.

Global warming could be slowed by cutting down on greenhouse gas emissions.

Governments and businesses are attempting to reduce the burning of fuels and are investigating alternative sources for energy.

## Ozone

Ozone ($\ce{O3}$) is a gas in the Earth's atmosphere that absorbs ultraviolet (UV) light from the sun.

Space-filling diagram of an ozone molecule ($\ce{O3}$).

In the atmosphere, ozone is at its highest concentration in the ozone layer, which is between 10 and 15 kilometres above the Earth's surface.

The ozone layer is essential to life as it absorbs a significant amount of UV light. UV light is dangerous to life on Earth.

The ozone layer prevents most of this light from reaching the Earth's surface.

Ozone is naturally produced when oxygen molecules ($\ce{O2}$) in the atmosphere are exposed to UV light. Laser printers and copiers found in offices and schools release ozone as well.

Ozone is beneficial high in the atmosphere but it is dangerous near the Earth's surface. Ozone is highly reactive and damages respiratory tissue (the lungs) if inhaled.

## Hole in the ozone layer

A 3D structure of dichlorofluoroethane, a CFC. It consists of carbon (black), hydrogen (white), fluorine (yellow-green) and chlorine (green).

Chlorofluorocarbons (CFCs) have depleted the ozone layer. They are pollutants that were released from aerosols and solvents.

Refrigerants, propellants and degreasing solvents all released significant amounts of CFCs into the atmosphere.

CFCs cause the breakdown of ozone molecules in the atmosphere. They have therefore created holes in the ozone layer, exposing the Earth to more UV light from the sun.

The breakdown of the ozone layer allows more UV light to reach the Earth's surface. This leads to health risks such as skin cancer.

Governments around the world have banned the use of CFCs.

The holes in the ozone layer are therefore beginning to slowly decrease in size, largely due to the banning of CFCs.