What are Greenhouse Gases? How does the Greenhouse Effect Work?

Greenhouse gases are responsible for creating the greenhouse effect on the Earth.  These gases have the tendency to trap heat in Earth’s atmosphere. The trapped heat affects the average global temperature.  It ensures than an optimal range of temperature for supporting life on Earth is maintained.

In order to understand the concept of greenhouse gases in detail, let’s first understand what exactly is a “greenhouse”. The word greenhouse refers to a structure made of glass or other transparent material which keeps the atmosphere within the structure warm enough for the plants to grow. Greenhouse structure basically regulates the climate within to nurture life and acts as a protective shield. Such greenhouses are usually built in areas with extreme climatic conditions or variations.

Similar to these greenhouses, the greenhouse gases also create a protective shield around the Earth. When surface of the Earth reflects infrared radiation, these greenhouses gases absorb the radiation and trap the heat. The natural process of warming of Earth’s surface due to these greenhouse gases is termed as the “greenhouse effect”.

How does the Greenhouse Effect Work?

When the radiation coming from the Sun reaches Earth’s atmosphere, a part of it is reflected back into space. The remaining part of the sun’s radiation is absorbed by the atmosphere eventually reaches the land (lithosphere) and oceans (hydrosphere). Solar radiation is then reflected back from the surface of the Earth towards the outer space. A part of this outgoing solar radiation is trapped by the greenhouse gases present in the atmosphere. The heat so trapped affects the global average temperature of the Earth. It ensures that the average temperature of Earth is warm enough to sustain and support life on it.

Impact of Greenhouses Gases on the Earth’s temperature

In order to understand the impact of greenhouse gases, let’s take examples of our Earth and the Moon. Both these celestial bodies are almost equidistant from the Sun. The temperature on the moon varies from 100 °Celsius (during the day) to as low as minus 173 °Celsius (during the night). This is primarily because the atmosphere of the moon has no mechanism to trap solar heat.

Our Earth, on the other hand,  maintains an average global temperature of 15 °Celsius. All thanks to the greenhouse effect!

If greenhouse gases were not present in the Earth’s atmosphere, the average temperature of Earth would have been around minus 18 °Celsius.

Greenhouse Gases: Natural & Industrial

Greenhouse gases can be categorized into natural greenhouse gases and industrial greenhouse gases.

Natural Greenhouse Gases

The natural greenhouse gases present in the atmosphere of the Earth are listed below.

1. Water Vapor

Water vapor holds that mark of being the most important greenhouse gas. It is the largest contributor to the greenhouse effect on Earth.

Water vapor amplifies the effect of other greenhouse gases like carbon dioxide. As the Earth becomes warmer, its atmosphere holds more water vapor resulting in amplification of the initial warming. Water vapor, thus, ends up functioning as a feedback to the climate.

The temperature of the Earth has a direct correlation with the amount of water vapor in the atmosphere. Higher the temperature, more the amount of water vapor. This is the reason why the atmospheric concentration of water vapor is highly variable and varies from 0.01% in cold regions to 3% by mass in saturated air at about 32 degrees Celsius.

2. Carbon Dioxide (CO₂)

This colorless and odorless gas is released through the natural processes like volcanic eruptions, respiration or triggered by human activities like burning of organic matters like fossil fuels, deforestation, etc. Despite being a minor component in the atmosphere as far as composition is concerned, carbon dioxide plays a dominant role in climate change. In fact, the carbon dioxide level in present times is unprecedented in human history.

Many scientists have acknowledged that the average temperature of Earth’s surface has already increased by 1 degree Celsius since 1880s. Anthropogenic activities since the industrial revolution have primarily contributed to the extraordinary increase in the atmospheric concentration of carbon dioxide by 47 percent. If significant efforts are not made towards controlling the carbon emission, it is likely that the concentration of carbon dioxide may reach 0.1% of the atmosphere by the end of this century.

3. Methane (CH₄)

Methane emission is naturally caused by the decomposition of plants, organic wastes, livestock and agricultural practices, etc. It is also released during the production and transportation of coal, natural gas, and oil. Methane is a short-lived gas and it stays in the atmosphere for nearly 12 years.

The radiative forcing produced in methane is greater than carbon dioxide making it more potent greenhouse gas than carbon dioxide. The global warming potential of methane is 84 times more than carbon dioxide in a 20 years time span. This is the reason why scientists are extremely worried due to the constant surge of atmospheric methane.

4. Nitrous Oxide (N₂O)

Nitrous oxide has both natural and man-made sources. In fact, the impact of many of these sources is difficult to measure. Naturally, nitrous oxide is produced from microbial activity in the soils and the ocean. Activities like the use of fertilizers in agriculture, combustion of fossil fuels, burning of biomass, and industrial production of nitric acid serve as the anthropogenic sources of nitric oxide.

This greenhouse gas is relatively short-lived than carbon dioxide. However, the global warming potential of nitrous oxide is 300 times more than that of carbon dioxide. It is also one of the causes of ozone layer depletion.

5. Ozone (O₃)

Technically, ozone is also a greenhouse gas. However, this natural greenhouse gas is not generally taken into consideration in the international climate-related negotiations. This is primarily because the protective benefits of ozone (especially in the stratosphere) outweighs its negative impact as a contributor to global warming.

Industrial Greenhouse Gases

Under this category, halocarbons are the most prominent gases.

Halocarbons

When we say halocarbons, it refers to chlorofluorocarbons or other compounds in which the hydrogen of a hydrocarbon is replaced by halogens like fluorine, chlorine, bromine, etc.  Some of them are also referred to as destroyers of the ozone layer. Chlorofluorocarbons like CFC-11, CFC-12 and CFC-113 are considered as some of the most powerful industrial greenhouse gases.

The Montreal Protocol on Substances that Deplete the Ozone Layer (1989) specifically emphasized on tight controlling on the release/production of halocarbons as these industrial greenhouse gases contribute to the depletion of stratospheric ozone.