A single item or activity can cause multiple different greenhouse gases to be emitted, the convention is to sum up the associated contribution of GHGs in terms of Carbon Dioxide Equivalent (CO2e). Carbon dioxide is taken as the gas of reference and given a 100-year GWP of 1. The mass emission of any gas multiplied by its GWP gives the equivalent emission of the gas as carbon dioxide.
However, the scientific evidence indicates that the warming caused by man-made emissions of CO2 and other greenhouse gases is increasing the amount of water vapour in the air by boosting the rate of evaporation. As the climate warms, air temperatures rise, leading to more evaporation from water sources which increases atmospheric moisture content. The increase in water vapour in the atmosphere contributes to even more warming. This increased water vapor content in the atmosphere is referred to as a feedback process.
Carbon dioxide is naturally present in the atmosphere as part of the Earth's carbon cycle and is constantly being exchanged among the atmosphere, ocean, and land surface as it is both produced and absorbed by many microorganisms, plants, and animals.
Since the Industrial Revolution anthropogenic emissions has rapidly increased the concentration of CO2 in the atmosphere and accounts for about 76% of human caused greenhouse gas emissions. Once emitted it remains in the atmosphere for a long period of time; 40% remains after 100 years, 20% after 1000 years and 10% as long as 10,000 years later.
Carbon dioxide also causes ocean acidification because it dissolves in water to form carbonic acid.
The primary source of atmospheric CO2 is from burning of fossil fuels (oil, natural gasses and coal), solid waste and other biological materials. Changes in landuse also play a role. Deforestation and soil degradation add CO2 to the atmosphere while forest regrowth talks it out of the atmosphere.
The concentration of CO2 in the atmosphere has risen from close to 280ppm (parts per million) in 1800, at first slowly and then progressively faster to a value of 367 ppm in 1999, to approximately 412 ppm today. This represents an increase of more than 47% since the beginning of the industrial age.
Methane accounts for around 16% of the impact of current human greenhouse-gas emissions. A significant source of human-made methane emissions is the production and transport of fossil fuels, such as oil, natural gas and coal. Methane is also emitted from livestock and agriculture practices and from organic waste in landfills. In Ireland, methane is primarily emitted from livestock and as a result of agricultural activities.
Methane doesn't persist in the atmosphere as long as CO2 but is particularly problematic as its impact is 25 times greater than CO2 over a 100-year period, according to the IPCC AR5 Report.
Nitrous Oxide (NO2)
Nitrous oxide account for 6% of human caused greenhouse gas emissions. It stays in the atmosphere about 114 years on average and has a global warming impact of 298 times that of CO2 on a 100-year time scale.
It is emitted during the combustion of fossil fuels and from industrial activities and agricultural especially nitrogen-fertilised soils and livestock waste.
In Ireland, transport is the principle source of NO2.
Fluorinated gases ("F gases") accounts for around 2% of the warming impact of current human greenhouse-gas emissions. F-gases are synthetic- entirely man-made and nearly exclusively result from human activities such as manufacturing and industrial processes.
These gases can be found in different products such as fridges or aerosol cans. While they are not ozone-depleting, they have a long atmospheris lifetime and their Global Warming Potential (GWP) is very high. At the 100 year point of comparison, their GWPs range from 1,000 to 8,000 and some variants top 10,000.