enhanced greenhouse effect

enhanced greenhouse effect The term ‘enhanced greenhouse effect’ has been used to describe anthropogenic impacts on the Earth's atmosphere which influence the global heat budget. It is generally suggested that these impacts will result in an increase in global temperatures and, as a result, the term ‘global warming’ has also been used to describe the phenomenon.

The Earth's atmosphere is naturally warmed as a result of its interaction with incoming and outgoing radiation. The Earth receives energy from the Sun, which mainly emits it at the short-wave (ultraviolet) end of the spectrum. This energy is readily transmitted through the atmosphere unaltered to heat the Earth's surface during the day. The Earth's surface, heated by the sun's energy, subsequently re-radiates this heat back into space as long-wave (infrared) energy. Certain components of the atmosphere (water vapour, carbon dioxide, methane, ozone) absorb some of this energy, and, as a result, the temperature of the atmosphere is raised. The energy is subsequently radiated from the atmosphere to the Earth's surface or out into space. After a short while the atmosphere achieves a balance, in which it receives and loses the same amount of heat, the trapped heat fluctuating only in response to variations in the levels of carbon dioxide, water vapour, ozone, and methane. As a result of this process it is estimated that the atmosphere's average temperature is raised from −17 °C to +15 °C.

This natural process of atmospheric warming (and hence the term ‘greenhouse effect’) has been likened to the way in which glass in greenhouses allows short-wave radiation into the structure but stops long-wave radiation from being transmitted out. The temperature in the greenhouse thus rises. It must, however, be remembered that the glass also prevents the air in a greenhouse from mixing with the colder air outside. Such a barrier is not present in the natural system. Many scientists would thus argue that the greenhouse is an inappropriate model and the term ‘greenhouse effect’ is consequently a rather poor one. It nevertheless continues to be widely used, particularly by the media and the general public.

Study of air samples trapped in polar ice has indicated that levels of carbon dioxide in the atmosphere have varied in the past. During Quaternary glaciations carbon dioxide levels ranged between 180 and 200 parts per million by volume (ppmv); during the warm interglacials they rose to 275 ppmv. The latter figure is taken as being a good estimate of carbon dioxide concentrations prior to the Industrial Revolution. Since the level of carbon dioxide in the atmosphere will control the amount of heat trapped, it is concluded that atmospheric temperatures will have fluctuated in response to these variations. It is now believed that human activities may be increasing the concentrations of this and other ‘greenhouse’ gases, thus enhancing the effect.

Most attention has been paid to the human impact on carbon dioxide concentrations, and the possible consequences of such changes on atmospheric temperature. Detailed measurements of carbon dioxide concentrations have been made only since 1957, but levels appear to have risen from around 290 ppmv in the 1880s to 353 ppmv in the 1990s. Much of this rise has been attributed to human activity. For example, the burning of fossil fuels (coal, oil), which are largely carbon-based, results in the generation of carbon dioxide, which is released into the atmosphere. Similarly, the clearing of vegetation usually ends with the material being burnt, which directly increases the amount of carbon dioxide in the atmosphere. Such clearing also reduces the rate of photosynthesis, which extracts carbon from the atmosphere, and it increases the rate of oxidation of carbon from the newly exposed soil. At present, clearing processes are believed to generate between 5 and 20 per cent of the anthropogenic generated carbon dioxide, but in the recent past (agricultural settlement of North America) they may have contributed 50 per cent of anthropogenically produced carbon dioxide. At present it is estimated that human activities put some six billion tonnes of carbon dioxide into the atmosphere each year. Much of this is taken up by natural sinks (oceans, terrestrial ecosystems). The detailed processes involved that operates in such sinks, and the full range of natural sinks are, however, rather poorly understood and it is recognized that less carbon dioxide appears to have remained within the atmosphere than current understanding of the systems would suggest.

Measurements indicate that concentrations of carbon dioxide in the atmosphere have increased rapidly in recent years (by 2 to 4 ppmv per year). If the present rate of increase continues, the concentrations will be double those of pre-industrial levels by the year 2075. It must, however, be recognized that such predictions are uncertain, since they depend on the rate at which fossil fuels and natural ecosystems are exploited, and on the measures taken to reduce such inputs.

Since carbon dioxide is an agent known to promote atmospheric warming, it has been concluded that increased concentrations should result in increased atmospheric temperatures. The extent of potential warming has usually been determined by using computerized General Circulation Models. The carbon dioxide concentration is raised within the model, which is then allowed to run until an equilibrium is reached. Models utilized by the International Panel on Climate Change (IPCC) indicate that a doubling of carbon dioxide concentrations would raise global temperatures by between 1.5 and 4.5 °C, with a best estimate 2.5 °C. These estimates mask wide-scale variations at a regional level, since in high latitudes temperatures are predicted to rise by as much as 8–12 °C in winter but by such less during the summer. Similarly, low-latitude areas are predicted to show only minor increases in temperature (1 °C). The models also make predictions relating to changes in precipitation. For example, much of Europe and the USA will become drier, while Canada becomes wetter in the winter and drier in the summer. Confidence in these predictions is, however, limited.

The predictions from most of the General Circulation Models are based solely upon changes in the carbon dioxide content of the atmosphere, and they thus fail to recognize that the concentrations of other greenhouse gases are also being altered. As a result the accuracy of the models is questionable and it is possible that the greenhouse effect may be enhanced to a greater extent.

Methane is principally produced through the anaerobic decay of organic matter and its concentration within the atmosphere is relatively low at 1.72 ppmv. Methane is, however, a far more effective greenhouse gas, being 21 more times more effective than carbon dioxide. It has been estimated that levels of methane are increasing at about 0.8–1.0 % per year, primarily as a result of agricultural practices. By far the largest source of anthropogenically produced methane are the world's rice paddies, which provide a suitable anaerobic environment when flooded. Other major sources include domestic animals, which produce considerable amounts of methane in their digestive systems, and biomass burning, which releases methane as the land is cleared for cultivation. Methane is also produced by the burning of coal and from the utilization of natural gas, as well as from the decay of organic wastes.

Another natural greenhouse gas is nitrous oxide, which is produced by the denitrification of soils. It has been suggested that levels have been increasing because of increased use of nitrogen fertilizers and from the burning of fossil fuels. The IPCC assessment has, however, indicated that the sources and sinks of this gas are at present very difficult to estimate, and, as a result, its possible contribution to the enhanced greenhouse effect can not be determined accurately.

In recent years, society has introduced new greenhouse gases into the atmosphere, namely CFCs (chlorofluorocarbons) and other halocarbons. These gases are released from aerosols, refrigeration units, insulating foams, and industrial plants. They are now recognized as one of the principal causes for the destruction of the ozone layer. They are, however, also very effective greenhouse gases. For example, when compared to carbon dioxide, the anthropogenically produced gas CFC-11 is some 120 00 times more effective in trapping heat in the atmosphere. The atmospheric concentration of these gases is very low, ranging between 2 and 484 parts per trillion by volume (pptv) but until recently they have have been increasing at a very rapid rate (4–15 per cent per annum). Although society has started to reduce rates of emission, in response to the ozone hole problems, such gases are estimated to have a long residence times in the atmosphere, in some instances up to 400 years. As a result they will continue to contribute to the enhanced greenhouse effect.

It is now generally accepted that concentrations of greenhouse gases have increased in the recent past, mainly as the result of human activities, and they are likely to increase in the future. The impact of such changes on the global heat budget remains uncertain. It is recognized that the atmospheric system is characterized by a wide range of feedback effects which could both enhance and reduce the greenhouse effect. At the same time it is recognized that many of the processes involved in the removal of greenhouse gases from the atmosphere are poorly understood. The General Circulation Models that have been used to estimate the possible impacts of an enhanced greenhouse effect also have their limitations. In the main they are based solely on changes in carbon dioxide and thus ignore the contribution of other gases. At the same time, the models have not effectively incorporated the ocean system, although this is essential if a full understanding of the global heat budget is to be achieved. At present there is no consensus about the possible impact of increased concentrations in greenhouse gases. Most scientists consider that warming will take places, but the likely rates and extent of warming, particularly at a regional level, remain poorly defined.

Some scientists have suggested that global warming has already begun (0.5 °C in the past hundred years). Most, however, have concluded that any changes that have taken place lie within the bounds of natural climate fluctuations, and thus the enhanced greenhouse effect remains unproved. Society has so far done little to reduce the possible effects, and it seems likely that while the levels of uncertainty surrounding this issue remain high, few changes will occur. If the models are correct, the enhanced greenhouse effect should be noticeable within the next twenty years.

Callum R. Firth

Bibliography

Kemp, D. D. (1995) Global environmental issues: a climatological approach (2nd edn). Routledge, London.