Greenhouse gases

Greenhouse gases (GHGs) are gases that, because of their physical properties, absorb long-wave radiation.

In the atmosphere they act like a blanket, holding in energy that has bounced off the earth, whilst letting short-wave radiation arriving from the sun pass through unimpeded. The result is that the earth holds onto more energy than it otherwise would and stays warm.

Glass does the same thing for a car parked in the sun.

cattle are a significant source of methane that is generated from the fermentation of grass in their stomachs — the technical term is enteric emissions

Humans have generated, released and manufactured GHGs for a long time, dramatically since the industrial revolution in the 1880’s, increasing the concentration of GHGs in the atmosphere and now considered the mechanism for anthropogenic climate change.

It is a delicate balance - without GHGs the earth would freeze, quite literally. Global temperatures would drop from today’s average of 14 degrees C to -18 degrees C.

Add enough GHGs to increase their concentration in the atmosphere and the average global temperature goes up. Keep doing it and we end up with global warming.

Convention has it that GHGs are emitted, we let them go up into the atmosphere and that burning coal and petroleum is the source.

But there are other ways – GHGs are

• released when forests are cleared (deforestation) and when agriculture results in poor soil management
• generated by burning fossil fuels, vegetation burning, waste treatment, rearing ruminant livestock (enteric fermentation), fertilizer use, some industrial processes
• manufactured chemicals that do not occur naturally but are used (or have been used) by humans for aerosol propellants, creation of foam packaging, refrigerants, solvents, fire retardants

Greenhouse gases that occur naturally and have the greatest influence on atmospheric warming include

• Carbon dioxide
• Methane
• Nitrous oxide
• Water vapour
• Tropospheric ozone

And those that humans manufacture include

• Chlorofluorocarbons (CFCs)
• Hydrochlorofluorocarbons (HCFCs)
• Perfluorocarbons
• Sulphur hexafluoride

Molecules of each of the GHGs have both a different intrinsic capability to absorb heat and residence time in the atmosphere - not every greenhouse gas is made equal.

In order to standardize the numbers and compare the warming effect of one gas with another, a global warming potential (sometimes called radiative forcing) of a given gas is calculated and compared with a similar mass of carbon dioxide- this is referred to as carbon dioxide equivalent CO2e

Nitrogen (78.1%), oxygen (20.9%) and argon (0.9%) account for 99.964% of the volume of the atmosphere.

This leaves just 0.036% to the GHGs – and carbon dioxide accounts for most of this - so although the list of gases that contribute to the greenhouse effect is long they are a tiny fraction of the atmosphere by volume.

This partly explains why emissions from human activities can be significant because, where GHGs are concerned, the amounts are small and the effect large.

Find out more about: Carbon dioxide | CO2e

management of grazing animals on this property on the tablelands of NSW, Australia has resulted in biosequestration — the capture of CO2 into vegetation and soil

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