The Greenhouse Effect (Leaving Cert CASD): Revision Notes

The Greenhouse Effect

What is the greenhouse effect?

The greenhouse effect is a natural process in which the Sun’s warmth is trapped in the Earth’s lower atmosphere, helping to keep the planet at a suitable temperature for life.

This happens because the atmosphere is more transparent to visible radiation from the Sun than to the infrared radiation emitted from the Earth’s surface. As a result, heat is retained rather than escaping into space. Without this effect, Earth would be much colder — about -18°C on average — making it impossible for most life forms to survive.

Greenhouse gases are special types of gases in our atmosphere that can absorb heat energy. They act like a blanket around Earth, trapping some of the heat that would otherwise escape to space. This natural process is essential for maintaining the warm temperatures we need for life.

How does the greenhouse effect work?

The greenhouse effect involves several key steps that work together to warm our planet:

The process works like this:

• Solar radiation arrives: Energy from the Sun travels to Earth as solar radiation, which powers our entire climate system
• Some energy is reflected: Part of this solar radiation bounces back to space from Earth's surface and atmosphere - this is called reflexion
• Energy absorption occurs: About half of the solar radiation is absorbed by Earth's surface, which warms it up
• Infrared radiation is emitted: The warmed Earth's surface gives off heat energy in the form of infrared radiation
• Greenhouse gases trap heat: Some of this infrared radiation is absorbed by greenhouse gas molecules and clouds in the atmosphere, then re-emitted in all directions. This effect warms both the Earth's surface and the lower atmosphere.

Greenhouse gases

Main greenhouse gases

The main greenhouse gases, listed in order of their contribution to Earth's greenhouse effect, are:

• Water vapour ($H_2O$) - the most important greenhouse gas
• Carbon dioxide ($CO_2$) - second most important contributor
• Nitrous oxide ($N_2O$) - third most significant
• Methane ($CH_4$) - fourth in importance
• Ozone ($O_3$) - fifth contributor

Global warming potential

Different greenhouse gases have different abilities to trap heat. Scientists measure this using global warming potential (GWP):

• Methane ($CH_4$) is 23 times more effective at trapping heat than $CO_2$
• Nitrous oxide ($N_2O$) is 296 times more effective than $CO_2$

It's also important to know that not all greenhouse gases stay in the atmosphere permanently. For example, $CO_2$ can dissolve into ocean waters, creating a balance between the air and water at the sea surface.

Anthropogenic greenhouse gases

Anthropogenic means "caused by human activities." Since the Industrial Revolution began in the mid-18th century, human activities have dramatically increased the concentrations of greenhouse gases in our atmosphere. Current $CO_2$ levels are many times higher than they were before industrial times.

Main sources of human-caused greenhouse gases

The primary sources of anthropogenic greenhouse gases include:

• Burning fossil fuels - This is the largest source, including coal, oil, and natural gas burned for energy
• Agriculture, forestry and other land use - Farming practices, deforestation, and land-use changes
• Cement manufacture - The production process releases significant amounts of $CO_2$
• Aerosols - Various industrial processes that release greenhouse gases

Enhanced greenhouse effect

The $CO_2$ released from burning fossil fuels is building up like an insulating blanket around Earth, trapping more of the Sun's heat in our atmosphere. When human actions increase greenhouse gas concentrations beyond natural levels, this creates what scientists call the enhanced greenhouse effect.

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