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Greenhouse Gases and the Greenhouse Effect

Key Concepts

The greenhouse effect is a natural process that produces a relatively warm environment near the earth's surface conducive to life on earth.
The natural greenhouse effect is caused by greenhouse gases which occur naturally in the earth's atmosphere.
The main natural greenhouse gases are carbon dioxide (CO_2(g)), methane (CH_4(g)), nitrous oxide (N_2(g)O), and water (H₂O_(g)).
The enhanced greenhouse effect is caused by human activities which release greenhouse gases into the atmosphere.
The main anthropogenic, or human-induced, greenhouse gases are carbon dioxide (CO_2(g)), nitrous oxide (N₂O_(g)), hydrofluorocarbons (HCFs), perfluorocarbons (PFCs), methane (CH_4(g)) and sulfur hexafluoride (SF_6(g)).
The enhanced greenhouse effect is widely believed to be responsible for the increase in global temperatures seen in the 20^th century (global warming).

Greenhouse Gases

Sources of greenhouse gases, the length of time they exist in the atmosphere, and the percentage of emissions they represent are shown in the table below:

Gas Name Gas Symbol Common Sources Sinks Atmospheric Lifetime (years) % Emissions

carbon dioxide CO₂ -combustion of fossil fuels
-deforestation
-gas flaring
-cement production -photosynthesis
-ocean surface 5-200 60

methane CH₄ -landfills
-production and distribution of natural gas and petroleum
-fermentation from digestive system of livestock
-rice cultivation
-natural wetlands
-combustion of fossil fuels -reaction with tropospheric hydroxyl (OH)
-removal by soils 12 20

nitrous oxide N₂O -combustion of fossil fuels
-fertilizers
manure
-nylon production
-biological sources in oceans and soils -removal by soils
-stratospheric photolysis 114 6

hydrofluorocarbons HFCs -aerosol propellants
-refrigeration gases
-foam-blowing agents
-solvents
-fire retardants
-aluminium smelting
-semiconductor production -CFCs, HCFCs removal by stratospheric photolysis
-HCFC, HFC reaction with tropospheric hydroxyl (OH) 2 - 50,000 (dependent on compound) <14%

perfluorocarbons PFCs -aluminium production
-semiconductor production >10,000 <1%

sulfur hexafluoride SF₆ -electrical transmission and distribution systems
-circuit breakers
-magnesium production 3,200 <1%

The Greenhouse Gas Effect

Greenhouse gases in the earth's atmosphere absorb infrared radiation from the ground.

This radiation (heat) is re-emitted in all directions. Heat radiated back towards the earth's surface leads to the warming of the surface.

Using data from ice cores and other sources, scientists have been able to determine the temperature variation and carbon dioxide concentrations on earth over time:

Since 1950, temperature variations have been observed that can not be explained solely on the basis of the presence of natural greenhouse gases.

Scientists have modeled this unusual temperature variation to take into account man-made sources of greenhouse gases and have found that this may explain the temperature variations being observed.

During the 21^st century, the earth is likely to experience:

higher maximum temperatures and more hot days
higher minimum temperatures and fewer cold days
more intense precipitation events like rain over many areas
melting of polar ice caps resulting in rising sea levels

Suggestions for combatting global warming have been:

reduction in combustion of fossil fuels and sourcing new fuel alternatives
reducing deforestation and replanting forests
reducing and/or eliminating the use of HCFs, HCFCs, perfluorocarbons and sulfur hexafluoride.

Finding alternatives to these greenhouse gases in chemical and industrial processes has led to the development of a new branch of chemistry called Green Chemistry.