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A wide range of gases known as greenhouse gases contribute to global warming/climate change. 

The most important ‘greenhouse gases’ are:

• carbon dioxide (CO₂)
• methane (CH₄)
• and nitrous oxide (N₂O).

Other greenhouse gases include the F- Gases:  hydrofluorocarbons (HFC), perfluorocarbons (PFC) and sulphur hexafluoride (SF₆).

• Global Warming Potential (GWP)
• Ozone
• Aerosols and Global Dimming
• Adding it all up and Radiative Forcing

Global Warming Potential (GWP)

The different gases have different atmospheric characteristics, including global warming potentials (GWP). The GWP of a gas is a measure of the cumulative warming over a specified time period usually 100 years, by a unit mass of this gas. This is expressed relative to carbon dioxide, which has a GWP of 1. The mass emission of any gas multiplied by its GWP gives the equivalent emission of the gas as carbon dioxide. This is known as CO₂ equivalent. This makes it easier to sum up the emissions and contribution of Greenhouse Gas (GHG) to climate change and determine options to address climate change.

Therefore, while CO₂, CH₄ and N₂O are important because they are normally emitted in large amounts, the fluorinated gases, HFC, PFC and SF6, are also important mainly because of their comparatively much larger GWP (global warming potentials) values.
 
The atmospheric lifetime of greenhouse gases is another key factor in determining what actions are required to combat climate change. Carbon dioxide is considered to stay in the atmosphere for 100 years. Other greenhouse gases have much longer lifetimes, for example sulphur hexafluoride has a lifetime of 3200 years. Methane has a relatively short lifetime of 12 years. These long lifetimes in the atmosphere mean that emissions today will have an impact on the climate for the rest of this century and, in the case of some gases, beyond. The long life of these gases is one of the reasons that action on reducing emissions is so vital to prevent long term climate change.

The lifetimes and GWP of some of the most important greenhouse gases are presented in the table below.

Source: IPCC Second Assessment Report, Working Group 1

Ozone

Ground level ozone is also an important greenhouse gas but it has a very short atmospheric lifetime of about one week. Ozone also causes damage to human health, crops and ecosystems. Actions to reduce ground level ozone levels are being taken under actions on air quality. These are primarily designed to address the more immediate impact of ozone and other standard air pollutants but will have some positive effects for climate change.
 
Similarly actions being taken under the Montreal Protocol to the Vienna Convention to address stratospheric ozone depletion by CFCs will have some positive effects for climate change since these actions reduce the emissions of ozone depleting substances, which are also greenhouse gases.

Water vapour
 
Water vapour is an important component of the atmosphere.  It is also a short lifetime greenhouse gas. The amount of water vapour that the atmosphere can hold is largely determined by its temperature. As temperatures increase  the amount of water vapour that the atmosphere can hold increases. Climate scientists see this as important positive feedback, which amplifies the warming caused by long lived greenhouse gases. It also influences rainfall characteristics as the increased levels of water vapour can result in more intense rainfall. More generally this is referred to as an intensified hydrological cycle. However, other factors such as aerosols also play important roles in the hydrological cycle.

Aerosols and Global Dimming

Aerosols are largely invisible microscopic particles that are an integral part of the atmosphere. They can be examined using powerful microscopes.  Aerosol particles are a complex mixture of salts, soot and organic materials. They absorb water vapour from the atmosphere and give rise to hazes and smogs. Also, as air rises high up in the atmosphere and cools, these aerosols become centres for condensation of cloud drops and cloud formation. Therefore, aerosols have a key role in the water cycle.
 
Aerosols influence climate in a number of ways. They can reflect sunlight back to space before it causes warming and can influence the characteristic of clouds and their lifetimes. These influences all have a cooling effect and are enhanced by human emissions of pollution such as sulphur dioxide (SO₂), nitrogen oxides (NOX) and ammonia (NH₃). The effect  is sometimes called global dimming.

Aerosols have short atmospheric lifetimes, typically from days to weeks.

Adding it all up and Radiative Forcing

As we have seen, a wide range of GHG and aerosols influence the global climate, or more precisely change the natural radiation balance of the earth. In order to add up and compare these different factors scientists use the concept of radiative forcing to quantify how much each of these is moving the earth away from its natural radiation balance. This is measured in Watts per meter squared and is a very important calculation.
 
The figure below has been prepared by the IPCC to demonstrate the issue of radiative forcing. These Radiative forcing result from one or more factors that affect climate and are associated with human activities or natural processes. The values represent the forcing in 2005 relative to the start of the industrial era (about 1750). Human activities cause significant changes in long-lived gases, ozone, water vapour, surface albedo, aerosols and contrails. The only increase in natural forcing of any significance between 1750 and 2005 occurred in solar irradiance. Positive forcing leads to warming of the climate and negative forcing leads to a cooling.
 
This figure sums up the various impacts of GHG aerosols, as well as solar radiation. It clearly shows that most of the current warming is caused by carbon dioxide, with methane and nitrousoxide being the next most important individual GHGs.
  
The figure also shows that part of the warming is being masked by some short lived aerosol species as well as aerosol impacts on clouds. Also variation in the solar radiation have had a small warming effect. More details on this chart are available from the IPCC.

The main GHGs

We take a look at the main GHG involved in climate change and the factors that have influenced their current levels in the atmosphere.

• Carbon Dioxide
• Methane
• Nitrous Oxide
• F-Gases