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Greenhouse gases (GHG)


Greenhouse gases are gases found in the atmosphere that are capable of absorbing infrared radiation emitted from the Earth’s surface.  Because these gases are good absorbers of radiation, they are also good emitters of radiation.  Greenhouse gases emit radiation back towards Earth, causing warming at the surface and lower atmosphere to occur.

Greenhouse gases, once present in the atmosphere, do not degrade quickly.  Most greenhouse gases persist for many years, and thus contribute to warming for an extending period of time.


Key GHGs


Greenhouse gases differ in their atmospheric concentration, their residency time in the atmosphere, and their efficiency at absorbing and emitting longwave radiation.  Water vapour, carbon dioxide, methane, nitrous oxide and halocarbons are all greenhouse gases and thus all play a role in warming our planet. (see also CO2 equivalents)

The key GHG are:

 

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Water vapour

Water vapour is the most abundant greenhouse gas in the atmosphere, and is a very effective absorber of radiation.  As a result, water vapour plays an important contributing role in global warming. 

Water vapour increases in the atmosphere as atmospheric temperatures increase.  This is because warmer air is able to hold more moisture. 

Unlike other greenhouse gases, the additional water vapour in the atmosphere was not put there directly by humans.  The increase in water vapour occurs because the climate is warming, and the increase then contributes to further warming.  This process is referred to as a positive feedback.

Also unlike other greenhouse gases, water vapour does not persist in the atmosphere.  Water vapour’s lifetime in the atmosphere can be measured in days, not decades or centuries as with other greenhouse gases.  The water vapour that is in the atmosphere today will not continue to contribute to warming well into the future.  

 

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Carbon Dioxide ( CO2)

Carbon dioxide levels in the atmosphere have increased from approximately 280 ppm (parts per million) in pre-industrial times to approximately 370 ppm today (1).  This represents an increase of more than 30%. 

        

Figure 2:  Carbon Dioxide levels (in ppm) and associated radiative forcing from 2005 back 20,000 years.  (IPCC3)

The increase in carbon dioxide in the atmosphere is due primarily to the burning of fossil fuels and land use changes.  Since the industrial revolution, humans have been relying increasingly on fossil fuels for transportation needs, industrial production and to heat our homes.  Land use changes, such as clearing land to make way for agricultural land or for urban development, has also contributed to an increase in carbon dioxide levels.  Clearing trees and replacing the land with cities or with agricultural land removes a valuable carbon sink.  Trees are responsible for the uptake of large amounts of carbon dioxide.  Once they are cleared, we have lost a valuable tool for offsetting some of the carbon dioxide that we are releasing into the environment. 

 

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Methane (NH4)

Methane levels in the atmosphere have increased from approximately 715 ppb (parts per billion) in pre-industrial times, to approximately 1774 ppb in 20053.  This represents a staggering increase of about 150%.

 

Figure 3:  Methane levels (in ppb) and associated radiative forcing from 2005 back 20,000 years.  (IPCC3)

The increase in methane in the atmosphere is caused primarily by human activities, such as livestock operations, rice agriculture, fossil fuels, biomass burning and decomposing garbage at landfills.  

 

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Nitrous Oxide (N2O)

Nitrous oxide levels in the atmosphere have increased from approximately 270 ppb in pre-industrial times to approximately 319 ppb in 20053.  This represents an increase of about 18%. 

       

Figure 4:  Nitrous oxide levels (in ppb) and associated radiative forcing from 2005 back 20,000 years.  (IPCC3)

The increase of nitrous oxide into the atmosphere can be traced to the burning of fossil fuels, as well as the use of chemical fertilizers.

 

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Halocarbons

Halocarbons are synthetic compounds that are created by humans.  Halocarbons include compounds like:

  • CFCs (chlorofluorocarbons)
  • HCFCs (hydrochlorofluorocarbons)
  • HFCs (hydrofluorocarbons)


Halocarbons are effective greenhouse gases as they absorb more counter radiation than other greenhouse gases.  These compounds also tend to persist in the environment for longer than other greenhouse gases as it takes longer for them to break down. 

CFCs are known ozone depleting substances.  As a result, the international community has taken steps to ban the production and release of CFCs.  The Montreal Protocol has yielded positive results in reducing the amount of CFCs released into the environment.  This ban did, however, result in a greater dependence on HCFCs and HFCs, which are powerful greenhouse gases.  Processes are in place to phase out a large portion of these products as well, but the fact that they persist in the atmosphere for lengthy periods of time will mean that these substances will continue to contribute to warming for centuries to come.

Click here for Manitoba greenhouse gas emission data.