The classification of greenhouse gases

20:40, Sep 26 2012

Anyone who denies that recent climate change is not the result of human actions has to explain the following: ice core measurements by independent scientists have confirmed beyond any doubt the concentration of carbon dioxide in the atmosphere has been steadily increasing since 1750.

This is the date when the Industrial Revolution and when the associated burning of fossil fuel really gathered pace. Fossil fuel burning generates greenhouse gases such as carbon dioxide which warm the planet. How do they do that?

Chemical elements (such as hydrogen, oxygen and nitrogen) can bond to form molecules. The water molecule, for example, results from the chemical bonding of an oxygen atom with two hydrogen atoms (H-O-H).

Some molecules in the Earth's atmosphere are greenhouse gases and some are not; that raises the question: what qualifies a gas molecule to be a greenhouse gas?

Light and heat are referred to as radiation. The light from the Sun is called solar radiation and comprises different wavelengths, depending on the type of radiation it is. Visible light has a different wavelength from ultraviolet light, which has a different wavelength from infrared radiation (heat). Solar radiation is mainly visible light and ultraviolet radiation. These are shorter wavelength forms of radiation than heat. Some solar radiation shining on the Earth is reflected by the clouds directly back into space (26 per cent) and some is reflected by the surface of the Earth (4 per cent). However, 70 per cent is absorbed by the oceans and continents and heats them up. In the cool nights this radiation is released as heat which rises into the atmosphere. Most of it passes directly into space but some interacts with molecules in the atmosphere that have a chemical bond length and electrical charge distribution perfect for capturing and absorbing the wavelength of heat. These gases which are transparent to visible and ultraviolet light but absorb heat are called greenhouse gases. The absorbed heat causes the greenhouse gas molecules to release heat into the atmosphere, thus warming the planet.

Atoms such as oxygen are able to bond with another oxygen atom to form an oxygen molecule. However when two of the same atoms form a molecule, the result is not a greenhouse gas: oxygen gas (O-O) and nitrogen gas (N-N) are therefore not greenhouse gases. Their bond lengths and charge distributions are not right for absorbing the wavelength of heat. Water vapour is a greenhouse gas because the H-O-H molecule is perfect for absorbing heat. Likewise, the carbon dioxide molecule (O-C-O) and methane molecule (in which a single C atom is bonded to four H atoms) are both strong greenhouse gases.


The greenhouse gas potential is different for different gases principally because some molecules linger longer in the atmosphere. For example, a single molecule of water vapour stays in the atmosphere for nine days on average, whilst a single molecule of carbon dioxide stays in the atmosphere from five to 200 years. Therefore, molecule for molecule, carbon dioxide has a greater greenhouse gas potential than water vapour.

Without greenhouse gases, the Earth would be a frozen wasteland unable to support human life; too much greenhouse gas and the Earth would become an oven.

The greenhouse gas with the greatest impact is water vapour. However, the effect of human activities on its concentration have been negligible since they are insignificant compared with the natural water cycle involving ocean evaporation and rainfall.

For thousands of years the greenhouse gas concentration has varied little, but human-generated emissions from fossil fuels are tipping this delicate balance; as a result the concentration, particularly of carbon dioxide, is gradually increasing.

The 1500 or so active volcanoes around the world are sources of carbon dioxide since a volcanic eruption flings large quantities into the atmosphere (approximately 200 million tonnes every year). Most of this is recaptured by reaction with water vapour in the atmosphere and silicates in rocks to form carbonates. This process, known as weathering, ensured that before the Industrial Revolution the concentration of carbon dioxide had not significantly changed for thousands of years. However, fossil fuel emissions from cars and power stations, for example, put 135 times more carbon dioxide into the atmosphere every year than all the volcanic eruptions in a year (Ref. US Geological Survey), hence the progressive rise in carbon dioxide concentration since 1750 and the accompanying warming of the planet.

The consequences of this steady rise in greenhouse concentration is the topic of next week's article.

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