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Greenhouse gases
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Greenhouse gases and global warming

Our planet is surrounded by a gaseous atmosphere, which comprises primarily oxygen and nitrogen. Both are important in sustaining life on earth. However what controls the global temperature is a group of gases called greenhouse gases which congregate in the upper atmosphere. These gases of which carbon dioxide is the most important allow sunlight to penetrate the earth’s atmosphere, which is then absorbed by the earth. When some of these rays are reflected back (at longer wavelengths) (Figure 1), these gases have the ability to trap some of the Earth’s radiation which would otherwise be transmitted to outer space. While one reason for the existence of life on earth is the presence of some greenhouse gases which raises the average earth temperature from -18 C to +12 C, the present concern is about the rate of increase in this average temperature and what impacts this can induce. Without these gases, the earth would be frozen (like Mars) so some concentration of these gases is essential for life on earth.

Ray diagram of solar energy

Figure 1 Ray diagram of solar energy flows showing effect of greenhouse gases

Carbon dioxide is the most abundant greenhouse gas and its production is primarily linked to our use of fossil fuel and results as a by-product of the combustion of oil, gas or coal which is used to provide energy for activities like transport, heating our homes or generating electricity.

Methane has a much stronger warming impact than carbon dioxide though a much shorter half life. It is the main constituent of natural gas, one of our main fossil fuels, which is widely used for space and water heating and increasingly for generating electricity. When burnt, methane combines with oxygen to form carbon dioxide and water vapour, which is also a greenhouse gas.

Nitrous oxides also have a much stronger warming impact than carbon dioxide and are released naturally by bacteria, digestion of ruminant animals like cows and in the combustion of fossil fuels.

Fluorinated gases have the highest warming impact and are a family of synthetic chemicals primarily used as refrigerants for air conditioners and in refridgerators. These chemicals are being phased out by international agreement and are being replaced by naturally occurring compounds with low global impact warming and low ozone depletion potential. These compounds include hydrocarbons like butane, ammonia and carbon dioxide. So paradoxically carbon dioxide has beneficial as well as adverse environmental properties. The other two greenhouse gases present naturally are water vapour (as noted above) and ozone.

Water vapour is created by evaporation from the seas and sources on land like lakes and rivers. Global warming will accelerate the rate of evaporation which will impact the earth’s water cycle and could increase global warming further.

Ozone is an unstable form (isotope) of oxygen and exists naturally in the upper atmosphere. Apart from its ability as a greenhouse gas to absorb the earth’s radiation, more importantly it protects living species by absorbing the ultraviolet portion of sunlight falling on the earth’s surface. As ozone is broken down by fluorinated gases, gaps have appeared in the ozone layer and so the global intention is to phase out all fluorinated gases as set out the Montreal convention (1987) on ozone depletion. Ozone can be created by the reaction between sunlight and ground level pollution so the there can be an excess of ozone if the level of pollution is high. Whilst ozone itself is a greenhouse gas, it can react with other greenhouse gases to shorten their lifespan.

Concentration of greenhouse gases

It is possible to determine the historic concentrations of these gases by various methods of which analysis of tree rings and the isotropic ratio of carbon 13 to carbon 14 in air trapped in ice cores are the most important. These data show that the concentration of carbon dioxide in the earth’s atmosphere is greater now than at any time in the past one million years.

The concentrations of the three principal greenhouse gases in the atmosphere are plotted in Figure 2. The sharp increase since 1800 has been studied and modelled in great depth by the scientists whose work has been collated by the Intergovernmental Panel on Climate Change (IPCC).This increase is above the natural fluctuations in their concentrations and coincides with the start of the industrial revolution in which industry grew very rapidly to produce products that people needed.

Rise in greenhouse gas emissions
Figure 2 Rise in greenhouse gas emissions over the past 2000 years (IPCC AR4 FAQ)
? Why do you think that global warming has become such a major issue?

Principal sources and sinks

Sources-The largest emissions of carbon dioxide arise through our combustion of fossil fuels to produce useful forms of energy such as heating our homes, moving our transport or generating electricity. Agriculture is the next biggest source via activities such as soil fertilization, farming of animals and growing crops like rice. A further important contribution is from changes in land use including clearing and burning of vegetation and deforestation as 40% of the Earth’s land surface has now been converted for grazing or farming (Figure 3).

Our emissions are increasing because the world’s population is growing as is our individual use of energy. So as we discuss in other SDGs we need to use energy more efficiently and derive our energy from renewable sources which do not result in greenhouse gas emissions.

Souce of carbon

Figure 3: Sources of carbon dioxide

Sinks-The biggest congregation of greenhouse gases is in the upper atmosphere, which is resulting in global warming. As already noted these gases include not only carbon dioxide, but also methane, nitrous oxides and florinated gases.The oceans are the second most important sink because of their huge volume and the varied life they support from plankton to whales. Forests, particularly tropical forests are also important sinks and the relative importance of the various sinks is illustrated in Figure 4. The ability of the oceans themselves to absorb CO2 will result in them becoming more acidic so decreasing the ability of marine organisms to form calcium carbonate, one of the key substances in building the skeletal of organisms. So like tropical rain-forests, it is important to preserve the biodiversity of the marine ecosystem.

Sinks of greenhouse gases
Figure 4: Sinks of greenhouse gases
? How do you think global warming will affect the ability of sinks to absorb greenhouse gases?

KITH # Activity Age range
1.13   Sources and sinks for greenhouse gases   
  Science, chemistry, biology
  11 – 18 


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