The Greenhouse Effect (College Board AP® Environmental Science): Study Guide

What are greenhouse gases?

• Greenhouse gases (GHGs) trap heat in the atmosphere to regulate Earth's climate through a natural process called the greenhouse effect

• The principal greenhouse gases are

  • carbon dioxide (CO₂)

  • methane (CH₄)

  • water vapor (H₂O)

  • nitrous oxide (N₂O)

  • chlorofluorocarbons (CFCs)

• Carbon dioxide (CO₂) is a colorless, odorless gas that's a natural part of the atmosphere

• It accounts for 20% of the greenhouse effect

  • Levels have risen from 315 parts per million (ppm) in 1950 to over 421 ppm in 2022

  • 2050 levels are expected to be 550 ppm 

  • The increase is due to human activities—burning fossil fuels and deforestation

    • Deforestation also decreases carbon storage, as trees remove CO₂ from the atmosphere during photosynthesis

• Methane is a colorless, odorless, flammable gas that's a main part of natural gas

• It accounts for 17% of the natural greenhouse effect 

  • Methane is 28 times more potent than CO₂ at warming the Earth

  • Produced naturally and through human activities such as:

    • The decay of organic matter in wetlands, forests, and oceans

    • Agriculture (particularly rice growing and livestock farming), fossil fuel extraction, and waste landfills

• Nitrous oxide (N₂O) is also known as laughing gas

  • It is a long-lived, potent GHG with a warming effect nearly 300 times that of carbon dioxide per molecule over a 100-year period

  • It remains in the atmosphere for around 121 years

  • Naturally present in the atmosphere as part of Earth's nitrogen cycle

    • This gas is emitted from agricultural activities, such as using nitrogen fertilizers and animal waste on farmland and pastures

    • Fossil fuel combustion

    • Industrial processes

  • From 1980 to 2020, nitrous oxide emissions grew 40%

  • In 2022, the concentration of atmospheric N₂O reached 336 parts per billion, which is much higher than previous predictions

• Chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) account for 1.5% of GHGs in the atmosphere

  • These are man-made chemicals that are

    • 10,000 times stronger than CO₂ at trapping heat

    • Increasing at a rate of 6% per year

    • Banned by many countries as they deplete the ozone layer (1 CFC atom can destroy thousands of ozone molecules)

  • HFCs are weak ozone-depleting substances but they are strong greenhouse gases

Is water vapor a greenhouse gas?

• Water vapor is the most abundant greenhouse gas 

• It accounts for about 95% of GHGs by volume

• Water vapor is responsible for 50% of the natural greenhouse effect

• While water vapor is a greenhouse gas, it doesn’t contribute to global climate change because it has a short residence time in the atmosphere of a few days or even hours

• Instead, increased water vapor is a consequence of global warming

• Increased water vapor in the atmosphere amplifies the warming caused by other greenhouse gases

  • A warmer atmosphere holds more water vapor, leading to more warming and greater evaporation

  • This is called a 'positive feedback loop,' and the effect more than doubles the warming that would happen due to increasing carbon dioxide alone

• Human activities indirectly affect temperature and moisture through:

  • Deforestation, land-use changes, and burning fossil fuels

What is the greenhouse effect?

• The greenhouse effect is a naturally occurring phenomenon where short-wave radiation (in the form of visible light and ultraviolet radiation) from the sun passes through to the Earth's surface

• Some of this energy is absorbed by the Earth's surface and re-emitted as long-wave infrared radiation

• This energy passes through the atmosphere, where some is emitted into space

• The remaining thermal energy is trapped by the greenhouse gases and then re-emitted in all directions back into the atmosphere and causes the temperature of the surface to rise

• This process is important to ensure that Earth is warm enough for life

• Without the greenhouse effect, the average temperature would be much colder, making the planet uninhabitable

  • For example, the average surface temperature of Earth is about 15 °C

  • Without the greenhouse effect, it would be about -18 °C

The natural greenhouse effect

The enhanced greenhouse effect

• The enhanced greenhouse effect is caused by human activities that produce excessive greenhouse gases (GHGs) into the atmosphere, causing heat trapping and global warming

• Human activity has increased CO₂ levels in the atmosphere by more than 100 parts per million (ppm) to 420ppm in 2020

• Increased amounts of greenhouse gases have led to less long-wave radiation escaping the atmosphere

• Increasing global average temperatures by over 1°C since pre-industrial times

Global warming potential

• Certain gases can retain heat energy longer than others because they have high thermal retention properties

• Greenhouse gases are known to have a high thermal retention property

• Each greenhouse gas has varying potency when it comes to their potential impact on global warming

• The global warming potential (GWP) of a gas is used to identify what potential impact it will have on global climate change

• The GWP value is the amount of warming that 1 tonne of a GHG would create in relation to 1 tonne of CO₂ over a 100-year timescale

• As the most common greenhouse gas (and one that stays in the atmosphere for a very long time) carbon dioxide is used as the standard baseline gas with a GWP of 1

• This makes it easy to compare the warming potential of other gases

• Global greenhouse gas emissions are converted to CO₂ equivalents (CO₂e) by multiplying each GHG by its 100-year value

• Methane (CH₄) survives in the atmosphere for 11.8 years and is approximately 27-30 times more potent than CO₂ at warming the climate

• Nitrous oxide (N₂O) resides in the atmosphere for 109 years and is 273 times more potent at warming the climate than CO₂

• Fluorinated gases (CFCs, HFCs, sulfur hexafluoride, perfluorocarbons) have a residency time in the atmosphere from a few weeks to thousands of years and their GWP varies from 12 to 25,000 times more potent than CO₂

Importance of GWP

• Scientists and policymakers use GWP to analyze the effects of different greenhouse gases on the climate

• It allows them to prioritize which gases to reduce emissions of and understand how various activities affect the climate overall