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Is CO₂ the most significant greenhouse gas?

Those that believe that the increase in CO₂ will have little effect on the climate (hypothesis 2), have this opinion, as they believe that water vapour not CO₂ is the most important greenhouse gas.  Soon et al. (1999), suggest that climate change prediction models do not handle water vapour feedback correctly, however substantial research in to water vapour feedback has taken place since these claims were made back in 1999 (Held and Soden, 2000; Hall and Manabe, 1999). The paper also argues that cooling from negative feedback as a result of greenhouse gases will even out temperatures after an initial increase. Low altitude cloud cover is an example of a negative feedback process. Additional water vapour in the atmosphere increases cloud cover, which acts to cool the earth ( http://www.newscientist.com ).

Critics also stress that greenhouse gases are present naturally, and in the past have helped keep the earth at ambient temperature for life to exist and thrive. The greenhouse gases are vital, without them the earth’s surface would be cooler than it is today by 30^oC, a very significant difference (UNFCC, 2002).

The IPCC (2001) discusses the effect of water vapour on climate. The range most often quoted for the equilibrium global mean surface temperature response to a doubling of CO₂ concentrations in the atmosphere is 1.4 ^oC to 5.8 ^oC. Increase in greenhouse gases reduces the emissions of outgoing long wave radiation. However, the amount of water vapour in the atmosphere varies with temperature. If temperatures increase the air has the ability to hold more water vapour. This increase in water vapour in the air reduces outgoing long wave radiation, which in turn makes the climate warmer, ultimately resulting in a positive water vapour feedback mechanism. Wentz and Schabel (2000) show a clear connection between water vapour volume and temperature, both at the surface and troposphere level. 

The IPCC (2001) accept that water vapour feedback accounts for the large warming predicted by the climate models when CO₂ concentrations are doubled. Water vapour feedback alone approximately doubles the effect on warming, compared to if there was a constant level of water vapour present in the atmosphere (Held and Soden, 2000). Also, other feedback mechanisms are affected by water vapour, including amplification of cloud cover, and ice albedo, which is a positive feedback, where less ice cover means more heat is absorbed. Predictions of up to 3.5 times more warming, based upon estimates of a highly positive cloud feedback have been made (Hall and Manabe, 1999).

It is now widely accepted in the scientific community that humans have altered the composition of the atmosphere, and that anthropogenic global warming is occurring. Increases in greenhouse gas emissions, which have occurred since the industrial revolution have been measured extensively, and cannot be convincingly disputed. Computer models used to stimulate global climate from 1860 to the present (see fig 3.0) only accurately reflect observed global temperature trends when both natural and human factors are included in the models calculations (UKCIP, 2002). The IPCC (2001) concludes that, “… most of the warming observed over the last 50 years is likely to have been due to increasing concentrations of greenhouse gases”. The problems begin when temperatures for before 1860 are predicted. The methods used to determine these past temperatures have their inaccuracies, and it is important that they are critically assessed, with their associated problems and short falls taken into account. It appears that the overall temperature trends are more important than individual predictions, and support an anthropogenic forcing on climate.

At present, an approximation of 6.5 billion tonnes of CO₂ being emitted globally each year has been quoted (UKCIP, 2002). Production is due to burning coal, oil and gas for energy. A further net annual emission of CO₂ at a rate of 1-2 billion tonnes can be attributed to changes in land use carried out by humans. However, there is still much uncertainty and differing opinions regarding future projections, and how the predicted increase in temperature will affect the planet. For example, what effect will the climate change have on the UK’s climate? Or how will the increased temperatures effect global food production? Scientists accept that the effect of doubling the concentration of CO₂ on temperature is not known exactly. This is taken in to account by the IPCC (2001), who state an increase in global mean surface temperature as a value anywhere between the ranges of 1.5-6 ^oC. Recent predictions have been refined, and appear to be in the middle of this range at approximately 3^oC. The scientific community believes a 2^oC rise is all that can be allowed before unpredictable and rapid changes begin to be experienced (Schellnhuber, 2004- cited in Kirby, 2004).

More Facts About Carbon Dioxide and the Greenhouse Gases:

• Carbon dioxide (CO2) is the most significant greenhouse gas because it is released in such vast quantities, mainly as a result of burning fossil fuels like coal, oil, and gas. 
• Carbon dioxide gas traps solar heat in the atmosphere, partly in the same way as glass traps solar heat in a sunroom or a greenhouse. 
• Carbon dioxide is one of the gasses known as greenhouse gasses , so-called because they trap the sun's heat within the atmosphere in much the same way as the glass in a greenhouse traps the heat.
• Carbon dioxide (CO2) - the principal greenhouse that contributes to climate change - comes largely from the use of fossil fuels to produce energy for homes, businesses and transport. 
• Carbon dioxide is not the only gas released by human activities that can cause warming. 
• Carbon dioxide, although not the most potent of the greenhouse gases is the one released in largest volume though everyday industry and living. 
• Carbon dioxide, methane and nitrous oxide in the atmosphere trap this reflected energy preventing it from escaping immediately into space. 
• Carbon dioxide is mainly generated by the use of fossil fuels such as petroleum, gas and coal, and therefore the energy sector is the main contributor to global warming.  Trees and other plants use carbon dioxide and give off oxygen.  Trees act as carbon sinks, removing carbon dioxide from the atmosphere.  Everything that saves energy saves carbon dioxide from being released.  For example, carbon dioxide acts as a fertilizer that makes some plants grow faster.  The carbon dioxide stays in the atmosphere and acts like a warm blanket that holds in the heat. 
• Greenhouse gases are composed primarily of water vapour, carbon dioxide and methane.  For example, carbon dioxide enters the atmosphere daily from decaying plant matter and forest fires.  The most common one that represents over 60% of the total amount of these gases is carbon dioxide (CO2). 
• Greenhouse gases are substances including carbon dioxide (CO2), methane, and chlorofluorocarbons.  Levels of carbon dioxide and other 'greenhouse gases' in the atmosphere have risen steeply since the industrial revolution.  There is more carbon dioxide in the atmosphere now than there has been in the past 650,000 years.  A little bit of carbon dioxide is a good thing. It keeps the planet warm and habitable. 
• Fossils fuels were formed over millions of years and a huge amount of the carbon dioxide formed became locked in organisms, and then minerals such as oil, coal and petroleum inside the Earth's crust.  Anything that requires energy and the energy is produced by burning fossil fuels, such as in normal electricity, transport and food and goods production, will have produced carbon dioxide in its manufacture. 
• Burning fossil fuels releases carbon dioxide stored millions of years ago as oil, coal or natural gas.  While some of the carbon dioxide released is absorbed into the ocean or taken up by plant life, in the short-term about half of it remains in the atmosphere. As the concentration of carbon dioxide in the atmosphere increases, these plants may grow faster and as a consequence take more carbon dioxide out of the atmosphere. This will tend to increase agricultural productivity, however, scientists believe that the increase in CO₂ removed will not be sufficient to slow down climate change in itself.
• How much more the temperature might rise due to climate change is not known, and what might happen as a result, is largely governed by the eventual concentration of carbon dioxide in the atmosphere, or stabilisation level. However, the world community is negotiating on measures (2007) to set a maximum permissible carbon dioxide level within the atmosphere and then limit CO₂ emissions to stabilise our atmosphere at that level.
• Not only does the melting of this ice raise water levels, but it releases carbon dioxide that has been trapped in ice for millions of years.  As more carbon dioxide or other greenhouse gases are added to the atmosphere, the temperature of the earth will rise, assuming nothing else changes. 
• When trees are cut down for development, agriculture, and other purposes, they not only can't use up the carbon dioxide in the air as they usually do, but they actually release it when they decay or are burned. 
• The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have grown by about 31%, 151% and 17%, respectively, between 1750 and 2000 (IPCC 2001).  This site provides information about the climate change issues such as: Carbon dioxide intensity, Global atmospheric concentration of green house and more.