Greenhouse Gases and Global Warming

The relative contributions of the various greenhouse gases to global warming may not be as clear as some sources would have us believe.

Thawing Ice on Hudson Bay - Jeff Schmaltz

Greenhouse gases are defined as those gases that act to trap outgoing long wave, or infra red, radiation as it leaves the Earth. The time taken for this radiation to escape the atmosphere is consequently prolongued and results in a net heating effect directly above the Earth’s surface. A reasonably persuasive correlation exists between increases in global temperatures and anthropogenic (human induced) greenhouse gas emissions since the Industrial Revolution.

What Are the Major Greenhouse Gases?

According to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) in 2007,the main greenhouse gases, in order of decreasing contribution to global warming, are carbon dioxide, methane, chlorofluorocarbons (CFCs), nitrous oxides and ozone (see figure 2). These figures do not actually reflect the relative heat absorbing qualities of each gas – if they were compared molecule for molecule , CFC’s, nitrous oxides and methane would all have higher heat trapping properties (or ‘Global Warming Potentials’) than carbon dioxide (see figure 2).

However, the calculations used to arrive at the overall warming contribution of each gas also incorporate their atmospheric concentrations. Hence carbon dioxide is the largest contributor to global warming due to its much higher concentrations in the atmosphere: 370 parts per million compared to 1.75 parts per million for methane , 0.75 parts per billion for CFCs and 310 parts per billion for nitrous oxide.

The actual contributions to global warming are therefore around 50% for carbon dioxide, 18% for methane, 14% for CFCs and 6% for nitrous oxides (see figure 2). Ozone’s contribution at 12% is sometimes a point of debate, due to the differing effects it has in the troposphere compared to the stratosphere.

Sources of Greenhouse Gases

Fig. 2: The Relative Importance of Greenhouse Gases -gcrio.org

The major sources of anthropogenic carbon dioxide are the combustion of fossil fuels and biomass and land clearing. Most scientists agree that atmospheric concentrations of this gas have increased from around 280 ppm to 370 ppm since the Industrial Revolution in the early 1800s.

Atmospheric concentrations of methane have increased by around one per cent per year since the 1960s, largely because of increased farming practices. Most methane is released in rice paddies and by grazing cattle, but it can also emanate from coal mining activities and by burning off of vegetation.

Nitrous oxides are produced by the combustion of fossil fuels and the use of nitrogenous fertilizers, while CFCs are present in refrigerants and propellants used in aerosol cans. Ozone is created in the troposphere (lower atmosphere) as a by product of photochemical smog ( nitrogen dioxide can form ozone in the presence of sunlight), and is made by natural processes in the stratosphere (upper atmosphere).

The Role of Water Vapour in Global Warming

Some climate ‘contrarians’ would argue that anthropogenic emissions of greenhouse gases are negligible when compared to the fact that water vapour contributes to greenhouse warming more than any other gas. Indeed, they often indicate that the IPCC and other scientific sources do not even include it in lists of common greenhouse gases.

Former climate Scientist, William Connelly, however, points out that although water vapour contributes up to around 70% of global warming, human activities do not markedly affect its atmospheric concentrations and it therefore has only a feedback, rather than a forcing, effect. This feedback effect is influenced by increases in anthropogenic greenhouse gases such as carbon dioxide and methane because the elevated atmospheric temperatures caused by these gases allow more water to evaporate, which in turn creates further warming.

Water vapour, then, is more a ‘submissive’ greenhouse gas than a dominant one, according to Connelly. NASA scientists have also recently reached similar conclusions using customised atmospheric infrared equipment.

Ozone and CFCs – Controversial Greenhouse Gases?

Because of the link between ozone and CFCs (stratospheric ozone is depleted because of the effect of CFCs), some climatologists argue that their warming effects could be cancelled out. This is because a reduction in the amount of ozone in the upper atmosphere will decrease the retention of outgoing infra red radiation. According to the IPCC Fourth Assessment Report, recent reductions in stratospheric ozone have indeed produced a slight cooling effect.

Moreover, the fact that ozone is distributed unevenly throughout the atmosphere has added to the confusion- while the IPCC have concluded that the warming effects of tropospheric ozone are about 25% of that contributed by carbon dioxide, the combined effects of tropospheric and stratospheric ozone tend to cloud this data.

Despite the question marks surrounding the net effects of water vapour, CFCs and ozone, scientists are in agreement about their heat trapping potential at the molecular level. Many are also unequivocal in their view that anthropogenic carbon dioxide has an overriding effect on climate patterns. Future atmospheric concentrations of each of the major greenhouse gases will no doubt dictate their overall effects on global warming.

References

Connelly, William, 2005, 'Water Vapor; Feedback or Forcing?', realclimate.org

HIeb, Monte, 2003, 'Water Vapour Rules the Greenhouse System', geocraft.com

IPCC, 2001,'Third Assessment Report: climate Change 2001', ipcc.ch

IPCC, 2007, 'Fourth Assessment Report; Climate Change 2007', ipcc.ch

Lashof, D. and Ahuja, D., 1990, 'Relative Contributions of Greenhouse Gases to Global Warming', nature.com

NASA, 2008, 'Water Vapour Confirmed as Major Player in Climate Change', nasa.gov

World Nuclear Association, 2007, 'Global warming- The Science', world-nuclear.org