The presence of an atmosphere is essential for life on Earth. Certain gases that make up the atmosphere, known as greenhouse gases, enable heat retention and maintain a viable temperature at the Earth's surface. This natural phenomenon is known as the greenhouse effect.
A greenhouse gas (GHG) is an atmospheric gas that has the ability to absorb and emit infrared radiation, which tends to retain heat in the atmosphere.
By retaining heat in the atmosphere, greenhouse gases contribute to the greenhouse effect.
The main greenhouse gases are water vapour |(\text{H}_2\text{O}),| carbon dioxide |(\text{CO}_2),| methane |(\text{CH}_4)| and dinitrogen oxide |(\text{N}_2\text{O}),| commonly referred to as nitrous oxide. They are produced naturally during the following processes:
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Water vapour |(\text{H}_2\text{O})| is produced by the evaporation of liquid water from the Earth's surface.
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Some carbon dioxide |(\text{CO}_2)| is produced by the natural decomposition of animal and plant matter.
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Methane |(\text{CH}_4)| is produced by the decomposition of plant matter.
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Dinitrogen oxide |(\text{N}_2\text{O})| is mainly produced by microbial activity in soil during denitrification.
The atmospheric concentration of these gases is regulated by the various processes involved in the biogeochemical cycles of water, carbon and nitrogen.
Water vapour is the most abundant greenhouse gas in the atmosphere. Water vapour alone is responsible for more than half of the greenhouse effect.
Not all gases in the atmosphere are GHGs, and not all GHGs have the same capacity to retain heat. This capacity can be compared using the Global Warming Potential (GWP). This is an index that indicates a gas's ability to retain heat in the atmosphere compared to carbon dioxide |(\text{CO}_2).| The following table shows the GWP index for several GHGs.
|
Greenhouse Gas |
Global warming potential |
|---|---|
|
Carbon dioxide |(\text{CO}_2)| |
1 |
|
Methane |(\text{CH}_4)| |
25 |
|
Dinitrogen oxide |(\text{N}_2\text{O})| |
298 |
Source: IPCC, 2021[1]. |
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According to this data:
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Methane |(\text{CH}_4)| can retain 25 times more heat than carbon dioxide |(\text{CO}_2).|
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Dinitrogen oxide |(\text{N}_2\text{O})| can retain 298 times more heat than carbon dioxide |(\text{CO}_2).|
The greenhouse effect is a natural phenomenon that allows some of the heat emitted by the Sun to be retained in the Earth's atmosphere.
By retaining heat in the atmosphere, the greenhouse effect reduces the difference in temperature between day and night, so that the average temperature at the Earth's surface is about |15^\circ\text{C}.| Without this effect, the average temperature at the Earth's surface would be around |-18^\circ\text{C}.| Under these conditions, most of the water would be in solid form, making it difficult to access. This would prevent the development of life on Earth as we know it.
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Some of the solar radiation is reflected by the atmosphere into space, while the rest enters the atmosphere.
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Some of the radiation that penetrates the atmosphere reaches the Earth's surface, which absorbs its energy. Land and ocean temperatures rise.
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Some of the energy absorbed by the Earth's surface is then transformed into heat and re-emitted in the form of infrared (IR) radiation towards the atmosphere.
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Some of the IR radiation emitted by the Earth's surface passes through the atmosphere and is released into space.
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Some of the IR radiation emitted by the Earth's surface is absorbed by GHG, increasing the temperature of the atmosphere.
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Some of the energy absorbed by GHGs is re-emitted in the form of IR radiation towards the Earth's surface.
The enhanced greenhouse effect is the intensification of the greenhouse effect caused by GHGs produced by human activity.
Since industrialization, human activity has increased the concentration of greenhouse gases.
As a result, more heat is retained in the atmosphere, raising the average temperature at the Earth's surface. This is known as global warming.
The following table shows the increase in the main greenhouse gases since the beginning of industrialization (1750) and the main human activities responsible for them.
|
Greenhouse gas |
Percent increase (%) |
Sources |
|---|---|---|
|
Carbon dioxide |
149% |
|
|
Methane |(\text{CH}_4)| |
262% |
|
|
Dinitrogen oxide |
124% |
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Source: IPCC, 2023 [2]. |
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The enhanced greenhouse effect increases the average temperature at the Earth's surface, causing glaciers, ice floes and pack ice to melt and permafrost to thaw. These consequences, in turn, have a number of repercussions for the environment.
The melting of glaciers, ice floes and pack ice reduces the albedo effect. This gradual disappearance of snow and ice decreases the amount of solar radiation reflected by the Earth's surface. As a result, the Earth's surface absorbs even more energy, contributing to a further rise in temperature. In addition, melting glaciers cause sea levels to rise and lower the salinity of the oceans, which in turn disrupts the thermohaline circulation. Rising sea levels also contribute to increased shoreline erosion and the risk of flooding.
Melting permafrost causes landslides and releases large quantities of methane |(\text{CH}_4),| increasing its concentration in the atmosphere and reinforcing the greenhouse effect even further.
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Intergovernmental Panel on Climate Change (IPCC). (May 3, 2021). Chapter 7: The Earth’s energy budget, climate feedbacks, and climate sensitivity - Supplementary Material. https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FGD_Chapter07_SM.pdf
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Intergovernmental Panel on Climate Change (IPCC). (2023). Synthesis report of the IPCC sixth assessment report (AR6). (Longer Report). https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf