Greenhouse gases are generally any gas molecules or compounds in the atmosphere which interact with outgoing infrared radiation from the sun and thus contribute to the Greenhouse Effect by warming the atmosphere. The major constituents of the atmosphere such as nitrogen (N₂) and oxygen (O₂) do not interact with infrared radiation and, thus are not greenhouse gases. The major greenhouse gas in the atmosphere is water vapour (H₂O) but it has a short life time in the atmosphere as part of the natural water cycle.

The human emissions of water vapour are not considered to have altered its effect. Hence, it is not regarded as an anthropogenic greenhouse gas. The six major types of anthropogenic greenhouse gases (listed in Annex A of the Kyoto Protocol) are: carbon dioxide (CO₂); methane (CH₄); methane (CH₄); nitrous oxide (N₂O); hydroflurocarbon (HFCs); perflurocarbons (PFCs) and sulphur hexafluoride (SF₆)

Greenhouse Effect

The Greenhouse Effect is the name given to the natural properties of the atmosphere that allow incoming solar energy to pass through the atmosphere without warming it, while outgoing infrared radiation warms the atmosphere. The natural Greenhouse Effect is essential for life on the Earth and keeps average temperature about 33 ⁰C warmer than if there were no greenhouse gases and clouds in the atmosphere.

Under ideal conditions the global energy balance is in equilibrium, i.e. 342 Wm^-2 incoming solar radiation is balanced by 235 Wm^-2 of outgoing long wave and 107 Wm-2 of reflected solar radiation. Disruption of this system causes global warming or cooling.

Emissions of different greenhouse gases have different effects on climate change due to the difference in the parts of infrared spectrum that they interact with and also due to the duration of their presence in the atmosphere. Some gases persist in the atmosphere for extremely long period of time and continue have effect on it. For example, while bulk of CO₂ from fossil fuels will be removed in several centuries, about 25% will continue to affect the atmosphere for many thousand years.

Measurement of Greenhouse Gases

To account for different effects of greenhouse gases in a single measurement, the Kyoto Protocol uses a unit known as “carbon dioxide equivalent” (CO_{2 –e} or CO_2-eq). This term is used in two different ways to provide a single metric, both for greenhouse gas emissions (in tonnes of CO_2-e) and the greenhouse gas concentration in the atmosphere (in ppm CO_2-e).

When used in the context of greenhouse gas emissions, it refers to the amount of CO₂ that would give the same warming effect as the effect of the greenhouse gas or greenhouse gases being emitted over a given timeframe. Generally, 100 years is the standard time timeframe that is used.

The amount of a greenhouse gas can be converted to carbon dioxide equivalents by multiplying it by its Global Warming Potential. The Global Warming Potential (GWP) of different GHG is different depending upon the chemical and physical properties of the molecules. The intergovernmental Panel on Climate Change have set the Global Warming Potential of CO₂ as 1. In contrast, GWP of methane (CH₄) over 100 years timeframe is 21 and therefore, emission of 1 tonne of methane (CH₄) equals to 21 tonnes of CO_2-e over 100 years.

Simplifying GHG Emissions

In reality, it is very difficult to conceptualize precisely what a “tonne of carbon dioxide” or other greenhouse gas is as many people are not familiar with the nature of greenhouse gases in a technical sense and they are invisible to human years. For example, CO₂ is a colourless and odourless gas, which cannot be perceived through direct senses. Furthermore, since the volume occupied by a gas varies with temperature and pressure, at sea level and 25 ⁰C, one tonne (1000 kg) of pure CO₂ occupies 556.2 m³ of space, which is equivalent to an invisible cube slightly larger than 8m8mx8m.

In an attempt to turn greenhouse gases into something visual, tangible, and measureable for the public, the Victorian government have used the analogy of ‘Black Balloons” escaping into the atmosphere as a visual image for greenhouse gases in major campaign for energy efficiency to reduce greenhouse gas emissions. The advertisements show electrical appliances producing black balloons, each of which represents 50g of greenhouse gas, floating up from houses into the sky.

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Sunanda Swain

Sunanda Swain is a senior environment and sustainability consultant who worked in the environment, health and safety, quality and sustainability areas in various operational sectors such as engineering and environmental consultancies, government regulatory agencies and industrial manufacturing companies, mostly in Australia and in the UAE.
She is an accredited sustainability assessor for green building certification under USGBC LEED rating system and Abu Dhabi UPC's Pearl Rating System. She is also an approved consultant for environmental studies by Dubai Municipality. Her key work areas include environmental consulting for Environmental Impact Assessment, audits for environmental compliance, sustainability advisory for green building certification and consulting for waste management for various infrastructure, building, and industrial projects.
Sunanda holds a master's degree in environmental management and a master's degree in chemistry.

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