Greenhouse Gases

What are the Greenhouse Gases?
Greenhouse gases are naturally occurring gases in the atmosphere that have the capacity to absorb long-wave radiation emanating from the Earth's surface. By absorbing this energy and re-radiating it, the gases cause the temperature of the Earth's lower atmosphere and surface to increase. The primary greenhouse gases in earth's atmosphere are

Water Vapor (H₂O)
Carbon Dioxide (CO₂)
Methane (CH_⁴)
Nitrous Oxide (N_²O)
Fluorinate Gases

Greenhouse gas concentrations are measured in parts per million, parts per billion, and even parts per trillion. Human activities have increased concentrations of all of these gases.

Global Warming Potential
Some greenhouse gases are more effective than others at making the planet warmer and can remain in the atmosphere for different amounts of time, ranging from a few years to thousands of years.
How greenhouse gases warm image

For each greenhouse gas, a Global Warming Potential (GWP) has been calculated to reflect how long it remains in the atmosphere, on average, and how strongly it absorbs energy. The GWP of greenhouse gases is a measure of how much heat each gas traps in the atmosphere relative to Carbon Dioxide.

GHG	Atmospheric Concentration	Atmospheric Lifetime	Global Warming Potential (Over a 100 year period)
Carbon Dioxide (CO²)	76%	1000’s of years	1
Methane (CH₄)	16%	10 years	25
Nitrous Oxide (N₂O)	6%	> 100 years	298
Fluorinated Gases	2%	1,000 - 10,000 years	1,000 -10,000

Carbon Dioxide Equivalent
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.
Emissions image

Water Vapour
Water vapour is the most abundant greenhouse gas in the atmosphere and is an effective greenhouse gas – it is warmer on a cloudy winter day than on a clear one. When compared to other greenhouse gases, water vapour stays in the atmosphere for a much shorter period of time, generally stay in the atmosphere for days before precipitating out.

For the most part, the addition of water vapour to the atmosphere cannot be directly attributed to human generated activities.

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
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 (CH₄)
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
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.