Science For Everyone: The Importance of Greenhouse Gases, And Their Role in Climate Change

Science For Everyone: This week, on ABP Live's science column, we discuss what greenhouse gases are, their importance, and what role they play in climate change. 

Greenhouse gases allow sunlight to pass through the atmosphere, but do not let some amount of Earth's heat escape the atmosphere. ( Image Source :Getty )

Science For Everyone: Welcome back to "Science For Everyone", ABP Live's weekly science column. Last week, we discussed how scientific experiments conducted on the International Space Station (ISS) benefit Earth. This week, we will discuss what greenhouse gases are, their importance, and what role they play in climate change. 

The atmospheric gases which trap heat in the atmosphere, and absorb and re-emit infrared energy from the atmosphere and the Earth’s surface, are called greenhouse gases. Greenhouse gases allow sunlight to pass through the atmosphere, but do not let some amount of Earth's heat escape the atmosphere.

The importance of greenhouse gases

The phenomenon of trapping of heat by greenhouse gases, which leads to a mean surface temperature 33 degrees Celsius greater than what would have been the temperature of Earth in the phenomenon's absence, is called the greenhouse effect. 

In the absence of the greenhouse effect, Earth's average temperature would have been minus 18 degrees Celsius, according to the World Meteorological Organization (WMO).

Solar radiation passes through the atmosphere, most of which is absorbed by the Earth's surface, while some part of it is reflected by the Earth and the atmosphere. 

The radiation warms the Earth's surface, which, in turn, emits infrared radiation. 

Some of the emitted infrared radiation passes through the Earth's atmosphere, while some of the radiation is absorbed by greenhouse gases, and then re-emitted. 

Thus, greenhouse gases absorb heat radiated from the Earth's surface, and emit the thermal energy in all directions. 

In this way, the greenhouse effect warms the Earth's surface, and also the planet's lower atmosphere. 

Trace amounts of certain gases in the atmosphere, such as water vapour, carbon dioxide, methane and nitrous oxide, create a natural greenhouse effect for Earth. These greenhouse gases allow solar radiation to reach the surface of Earth, but absorb the infrared radiation emitted by the planet. This causes the Earth's surface to heat up. 

The name comes from greenhouses, which are enclosed spaces with windows that allow sunlight to enter to create warmth for plants, but do not allow the heat to escape. 

Types of greenhouse effect

Greenhouse effect is of two types: natural greenhouse effect and enhanced greenhouse effect. 

Natural amounts of greenhouse gases result in the natural greenhouse effect, and are important for survival. Earth will be approximately 33 degrees Celsius cooler in the absence of the natural greenhouse effect. 

How anthropogenic activities have increased the concentrations of greenhouse gases

Human activities induce increased concentrations of some greenhouse gases, which result in additional radiative forcing, a phenomenon in which the amount of energy entering the Earth's atmosphere is different from the amount of energy that leaves it. This entire process is known as the enhanced greenhouse effect. 

Human activities have been releasing extra greenhouse gases into the atmosphere since about 1850, which has slowly resulted in an increase in average temperatures across Earth.

Due to anthropogenic activities, the concentrations of greenhouse gases such as carbon dioxide, nitrous oxide, ozone, methane, hydrochlorofluorocarbons and hydrofluorocarbons have increased in the lower atmosphere. 

The Global Atmosphere Watch (GAW) is responsible for observing, analysing and publishing greenhouse gas data collected by 50 countries around the globe, from the High Arctic to the South Pole. 

Carbon dioxide, nitrous oxide, molecular hydrogen, methane, halocarbons, and sulphur hexafluoride are the greenhouse gases monitored. 

More about different greenhouse gases

While water vapour is the most abundant greenhouse gas, the water vapour produced directly by human activity contributes very little to the amount of water vapour in the atmosphere, scientists say. 

Ozone is also a greenhouse gas, but it can be helpful or harmful depending on where it is found in the Earth's atmosphere, according to the US Energy Information Administration.

Ozone naturally occurs in the stratosphere, and blocks ultraviolet light from the Sun from reaching the Earth's surface. The protective effects of ozone offset its contribution to the greenhouse effect. 

However, the use of chlorofluorocarbons in the past has created a hole in the ozone, because of which countries around the world came together and banned the sale, distribution and use of chlorofluorocarbons since 1994. 

Anthropogenic activities are resulting in increased concentrations of carbon dioxide, nitrous oxide, methane, and fluorinated gases in the atmosphere.

Carbon dioxide: Burning of fossil fuels such as coal, natural gas and oil, combustion of solid waste, trees and other biological materials, use of vehicles for transportation, electricity generation, and different chemical reactions cause carbon dioxide to enter the atmosphere. When plants absorb carbon dioxide during photosynthesis, as part of the biological carbon cycle, the gas is removed or sequestered from the atmosphere. 

Nitrous oxide: Agricultural and industrial activities, land use, wastewater treatment, and the combustion of fossil fuels and solid waste emit nitrous oxide into the atmosphere. 

Nitrous oxide is hundreds of times more effective in trapping heat, compared to carbon dioxide, and can remain in the atmosphere for nearly a century. Only about one per cent of atmospheric nitrogen oxide is converted by green plants into ammonia each year. 

Methane: The production and transport of coal, natural gas and oil, livestock and agricultural practices, the decay of organic waste in landfills, and land use cause methane emissions.

Methane is more effective at trapping heat than carbon dioxide, but does not remain in the atmosphere for long, because it gets broken down when it reacts with hydroxyl radicals, or neutral charged OH molecules, in the atmosphere. 

Fluorinated gases: Fluorinated gases included perfluorocarbons, nitrogen trifluoride, hydrofluorocarbons, and sulphur hexafluoride. These are synthetic, powerful greenhouse gases that are emitted as a result of commercial, household and industrial processes, and are sometimes used as substitutes for stratospheric ozone-depleting substances such as chlorofluorocarbons, halons and hydrochlorofluorocarbons. 

While fluorinated gases are emitted in smaller quantities compared to other greenhouse gases, they are potent greenhouse gases. 

Their global warming potentials (GWPs) range from thousands to tens of thousands. 

Fluorinated gases are sometimes referred to as high-GWP gases. This is because they trap substantially more heat than carbon dioxide, for a given amount of mass. 

Role of greenhouse gases in climate change 

The effect of greenhouse gases on climate change depends on three main factors, according to the US Environmental Protection Agency (EPA). These factors are: how abundant greenhouse gases are in the atmosphere, how long greenhouse gases stay in the atmosphere, and how strongly greenhouse gases impact the atmosphere. 

How abundant greenhouse gases are in the atmosphere

The amount of a particular gas in the atmosphere is called concentration or abundance. More the emissions of greenhouse gases, the higher their atmospheric concentrations will be. Concentrations of greenhouse gases are measured in parts per million, parts per billion, and parts per trillion. 

How long greenhouse gases stay in the atmosphere

Greenhouse gases can stay in the atmosphere for different amounts of time, long enough to become well-mixed. Therefore, the combined amount of all greenhouse gases measured in the atmosphere is roughly the same all over the world, irrespective of the emission source. 

How strongly greenhouse gases impact the atmosphere

Some greenhouse gases are more effective than others at making Earth warmer and thickening the planet's atmospheric blanket.

The GWP of a greenhouse gas is a measure of how much energy the emissions of one tonne of the gas will absorb over a given period of time, typically over a 100-year-period, relative to the emissions of one tonne of carbon dioxide, according to the EPA. 

This allows scientists to compare the global warming impact of greenhouse gas to that of other gases. 

Greenhouse gases which have a higher GWP absorb more energy, per tonne of gas emitted, compared to gases with a lower GWP. Therefore, greenhouse gases with a higher GWP contribute more to the warming of Earth, compared to gases with a lower GWP.

Greenhouse gases are important to keep the Earth warm, but humans should switch to sustainable practices, and plant more trees, to reduce the enhanced greenhouse effect, which significantly contributes to climate change.